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help designing gimmick capacitor
Hi all,
I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. Any ideas to minimize input capacitance to the box? Here's the amp in box. http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic; http://www.crystal-radio.eu/enfetamp.htm Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. |
help designing gimmick capacitor
On Feb 20, 10:20*am, "amdx" wrote:
Hi all, *I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. *The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about *rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) *Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. * Any ideas to minimize input capacitance to the box? *Here's the amp in box.http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic;http://www.crystal-radio..eu/enfetamp.htm * * * * * * * *Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. The old-tyme gimmick was two wires, twisted together. That would contain the field better than your open plates. Another alternative: you could surround your cap with a shield and drive the shield from the FET source, bootstrapping it. -- Cheers, James Arthur |
help designing gimmick capacitor
wrote in message ... On Feb 20, 10:20 am, "amdx" wrote: Hi all, I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. Any ideas to minimize input capacitance to the box? Here's the amp in box.http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic;http://www.crystal-radio.eu/enfetamp.htm Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. ........................................ The old-tyme gimmick was two wires, twisted together. That would contain the field better than your open plates. Another alternative: you could surround your cap with a shield and drive the shield from the FET source, bootstrapping it. -- Cheers, James Arthur Hi James, The twisted pair may be the easiest, but as a thought, if I took a piece of coax and removed the center conductor leaving just the shield, then inserted a twisted pair as my capacitor. I then connect the shield the FET source, Ok, I have never bootstrapped before, but wouldn't I then have a higher potential than desired, even a possibility of oscillation? Mikek PS. Do I have any concern about inductive coupling with a twisted pair? |
help designing gimmick capacitor
On Feb 20, 11:14*am, "amdx" wrote:
wrote in message ... On Feb 20, 10:20 am, "amdx" wrote: Hi all, I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. Any ideas to minimize input capacitance to the box? Here's the amp in box.http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic;http://www.crystal-radio.eu/enfetamp.htm Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. ....................................... The old-tyme gimmick was two wires, twisted together. *That would contain the field better than your open plates. Another alternative: you could surround your cap with a shield and drive the shield from the FET source, bootstrapping it. r * Hi James, * The twisted pair may be the easiest, but as a thought, if I took a piece of coax and removed the center conductor leaving just the shield, then inserted a twisted pair as my capacitor. That's too tight a box, and the dielectric screws things up--you'll be making shunt caps to the shield. *I then connect the shield the FET source, Ok, I have never bootstrapped before, but wouldn't I then have a higher potential than desired, even a possibility of oscillation? Higher potential? No. Possibility of oscillation? Yes, but not likely. Keep the shield well clear of the gimmick to minimize that. Just the twisted gimmick itself is probably all you'll need. The bootstrapping is lagniappe. * * * * * * * * * * * *Mikek *PS. Do I have any concern about inductive coupling with a twisted pair? Nope. -- Cheers, James Arthur |
help designing gimmick capacitor
On Sun, 20 Feb 2011 09:20:12 -0600, "amdx" wrote:
Hi all, I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. Any ideas to minimize input capacitance to the box? Here's the amp in box. http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic; http://www.crystal-radio.eu/enfetamp.htm Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. Use a real surface-mount 0.3 pF cap, or a homemade coaxial cap. The 1 cm square plates are too big and have their own capacitance to the world. Bootstrap the drain of Q1. "T" means transformer, which shows that this circuit was done by an amateur. All that tricky stuff could be replaced by one opamp. It could have close to zero Cin with a little positive feedback. John |
help designing gimmick capacitor
On Feb 20, 12:39*pm, John Larkin
wrote: On Sun, 20 Feb 2011 09:20:12 -0600, "amdx" wrote: Hi all, I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about *rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. *Any ideas to minimize input capacitance to the box? Here's the amp in box. http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic; http://www.crystal-radio.eu/enfetamp.htm * * * * * * * Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. Use a real surface-mount 0.3 pF cap, or a homemade coaxial cap. The 1 cm square plates are too big and have their own capacitance to the world. For that matter the tiny input cap in Mike's circuit is counterproductive--it divides the signal down and makes the gain unpredictable. Better: use 10pF coupling, lose less at the input, and use less gain later. Bootstrap the FET so the input sees very low C. Do those and you don't even need a gimmick. Bootstrap the drain of Q1. "T" means transformer, which shows that this circuit was done by an amateur. All that tricky stuff could be replaced by one opamp. It could have close to zero Cin with a little positive feedback. John I like the one at the top of pg. 2: http://www.national.com/an/AN/AN-32.pdf 1970--a classic. -- Cheers, James Arthur |
help designing gimmick capacitor
On Sun, 20 Feb 2011 09:20:12 -0600, "amdx" wrote:
http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. Ever consider simply using a proper air core variable cap? |
help designing gimmick capacitor
On Sun, 20 Feb 2011 10:13:12 -0800 (PST),
