Wes Hayward hybrid cascode amplifiers from 2007 QST.
Has anyone else tried these circuits?
I have built 2 examples, a 9 MHZ AGC controlled amplifier and a 20 dB wideband amp for a noise cancelling system. BIG problems! The diode string in the source of the FET always biases the J310 OFF. Result, no gain or a loss!!! It took me an hour or two to twig what was going on. The trouble is that I cannot see how it can possibly work when the gate is referenced to ground. It is equivalent biasing off a valve! Not only that, but the AGC circuit published at the same time had extreme instability in the front end of the AGC amplifier. I am a VERY experienced electronics person (60 years) and was unable to find any obvious reason for this. SO Now I have actually began to lose faith in one of my favourite engineers. Or, have I done something really stupid in my old age. FYI the circuits were built "dead bug" style with extensive shielding and full decoupling. |
Wes Hayward hybrid cascode amplifiers from 2007 QST.
On Tue, 6 Nov 2012 02:18:14 -0800 (PST), clifford wright
wrote: Has anyone else tried these circuits? No, but I saw that in theory they might work. I have built 2 examples, a 9 MHZ AGC controlled amplifier and a 20 dB wideband amp for a noise cancelling system. BIG problems! The diode string in the source of the FET always biases the J310 OFF. Result, no gain or a loss!!! The spread in FETs is rather high so you might have had the bad luck that the ones you used have a cut-off Vgs lower than ~ 1.8V. One issue is strange though: maximum gain occurs not at the limit of V(AGC) of 4V but at a higher voltage. At 8V at the base, one stage (J310 +2N3409) provides for some 16 dB (voltage gain) then but about 10 dB of it is due to transformation from 50 to 2200 ohm at the input LC. It took me an hour or two to twig what was going on. The trouble is that I cannot see how it can possibly work when the gate is referenced to ground. It is equivalent biasing off a valve! Not only that, but the AGC circuit published at the same time had extreme instability in the front end of the AGC amplifier. I am a VERY experienced electronics person (60 years) and was unable to find any obvious reason for this. It could be useful to try the circuit with a FET with much higher cut-off gate voltage. An example is BF246C and BF247C. It should work with the diodes in series with the source resistor. SO Now I have actually began to lose faith in one of my favourite engineers. Or, have I done something really stupid in my old age. FYI the circuits were built "dead bug" style with extensive shielding and full decoupling. I wonder why you didn't try to get an oldie like the Plessey SL312. Look it up, and see that it's perfect for the job - it provides some 30 dB gain without an input up-transformation and with a larger AGC range. Regards, Jan |
Wes Hayward hybrid cascode amplifiers from 2007 QST.
On Thursday, November 8, 2012 6:37:09 PM UTC-8, Arid ace wrote:
I wonder why you didn't try to get an oldie like the Plessey SL312. Look it up, and see that it's perfect for the job - it provides some 30 dB gain without an input up-transformation and with a larger AGC range. Is the SL312 still available? Texas Instruments' TL026 is still new from DigiKey and Mouser, even through-hole parts. Av of 55 and 50dB of AGC range. Cheers, Dana K6JQ |
Wes Hayward hybrid cascode amplifiers from 2007 QST.
On Thu, 22 Nov 2012 02:27:13 -0800 (PST), Dana wrote:
On Thursday, November 8, 2012 6:37:09 PM UTC-8, Arid ace wrote: I wonder why you didn't try to get an oldie like the Plessey SL312. Look it up, and see that it's perfect for the job - it provides some 30 dB gain without an input up-transformation and with a larger AGC range. Is the SL312 still available? Texas Instruments' TL026 is still new from DigiKey and Mouser, even through-hole parts. Av of 55 and 50dB of AGC range. Cheers, Dana K6JQ My error - the IC mentioned should be SL612C and nowadays it's available from China via eBay: http://www.ebay.com/itm/120922014405... 4.m1497.l2649 It has a much wider AGC range than the TL026, up to 70 dB IOW higher than the stage gain. Jan |
Wes Hayward hybrid cascode amplifiers from 2007 QST.
On Tuesday, December 4, 2012 6:54:15 PM UTC-8, Arid ace wrote:
My error - the IC mentioned should be SL612C and nowadays it's available from China via eBay: http://www.ebay.com/itm/120922014405... 4.m1497.l2649 It has a much wider AGC range than the TL026, up to 70 dB IOW higher than the stage gain. Let's see. SL612, $8+ each. TL026 $1.50 each. -3dB bandwidth - SL612 up to maybe 15MHz. TL026 up to 50MHz. SL612 AGC range 70dB, gain of 34dB. TL026 AGC range 50dB, gain of 38dB. SL612 obsolete. TL026 in production. An AGC range far in excess of stage gain isn't much use if the stage is overloading anyway. Why again would I consider the SL612? :-) Dana K6JQ |
Wes Hayward hybrid cascode amplifiers from 2007 QST.