wrote: On Feb 20, 12:39*pm, John Larkin wrote: On Sun, 20 Feb 2011 09:20:12 -0600, "amdx" wrote: Hi all, I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about *rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. *Any ideas to minimize input capacitance to the box? Here's the amp in box. http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic; http://www.crystal-radio.eu/enfetamp.htm One gotcha: with +12 on the drain of the jfet, expect a lot of gate current. Hot carriers or some such. * * * * * * * Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. Use a real surface-mount 0.3 pF cap, or a homemade coaxial cap. The 1 cm square plates are too big and have their own capacitance to the world. For that matter the tiny input cap in Mike's circuit is counterproductive--it divides the signal down and makes the gain unpredictable. Better: use 10pF coupling, lose less at the input, and use less gain later. Bootstrap the FET so the input sees very low C. Do those and you don't even need a gimmick. Bootstrap the drain of Q1. "T" means transformer, which shows that this circuit was done by an amateur. All that tricky stuff could be replaced by one opamp. It could have close to zero Cin with a little positive feedback. John I like the one at the top of pg. 2: http://www.national.com/an/AN/AN-32.pdf 1970--a classic. If the jfet drives a non-inverting opamp with gain, one could over-bootstrap the drain to hit zero or even negative input capacitance; tweak that with a pot or a small variable cap. Then, as you say, dump the 0.3 pF input cap and have predictable gain. Phil Hobbs likes this: +-----------+-----Vcc | | | | | R | | c | b----+-----+ ~~ +3V e | | | | | | | | d C R in----------g | | s | | | | gnd | | +-----+-----------out | | | R or current sink | | | Vee John |
help designing gimmick capacitor
"John Larkin" wrote in message ... On Sun, 20 Feb 2011 09:20:12 -0600, "amdx" wrote: Hi all, I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. Any ideas to minimize input capacitance to the box? Here's the amp in box. http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic; http://www.crystal-radio.eu/enfetamp.htm Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. Use a real surface-mount 0.3 pF cap, or a homemade coaxial cap. The 1 cm square plates are too big and have their own capacitance to the world. Bootstrap the drain of Q1. "T" means transformer, which shows that this circuit was done by an amateur. All that tricky stuff could be replaced by one opamp. It could have close to zero Cin with a little positive feedback. John Bootstrap the drain of Q1. You need to walk me through that, (I'm an amateur) Ah, he's done some nice work on the subject of crystal radios and high Q inductors. http://www.crystal-radio.eu/index.html Page down to experiment with LC circuits. It could have close to zero Cin with a little positive feedback. How much closer? If the input cap is 0.3pf what do you the input impedance is? Input is 0.3pf, 20 Meg to ground driving FET gate. Thanks, Mikek |
help designing gimmick capacitor
"FigureItOut" wrote in message ... On Sun, 20 Feb 2011 09:20:12 -0600, "amdx" wrote: http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. Ever consider simply using a proper air core variable cap? You have any 0.3pf air variables. Why is this one improper? Mikek |
help designing gimmick capacitor
On Feb 20, 2:44*pm, "amdx" wrote:
"John Larkin" wrote in message ... On Sun, 20 Feb 2011 09:20:12 -0600, "amdx" wrote: Hi all, I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about *rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. *Any ideas to minimize input capacitance to the box? Here's the amp in box. http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic; http://www.crystal-radio.eu/enfetamp.htm * * * * * * * Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. Use a real surface-mount 0.3 pF cap, or a homemade coaxial cap. The 1 cm square plates are too big and have their own capacitance to the world. Bootstrap the drain of Q1. "T" means transformer, which shows that this circuit was done by an amateur. All that tricky stuff could be replaced by one opamp. It could have close to zero Cin with a little positive feedback. John * * Bootstrap the drain of Q1. You need to walk me through that, (I'm an amateur) Ah, he's done some nice work on the subject of crystal radios and high Q inductors.http://www.crystal-radio.eu/index.html Page down to experiment with LC circuits. * It could have close to zero Cin with a little positive feedback. *How much closer? If the input cap is 0.3pf what do you the input impedance is? Input is 0.3pf, 20 Meg to ground driving FET gate. * * * * * * * * * * * * * * * * * Thanks, Mikek We're talking about something roughly like this: +12V -o-----o-----------. | | | | | | | | R5 3.3M | |Q2 | | \| | V1 | |---o-----o ~~ ~=+3V | .| | | | | | R6 1M R3 | | | C1 | | |C3 === 10pF | g |-'d --- in--||--o--|--| --- | | |-.s |100n R1 10M | |Q1 | | | | | o--o-||--o-----o-----------out | C2 | R2 10M 100nF| | | === R4 470 R | | === C2 drives the center of the input bias resistor, which cancels the loading caused by R1-R2. This is called "bootstrapping". Q2 does the same thing for the drain terminal--it causes the drain terminal to go up and down with the input signal. That saves the input signal from having to charge Q1's gate-to-drain capacitance, effectively making that capacitance disappear. This front-end has *much* higher impedance than the original, and a predictable gain that's close to 1. R3 is to bias the FET output to (V1)/2, for maximum dynamic range. Higher V1 would give bigger dynamic range. A cheapskate could put a resistor in Q2's collector and use it as a voltage-gain stage too. -- Cheers, James Arthur |
help designing gimmick capacitor
On Feb 20, 3:30*pm, wrote:
On Feb 20, 2:44*pm, "amdx" wrote: "John Larkin" wrote in message .. . On Sun, 20 Feb 2011 09:20:12 -0600, "amdx" wrote: Hi all, I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about *rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. *Any ideas to minimize input capacitance to the box? Here's the amp in box. http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic; http://www.crystal-radio.eu/enfetamp.htm * * * * * * * Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. Use a real surface-mount 0.3 pF cap, or a homemade coaxial cap. The 1 cm square plates are too big and have their own capacitance to the world. Bootstrap the drain of Q1. "T" means transformer, which shows that this circuit was done by an amateur. All that tricky stuff could be replaced by one opamp. It could have close to zero Cin with a little positive feedback. John * * Bootstrap the drain of Q1. You need to walk me through that, (I'm an amateur) Ah, he's done some nice work on the subject of crystal radios and high Q inductors.http://www.crystal-radio.eu/index.html Page down to experiment with LC circuits. * It could have close to zero Cin with a little positive feedback. *How much closer? If the input cap is 0.3pf what do you the input impedance is? Input is 0.3pf, 20 Meg to ground driving FET gate. * * * * * * * * * * * * * * * * * Thanks, Mikek We're talking about something roughly like this: * * +12V -o-----o-----------. * * * * * *| * * | * * * * * | * * * * * *| * * | * * * * * | * * * * * *| * * | * * * R5 3.3M * * * * * *| * * |Q2 * * * * | * * * * * *| * * *\| * * * * | * * * *V1 * * * * * *| * * * |---o-----o * ~~ ~=+3V * * * * * *| * * .| * | * * | * * * * * *| * * | * * | *R6 1M * * * * * R3 * * | * * | * * | * * C1 * * | * * | * * |C3 *=== * * 10pF * | g |-'d * --- in--||--o--|--| * * *--- * * * * | *| * |-.s * *|100n * * R1 10M | * * |Q1 * | * * * * | *| * * | * * | * * * * o--o-||--o-----o-----------out * * * * | * *C2 *| * * R2 10M *100nF| * * * * | * * * *| * * * *=== * R4 470 R * * * * * * * * *| * * * * * * * * *| * * * * * * * * === C2 drives the center of the input bias resistor, which cancels the loading caused by R1-R2. *This is called "bootstrapping". Q2 does the same thing for the drain terminal--it causes the drain terminal to go up and down with the input signal. *That saves the input signal from having to charge Q1's gate-to-drain capacitance, effectively making that capacitance disappear. This front-end has *much* higher impedance than the original, and a predictable gain that's close to 1. R3 is to bias the FET output to (V1)/2, for maximum dynamic range. Higher V1 would give bigger dynamic range. A cheapskate could put a resistor in Q2's collector and use it as a voltage-gain stage too. Oh, I didn't calculate the biasing, so R4's probably wrong. --James |
help designing gimmick capacitor
wrote in message ... On Feb 20, 12:39 pm, John Larkin wrote: On Sun, 20 Feb 2011 09:20:12 -0600, "amdx" wrote: Hi all, I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. Any ideas to minimize input capacitance to the box? Here's the amp in box. http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic; http://www.crystal-radio.eu/enfetamp.htm Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. Use a real surface-mount 0.3 pF cap, or a homemade coaxial cap. The 1 cm square plates are too big and have their own capacitance to the world. .................................................. ............ For that matter the tiny input cap in Mike's circuit is counterproductive--it divides the signal down and makes the gain unpredictable. It's not so inpredictable, I set the amplifier gain at 17 and then adjusted the capacitor spacing for a total amplifier gain of 1. I'm not being argumentive, just trying understand. Better: use 10pF coupling, lose less at the input, and use less gain later. Bootstrap the FET so the input sees very low C. Do those and you don't even need a gimmick. I need to know more about bootstraping. Also doesn't the 0.3pf cap reduce the loading effect of the 20 Meg resistor? Thanks, Mikek |
help designing gimmick capacitor
amdx Inscribed thus:
Hi all, I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. Any ideas to minimize input capacitance to the box? Here's the amp in box. http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic; http://www.crystal-radio.eu/enfetamp.htm Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. Change to a co-axial cap. -- Best Regards: Baron. |
help designing gimmick capacitor
On Feb 20, 3:56*pm, "amdx" wrote:
wrote in message ... On Feb 20, 12:39 pm, John Larkin wrote: On Sun, 20 Feb 2011 09:20:12 -0600, "amdx" wrote: Hi all, I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. Any ideas to minimize input capacitance to the box? Here's the amp in box. http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic; http://www.crystal-radio.eu/enfetamp.htm Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. Use a real surface-mount 0.3 pF cap, or a homemade coaxial cap. The 1 cm square plates are too big and have their own capacitance to the world. .................................................. ........... For that matter the tiny input cap in Mike's circuit is counterproductive--it divides the signal down and makes the gain unpredictable. * It's not so inpredictable, I set the amplifier gain at 17 and then adjusted the capacitor spacing for a total amplifier gain of 1. *I'm not being argumentive, just trying understand. The input gain of the original circuit depends on the voltage divider comprising your gimmick cap, on the one hand, and the FET's capacitance on the other. No two FETs will have the same capacitance, so you can't know in advance exactly what the circuit's gain will be. That's why you have to tweak it via the input capacitor right now. Better: use 10pF coupling, lose less at the input, and use less gain later. *Bootstrap the FET so the input sees very low C. *Do those and you don't even need a gimmick. I need to know more about bootstraping. Also doesn't the 0.3pf cap reduce the loading effect of the 20 Meg resistor? * * * * * * * * * * * *Thanks, Mikek Not really. At 1MHz, 0.3pf has a reactance of 530K, so it just adds 530K in series with 20M. It also attenuates your signal by a factor of 17, if the writeup is accurate. The author says the amp's input capacitance is 1.4pF. That means either there's a lot of stray capacitance, or the input coupling cap is actually a lot larger than 0.3pF. --James |
help designing gimmick capacitor
wrote in message ... On Feb 20, 3:56 pm, "amdx" wrote: wrote in message ... On Feb 20, 12:39 pm, John Larkin wrote: On Sun, 20 Feb 2011 09:20:12 -0600, "amdx" wrote: Hi all, I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. Any ideas to minimize input capacitance to the box? Here's the amp in box. http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic; http://www.crystal-radio.eu/enfetamp.htm Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. Use a real surface-mount 0.3 pF cap, or a homemade coaxial cap. The 1 cm square plates are too big and have their own capacitance to the world. .................................................. ........... For that matter the tiny input cap in Mike's circuit is counterproductive--it divides the signal down and makes the gain unpredictable. It's not so inpredictable, I set the amplifier gain at 17 and then adjusted the capacitor spacing for a total amplifier gain of 1. I'm not being argumentive, just trying understand. ........................................ The input gain of the original circuit depends on the voltage divider comprising your gimmick cap, on the one hand, and the FET's capacitance on the other. No two FETs will have the same capacitance, so you can't know in advance exactly what the circuit's gain will be. That's why you have to tweak it via the input capacitor right now. Hmm. I might have got a hot FET as my cap is spaced 5 mm vs his 3mm. I calculated 0.177pf for my cap. Had adjusted the circuit voltage gain after the FET circuit for 17. Then adjusted the cap for a total gain of 1. Better: use 10pF coupling, lose less at the input, and use less gain later. Bootstrap the FET so the input sees very low C. Do those and you don't even need a gimmick. I need to know more about bootstraping. Also doesn't the 0.3pf cap reduce the loading effect of the 20 Meg resistor? Thanks, Mikek Not really. At 1MHz, 0.3pf has a reactance of 530K, so it just adds 530K in series with 20M. It also attenuates your signal by a factor of 17, if the writeup is accurate. Oh, I had assumed a much higher impedance for that small cap. Best not to assume :-) The author says the amp's input capacitance is 1.4pF. That means either there's a lot of stray capacitance, or the input coupling cap is actually a lot larger than 0.3pF. I'll be trying the twisted pair to reduce that stray capacitance. Mikek |
help designing gimmick capacitor
On Sun, 20 Feb 2011 13:44:37 -0600, "amdx" wrote:
"John Larkin" wrote in message .. . On Sun, 20 Feb 2011 09:20:12 -0600, "amdx" wrote: Hi all, I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. Any ideas to minimize input capacitance to the box? Here's the amp in box. http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic; http://www.crystal-radio.eu/enfetamp.htm Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. Use a real surface-mount 0.3 pF cap, or a homemade coaxial cap. The 1 cm square plates are too big and have their own capacitance to the world. Bootstrap the drain of Q1. "T" means transformer, which shows that this circuit was done by an amateur. All that tricky stuff could be replaced by one opamp. It could have close to zero Cin with a little positive feedback. John Bootstrap the drain of Q1. You need to walk me through that, (I'm an amateur) If the source of the fet follower drives an opamp with a gain of, say, +2, you could take the amp's output, run it through a pot, and AC couple that into the drain. With the pot at zero gain, there's no bootstrapping, so the fet's Cg-d loads the input 100%. With the pot at mid-rotation, the drain is forced to swing up and down just about as much as the input signal does. Since both ends of Cg-d are at the same signal level, there's almost no current in that capacitance, so it sort of disappears, as far as the gate is concerned. Turn the pot a little more, and a small excursion at the gate results in a *bigger* excursion at the drain. So the current in Cg-d flows in the opposite direction from a real capacitor, and Cg-d becomes a negative capacitor. It can then be used to cancel all other capacitance at the gate node, making the box have a net nearly-zero input capacitance. Or just add a small variable cap from the opamp output back into the gate, and adjust for near zero input capacitance. When it oscillates, you've gone too far. John |
help designing gimmick capacitor
On Sun, 20 Feb 2011 07:52:18 -0800 (PST),
wrote: The old-tyme gimmick was two wires, twisted together. That would contain the field better than your open plates. Another alternative: you could surround your cap with a shield and drive the shield from the FET source, bootstrapping it. Does the term SNIP have any meaning to you? Jim |
help designing gimmick capacitor
On Sun, 20 Feb 2011 21:31:42 -0800, RST Engineering
wrote: On Sun, 20 Feb 2011 07:52:18 -0800 (PST), wrote: The old-tyme gimmick was two wires, twisted together. That would contain the field better than your open plates. Another alternative: you could surround your cap with a shield and drive the shield from the FET source, bootstrapping it. Does the term SNIP have any meaning to you? Jim Save up and buy yourself some scroll bars. John |
help designing gimmick capacitor
amdx wrote:
Hi all, I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. Any ideas to minimize input capacitance to the box? Here's the amp in box. http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic; http://www.crystal-radio.eu/enfetamp.htm Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. Response before looking at others: 1. use one 10Meg and connect it to source OR cap bootstrap the 10meg input divider to the source. 2. "wrap" a shield around the input wire & plate and drive the shield from the FET source or T2 emitter. 3 What the heck is that HUGE block attached to the input plate? Ditech it; you will see a significant decrease in input capacitance. |
help designing gimmick capacitor
On Feb 21, 12:31*am, RST Engineering wrote:
On Sun, 20 Feb 2011 07:52:18 -0800 (PST), wrote: The old-tyme gimmick was two wires, twisted together. *That would contain the field better than your open plates. Another alternative: you could surround your cap with a shield and drive the shield from the FET source, bootstrapping it. Does the term SNIP have any meaning to you? Jim I quoted the original post. Did you think the original was irrelevant? I don't agree. I try to quote enough so that a newcomer can, in one post, understand both the full question I'm responding to, and my reply. -- Cheers, James Arthur |
help designing gimmick capacitor
On Feb 20, 3:30*pm, wrote:
On Feb 20, 2:44*pm, "amdx" wrote: "John Larkin" wrote in message .. . On Sun, 20 Feb 2011 09:20:12 -0600, "amdx" wrote: Hi all, I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about *rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. *Any ideas to minimize input capacitance to the box? Here's the amp in box. http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic; http://www.crystal-radio.eu/enfetamp.htm * * * * * * * Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. Use a real surface-mount 0.3 pF cap, or a homemade coaxial cap. The 1 cm square plates are too big and have their own capacitance to the world. Bootstrap the drain of Q1. "T" means transformer, which shows that this circuit was done by an amateur. All that tricky stuff could be replaced by one opamp. It could have close to zero Cin with a little positive feedback. John * * Bootstrap the drain of Q1. You need to walk me through that, (I'm an amateur) Ah, he's done some nice work on the subject of crystal radios and high Q inductors.http://www.crystal-radio.eu/index.html Page down to experiment with LC circuits. * It could have close to zero Cin with a little positive feedback. *How much closer? If the input cap is 0.3pf what do you the input impedance is? Input is 0.3pf, 20 Meg to ground driving FET gate. * * * * * * * * * * * * * * * * * Thanks, Mikek We're talking about something roughly like this: * * +12V -o-----o-----------. * * * * * *| * * | * * * * * | * * * * * *| * * | * * * * * | * * * * * *| * * | * * * R5 3.3M * * * * * *| * * |Q2 * * * * | * * * * * *| * * *\| * * * * | * * * *V1 * * * * * *| * * * |---o-----o * ~~ ~=+3V * * * * * *| * * .| * | * * | * * * * * *| * * | * * | *R6 1M * * * * * R3 * * | * * | * * | * * C1 * * | * * | * * |C3 *=== * * 10pF * | g |-'d * --- in--||--o--|--| * * *--- * * * * | *| * |-.s * *|100n * * R1 10M | * * |Q1 * | * * * * | *| * * | * * | * * * * o--o-||--o-----o-----------out * * * * | * *C2 *| * * R2 10M *100nF| * * * * | * * * *| * * * *=== * R4 470 R * * * * * * * * *| * * * * * * * * *| * * * * * * * * === C2 drives the center of the input bias resistor, which cancels the loading caused by R1-R2. *This is called "bootstrapping". Q2 does the same thing for the drain terminal--it causes the drain terminal to go up and down with the input signal. *That saves the input signal from having to charge Q1's gate-to-drain capacitance, effectively making that capacitance disappear. This front-end has *much* higher impedance than the original, and a predictable gain that's close to 1. R3 is to bias the FET output to (V1)/2, for maximum dynamic range. Higher V1 would give bigger dynamic range. A cheapskate could put a resistor in Q2's collector and use it as a voltage-gain stage too. -- Cheers, James Arthur- Hide quoted text - - Show quoted text - Thanks for the explanation James. The bootstrapping of the base does nothing for the input capacitance.(?) Is that correct? George H. |
help designing gimmick capacitor
wrote in message ... On Feb 20, 11:14 am, "amdx" wrote: wrote in message ... On Feb 20, 10:20 am, "amdx" wrote: Hi all, I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. Any ideas to minimize input capacitance to the box? Here's the amp in box.http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic;http://www.crystal-radio.eu/enfetamp.htm Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. ....................................... The old-tyme gimmick was two wires, twisted together. That would contain the field better than your open plates. Another alternative: you could surround your cap with a shield and drive the shield from the FET source, bootstrapping it. r Hi James, The twisted pair may be the easiest, but as a thought, if I took a piece of coax and removed the center conductor leaving just the shield, then inserted a twisted pair as my capacitor. That's too tight a box, and the dielectric screws things up--you'll be making shunt caps to the shield. I then connect the shield the FET source, Ok, I have never bootstrapped before, but wouldn't I then have a higher potential than desired, even a possibility of oscillation? ........................... Just the twisted gimmick itself is probably all you'll need. The bootstrapping is lagniappe. I tried a twisted pair of #26 wire and got way more capacitance then I wanted. I ended up with just about 1/4" layed side by side as about equal to 0.3pf. When I put the cover on there was much less effect on the signal than with the previously used plate capacitor. The side by side wire is not stable for long term use, so I'll find a way to stablize it and call this one a wrap. I'll try tying a low capacitance knot ;-) I have two more boards and boxes, I'll build a bootstrapped version for the next design. I'll be back, I might try one with a gain of 1 and one with a gain of maybe 10. Mikek |
help designing gimmick capacitor
On Feb 21, 11:18*am, "amdx" wrote:
wrote: On Feb 20, 11:14 am, "amdx" wrote: Just the twisted gimmick itself is probably all you'll need. *The bootstrapping is lagniappe. * I tried a twisted pair of #26 wire and got way more capacitance then I wanted. Just half a twist is likely enough--regular twisted pair is way too tightly coupled. [...] I have two more boards and boxes, I'll build a bootstrapped version for the next design. * * I'll be back, I might try one with a gain of 1 and one with a gain of maybe 10. * * * * * * * * * * * * * * * * * * * Mikek -- Cheers, James Arthur |
help designing gimmick capacitor
On Feb 21, 11:16*am, George Herold wrote:
On Feb 20, 3:30*pm, wrote: On Feb 20, 2:44*pm, "amdx" wrote: "John Larkin" wrote in message .. . On Sun, 20 Feb 2011 09:20:12 -0600, "amdx" wrote: Hi all, I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about *rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. *Any ideas to minimize input capacitance to the box? Here's the amp in box. http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic; http://www.crystal-radio.eu/enfetamp.htm * * * * * * * Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. Use a real surface-mount 0.3 pF cap, or a homemade coaxial cap. The 1 cm square plates are too big and have their own capacitance to the world. Bootstrap the drain of Q1. "T" means transformer, which shows that this circuit was done by an amateur. All that tricky stuff could be replaced by one opamp. It could have close to zero Cin with a little positive feedback. John * * Bootstrap the drain of Q1. You need to walk me through that, (I'm an amateur) Ah, he's done some nice work on the subject of crystal radios and high Q inductors.http://www.crystal-radio.eu/index.html Page down to experiment with LC circuits. * It could have close to zero Cin with a little positive feedback. *How much closer? If the input cap is 0.3pf what do you the input impedance is? Input is 0.3pf, 20 Meg to ground driving FET gate. * * * * * * * * * * * * * * * * * Thanks, Mikek We're talking about something roughly like this: * * +12V -------+-----------. * * * * * * * * | * * * * * | * * * * * * * * | * * * * * | * * * * * * * * | * * * R5 220K * * * * * * * * |Q2 * * * * | * * * * * * * * *\| * * * * | * * * *V1 * * * * * * * * * |---+-----+ * ~~ ~=7V * * * * * * * * .| * | * * | * * * * * * * * | * * | *R6 330K * * * * * * * | * * | * * | * * C1 * * * * | * * |C3 *=== * * 10pF * g |-'d * --- in--||--+-----| * * *--- * * * * | * * |-.s * *|100n * * R1 10M * * |Q1 * | * * * * | * * * | * * | * * * * +----||--+-----+------ out * * * * | * *C2 *| * * R2 10M *100nF| * * * * | * * * *| * * * *=== * R4 2.2K * * * * * * * * *| * * * * * * * * *| * * * * * * * * === edited, simplified, R4-6 values corrected C2 drives the center of the input bias resistor, which cancels the loading caused by R1-R2. *This is called "bootstrapping". Q2 does the same thing for the drain terminal--it causes the drain terminal to go up and down with the input signal. *That saves the input signal from having to charge Q1's gate-to-drain capacitance, effectively making that capacitance disappear. This front-end has *much* higher impedance than the original, and a predictable gain that's close to 1. Thanks for the explanation James. *The bootstrapping of the base does nothing for the input capacitance.(?) Is that correct? If you mean "gate" and Q1, that's inherently bootstrapped by the voltage-follower configuration: the source already rises and falls with the input, leaving the Miller capacitance as by far the biggest nasty. The best, easy bootstrap for the FET input capacitance is getting the FET follower gain up closer to unity. That means using a current source for the source load instead of R4. The input bootstrap above--C2 to the junction of R1-R2--serves to reduce the a) stray capacitive loading posed by R1, and b) raise the a.c. input impedance to much higher than the 20M input resistor in parallel with the FET gate impedance. -- Cheers, James Arthur |
help designing gimmick capacitor
amdx Inscribed thus:
wrote in message ... On Feb 20, 11:14 am, "amdx" wrote: wrote in message ... On Feb 20, 10:20 am, "amdx" wrote: Hi all, I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. Any ideas to minimize input capacitance to the box? Here's the amp in box.http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic;http://www.crystal-radio.eu/enfetamp.htm Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. ....................................... The old-tyme gimmick was two wires, twisted together. That would contain the field better than your open plates. Another alternative: you could surround your cap with a shield and drive the shield from the FET source, bootstrapping it. r Hi James, The twisted pair may be the easiest, but as a thought, if I took a piece of coax and removed the center conductor leaving just the shield, then inserted a twisted pair as my capacitor. That's too tight a box, and the dielectric screws things up--you'll be making shunt caps to the shield. I then connect the shield the FET source, Ok, I have never bootstrapped before, but wouldn't I then have a higher potential than desired, even a possibility of oscillation? .......................... Just the twisted gimmick itself is probably all you'll need. The bootstrapping is lagniappe. I tried a twisted pair of #26 wire and got way more capacitance then I wanted. I ended up with just about 1/4" layed side by side as about equal to 0.3pf. When I put the cover on there was much less effect on the signal than with the previously used plate capacitor. The side by side wire is not stable for long term use, so I'll find a way to stablize it and call this one a wrap. I'll try tying a low capacitance knot ;-) I have two more boards and boxes, I'll build a bootstrapped version for the next design. I'll be back, I might try one with a gain of 1 and one with a gain of maybe 10. Mikek Try a glass cased diode with one end cut off so the case is empty. Use the wire at one end for one connection, use a piece of thick wire inside the tube for the other connection. Adjust for "C". Secure with a drop of adhesive. -- Best Regards: Baron. |
help designing gimmick capacitor
amdx wrote:
The side by side wire is not stable for long term use, so I'll find a way to stablize it and call this one a wrap. I'll try tying a low capacitance knot ;-) Heat shrink? Good Luck! Rich |
help designing gimmick capacitor
On Feb 21, 12:03*pm, wrote:
On Feb 21, 11:16*am, George Herold wrote: On Feb 20, 3:30*pm, wrote: On Feb 20, 2:44*pm, "amdx" wrote: "John Larkin" wrote in message .. . On Sun, 20 Feb 2011 09:20:12 -0600, "amdx" wrote: Hi all, I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about *rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. *Any ideas to minimize input capacitance to the box? Here's the amp in box. http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic; http://www.crystal-radio.eu/enfetamp.htm * * * * * * * Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. Use a real surface-mount 0.3 pF cap, or a homemade coaxial cap. The 1 cm square plates are too big and have their own capacitance to the world. Bootstrap the drain of Q1. "T" means transformer, which shows that this circuit was done by an amateur. All that tricky stuff could be replaced by one opamp. It could have close to zero Cin with a little positive feedback. John * * Bootstrap the drain of Q1. You need to walk me through that, (I'm an amateur) Ah, he's done some nice work on the subject of crystal radios and high Q inductors.http://www.crystal-radio.eu/index.html Page down to experiment with LC circuits. * It could have close to zero Cin with a little positive feedback. *How much closer? If the input cap is 0.3pf what do you the input impedance is? Input is 0.3pf, 20 Meg to ground driving FET gate. * * * * * * * * * * * * * * * * * Thanks, Mikek We're talking about something roughly like this: * * +12V -------+-----------. * * * * * * ** * | * * * * * | * * * * * * ** * | * * * * * | * * * * * * ** * | * * * R5 220K * * * * * * ** * |Q2 * * * * | * * * * * * ** * *\| * * * * | * * * *V1 * * * * * * ** * * |---+-----+ * ~~ ~=7V * * * * * * ** * .| * | * * | * * * * * * ** * | * * | *R6 330K * * * * * * ** * | * * | * * | * * C1 * * * * * | * * |C3 *=== * * 10pF * * g |-'d * --- in--||--+-----| * * *--- * * * * | * ** |-.s * *|100n * * R1 10M * * * |Q1 * | * * * * | * ** * | * * | * * * * +----||--+-----+------ out * * * * | * *C2 *| * * R2 10M *100nF| * * * * | * * * *| * * * *=== * R4 2.2K * * * * * * * * *| * * * * * * * * *| * * * * * * * * === edited, simplified, R4-6 values corrected C2 drives the center of the input bias resistor, which cancels the loading caused by R1-R2. *This is called "bootstrapping". Q2 does the same thing for the drain terminal--it causes the drain terminal to go up and down with the input signal. *That saves the input signal from having to charge Q1's gate-to-drain capacitance, effectively making that capacitance disappear. This front-end has *much* higher impedance than the original, and a predictable gain that's close to 1. Thanks for the explanation James. *The bootstrapping of the base does nothing for the input capacitance.(?) Is that correct? If you mean "gate" and Q1, that's inherently bootstrapped by the voltage-follower configuration: the source already rises and falls with the input, leaving the Miller capacitance as by far the biggest nasty. *The best, easy bootstrap for the FET input capacitance is getting the FET follower gain up closer to unity. *That means using a current source for the source load instead of R4. Uh yes... Gate of Q1. (thanks for reading my mind.) George H. The input bootstrap above--C2 to the junction of R1-R2--serves to reduce the a) stray capacitive loading posed by R1, and b) raise the a.c. input impedance to much higher than the 20M input resistor in parallel with the FET gate impedance. -- Cheers, James Arthur- Hide quoted text - - Show quoted text - |
help designing gimmick capacitor
amdx wrote:
wrote in message ... On Feb 20, 11:14 am, "amdx" wrote: wrote in message ... On Feb 20, 10:20 am, "amdx" wrote: Hi all, I finished the amp that had the 5 Ghz transistor, I changed it to a slower one. The objective of this amp is to cause minimal loading of the circuit it is measuring. When I install the box cover the voltage gain drops by 7%, so I think the input capacitor plate is being loaded by the cover. The input capacitor plates can be seen here; http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. I have thoughts about rectangular plates 0.25 cm x 4 cm to get more distance from the top cover, (and the bottom.) Or a real gimmick cap where I twist a couple of 39 Gauge wires together and attach opposite ends to input and output. Any ideas to minimize input capacitance to the box? Here's the amp in box.http://i395.photobucket.com/albums/p...erampinbox.jpg This is the original circuit page with schematic;http://www.crystal-radio.eu/enfetamp.htm Thanks, Mike PS, I was having trouble getting some close-up pictures, I grabbed a magnifying glass and took some pictures through that, works good. ....................................... The old-tyme gimmick was two wires, twisted together. That would contain the field better than your open plates. Another alternative: you could surround your cap with a shield and drive the shield from the FET source, bootstrapping it. r Hi James, The twisted pair may be the easiest, but as a thought, if I took a piece of coax and removed the center conductor leaving just the shield, then inserted a twisted pair as my capacitor. That's too tight a box, and the dielectric screws things up--you'll be making shunt caps to the shield. I then connect the shield the FET source, Ok, I have never bootstrapped before, but wouldn't I then have a higher potential than desired, even a possibility of oscillation? .......................... Just the twisted gimmick itself is probably all you'll need. The bootstrapping is lagniappe. I tried a twisted pair of #26 wire and got way more capacitance then I wanted. I ended up with just about 1/4" layed side by side as about equal to 0.3pf. When I put the cover on there was much less effect on the signal than with the previously used plate capacitor. The side by side wire is not stable for long term use, so I'll find a way to stablize it and call this one a wrap. I'll try tying a low capacitance knot ;-) I have two more boards and boxes, I'll build a bootstrapped version for the next design. I'll be back, I might try one with a gain of 1 and one with a gain of maybe 10. Mikek Someone suggested the use of a chip cap for the input; much more stab;e - and smaller than the plate setup.. |
help designing gimmick capacitor
On Sun, 20 Feb 2011 14:26:42 -0600, "amdx" wrote:
"FigureItOut" wrote in message .. . On Sun, 20 Feb 2011 09:20:12 -0600, "amdx" wrote: http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. Ever consider simply using a proper air core variable cap? You have any 0.3pf air variables. You could manage .3 with the empty tank alone. Why is this one improper? Mikek Because applying the lid changed the value. Duh. That air gap is so big that even local electrostatic fields would **** with it. |
help designing gimmick capacitor
On 2/23/2011 8:38 PM, FigureItOut wrote:
On Sun, 20 Feb 2011 14:26:42 -0600, wrote: wrote in message ... On Sun, 20 Feb 2011 09:20:12 -0600, wrote: http://i395.photobucket.com/albums/p...mspaced5mm.jpg The plates are 1 cm x 1 cm spaced 5 mm apart. Ever consider simply using a proper air core variable cap? You have any 0.3pf air variables. You could manage .3 with the empty tank alone. Can you back that up with calculations using believable numbers? Why is this one improper? Mikek Because applying the lid changed the value. Duh. That air gap is so big that even local electrostatic fields would **** with it. Plagiarism from an earlier post aside, you are on the verge of offering some legitimate advice. But your delivery sucks more than a vacuum. Maybe you have a black hole. You should look into that. Literally. |
help designing gimmick capacitor
On Wed, 23 Feb 2011 20:56:40 -0600, John - KD5YI
wrote: Can you back that up with calculations using believable numbers? **** you, you ****ing retard. Did you not read the post? He said that putting the lid on changed the value. You would have to be a complete dolt (you are) to miss that one! Again, come back when you are not being your usual mouthy little bitch self. Plagiarism? You're an idiot, and a goddamned liar, boy. I was making feedback circuit caps before you even knew what an antenna match was. **** off, Johnny. Die, Johnny. Die painfully, Johnny. Go mouth someone more deserving of the **** you spew... your mother. |
help designing gimmick capacitor
On Thu, 24 Feb 2011 04:23:09 -0800 FigureItOut
wrote in Message id: : On Wed, 23 Feb 2011 20:56:40 -0600, John - KD5YI wrote: Can you back that up with calculations using believable numbers? **** you, you ****ing retard. Did you not read the post? He said that putting the lid on changed the value. You would have to be a complete dolt (you are) to miss that one! Again, come back when you are not being your usual mouthy little bitch self. Plagiarism? You're an idiot, and a goddamned liar, boy. I was making feedback circuit caps before you even knew what an antenna match was. **** off, Johnny. Die, Johnny. Die painfully, Johnny. Go mouth someone more deserving of the **** you spew... your mother. How do you type with a straitjacket on? |
help designing gimmick capacitor
On Thu, 24 Feb 2011 04:23:09 -0800, FigureItOut
wrote: On Wed, 23 Feb 2011 20:56:40 -0600, John - KD5YI wrote: Can you back that up with calculations using believable numbers? **** you, you ****ing retard. Did you not read the post? He said that putting the lid on changed the value. You would have to be a complete dolt (you are) to miss that one! Again, come back when you are not being your usual mouthy little bitch self. Plagiarism? You're an idiot, and a goddamned liar, boy. I was making feedback circuit caps before you even knew what an antenna match was. So what made you take up pushing a mop? **** off, Johnny. Die, Johnny. Die painfully, Johnny. Go mouth someone more deserving of the **** you spew... your mother. |
help designing gimmick capacitor
On 2/24/2011 6:23 AM, FigureItOut wrote:
On Wed, 23 Feb 2011 20:56:40 -0600, John - wrote: To put this back in context, you said: You could manage .3 with the empty tank alone. And I said: Can you back that up with calculations using believable numbers? I read the post. But I challenged you to prove what you asserted knowing that you could not. Again, come back when you are not being your usual mouthy little bitch self, NymNuts, AlwaysWrong, PomegranateHead. |
help designing gimmick capacitor
On Thu, 24 Feb 2011 13:03:29 -0600, John - KD5YI
wrote: On 2/24/2011 6:23 AM, FigureItOut wrote: On Wed, 23 Feb 2011 20:56:40 -0600, John - wrote: To put this back in context, It was never out of context, you retarded ****. you said: You could manage .3 with the empty tank alone. And I could. And I said: Can you back that up with calculations using believable numbers? Then, you mouthed off like a little petty bitch with some lame claim. You lose. **** off. Your mother is a slut. I read the post. Good for you. Stop acting like you didn't then. But I challenged you to prove what you asserted knowing that you could not. I do not need to prove a goddamned thing to a retarded twit like you. Again, come back when you are not being your usual mouthy little bitch self, JW, the ****ing total retard. You're a post-edit little **** too. The Pomegranite retard is someone else, you dumb ****. Write your own lines and stop ****ing with mine, pussy boy. Better yet, **** off and die. I do not need to waste my time on a twit like you. Nor will I. |
help designing gimmick capacitor
On Thu, 24 Feb 2011 16:40:58 -0800, FigureItOut wrote:
On Thu, 24 Feb 2011 13:03:29 -0600, John - KD5YI wrote: On 2/24/2011 6:23 AM, FigureItOut wrote: On Wed, 23 Feb 2011 20:56:40 -0600, John - KD5YI wrote: To put this back in context, It was never out of context, you retarded ****. you said: You could manage .3 with the empty tank alone. And I could. And I said: Can you back that up with calculations using believable numbers? Then, you mouthed off like a little petty bitch with some lame claim. You lose. **** off. Your mother is a slut. I read the post. Good for you. Stop acting like you didn't then. But I challenged you to prove what you asserted knowing that you could not. I do not need to prove a goddamned thing to a retarded twit like you. Again, come back when you are not being your usual mouthy little bitch self, JW, the ****ing total retard. You're a post-edit little **** too. The Pomegranite retard is someone else, you dumb ****. Write your own lines and stop ****ing with mine, pussy boy. Better yet, **** off and die. I do not need to waste my time on a twit like you. Nor will I. It sure doesn't take too much depth in the followups of this NG before it degenerates into a ****ing contest that two or more uncouth jerks should have taken to private email. |
help designing gimmick capacitor
On 2/24/2011 6:40 PM, FigureItOut wrote:
To put this back in context, you said: You could manage .3 with the empty tank alone. And I could. And I said: Can you back that up with calculations using believable numbers? But I challenged you to prove what you asserted knowing that you could not. I do not need to prove a goddamned thing to a retarded twit like you. Since you can't anyway, that statement is an attempt to show your superiority in the face of humiliation. The Pomegranite retard is my lover, you dumb ****. I do not need to waste my time on a twit like you. Nor will I. Good! Then I'm sure you will not waste your time responding to this one. *Yawn* Anyhow, I'm done with you, too. |
help designing gimmick capacitor
On 2/24/2011 8:23 AM, Pomegranate ******* wrote:
On Thu, 24 Feb 2011 04:23:09 -0800, FigureItOut wrote: On Wed, 23 Feb 2011 20:56:40 -0600, John - wrote: Can you back that up with calculations using believable numbers? **** you, you ****ing retard. Did you not read the post? He said that putting the lid on changed the value. You would have to be a complete dolt (you are) to miss that one! Again, come back when you are not being your usual mouthy little bitch self. Plagiarism? You're an idiot, and a goddamned liar, boy. I was making feedback circuit caps before you even knew what an antenna match was. So what made you take up pushing a mop? **** off, Johnny. Die, Johnny. Die painfully, Johnny. Go mouth someone more deserving of the **** you spew... your mother. Pomegranate ******* - I mistakenly referred to you in this thread in an unflattering way, as you will eventually see. I regret that reference and apologize for any embarassment which you certainly do not deserve. Sorry. John |
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