On Thu, 6 Dec 2012 00:45:17 -0800 (PST), Dana wrote:
On Tuesday, December 4, 2012 6:54:15 PM UTC-8, Arid ace wrote: My error - the IC mentioned should be SL612C and nowadays it's available from China via eBay: http://www.ebay.com/itm/120922014405... 4.m1497.l2649 It has a much wider AGC range than the TL026, up to 70 dB IOW higher than the stage gain. Let's see. SL612, $8+ each. TL026 $1.50 each. -3dB bandwidth - SL612 up to maybe 15MHz. TL026 up to 50MHz. SL612 AGC range 70dB, gain of 34dB. TL026 AGC range 50dB, gain of 38dB. SL612 obsolete. TL026 in production. An AGC range far in excess of stage gain isn't much use if the stage is overloading anyway. Why again would I consider the SL612? :-) Dana K6JQ The data sheet of the TL026 doesn't mention the maximum input signal that can be processed. For the SL612 that is 250 mV rms. Then there's the supply current: less than 5 mA for the SL612 but for the TL026 it's 6 times as much. Not to mention the issue of stability. For an IF higher than 10 MHz, the SL611 has -3 dB at 80 MHz and for the SL610 that is 120 MHz. On SW, an IF amp with low NF allows for a passive mixer as long as the overall NF stays below ~10 dB. The SL612 has a 3 dB NF and a passive mixer like the VAY1 a loss of ~ 5.5 dB so even with some input filter losses, one can design a radio with high dynamic range, low power consumption and NF 10dB. Who would need a TL026? :-D Jan |
Wes Hayward hybrid cascode amplifiers from 2007 QST.
On Thursday, December 6, 2012 7:00:22 PM UTC-8, Arid ace wrote:
Who would need a TL026? :-D Just someone that wants a part that's still in production :-) Cheers - Dana K6JQ |
Wes Hayward hybrid cascode amplifiers from 2007 QST.
On Thursday, December 6, 2012 7:00:22 PM UTC-8, Arid ace wrote:
The data sheet of the TL026 doesn't mention the maximum input signal that can be processed. For the SL612 that is 250 mV rms. Actually, the datasheet mentions it - ~200mV. Dana K6JQ |
Wes Hayward hybrid cascode amplifiers from 2007 QST.
On Sat, 8 Dec 2012 01:39:46 -0800 (PST), Dana wrote:
On Thursday, December 6, 2012 7:00:22 PM UTC-8, Arid ace wrote: The data sheet of the TL026 doesn't mention the maximum input signal that can be processed. For the SL612 that is 250 mV rms. Actually, the datasheet mentions it - ~200mV. Dana K6JQ With so many data sheet archives online, I could have hit the wrong one. But what didn't fail to get my attention was the remark "Other Characteristics Similar to NE592 and uA733". With those ICs I'm not only familiar but they are from the era when customarily, the equivalent internal diagram was published so uninitiated application engineers could use the IC in a fashion, not intended by the designer. The NE592 and uA733 employ negative feedback which is at odds with voltage controlled gain - unless the gain control is before the stage(s) with feedback, and then the S/N under application of feedback deteriorates. So the TL026 isn't my choice even if it's modern ;-) Jan |
Wes Hayward hybrid cascode amplifiers from 2007 QST.
On Saturday, December 8, 2012 6:31:35 PM UTC-8, Arid ace wrote:
With so many data sheet archives online, I could have hit the wrong one. Since the TL026 is a currently-produced part by TI, there's no need to sift through online archives. Just go to http://www.ti.com/ and look up the part number. But what didn't fail to get my attention was the remark "Other Characteristics Similar to NE592 and uA733". Fair enough. Of course, the TL026 *isn't* either of those parts, and the statement suggests to me that the TL026 has input and output characteristics similar to those non-AGC parts - relatively easy to use. Nothing at all about internal architecture. Perhaps you're reading something that isn't in the datasheet at all. Given how cheap the TL026 is, perhaps you might try one instead of an SL612 and see if your assumption is true. But, seriously, you're quite passionate about the SL612 and that's a wonderful thing. Cheers, Dana K6JQ |
Wes Hayward hybrid cascode amplifiers from 2007 QST.
On Wed, 12 Dec 2012 17:23:35 -0800 (PST), Dana wrote:
On Saturday, December 8, 2012 6:31:35 PM UTC-8, Arid ace wrote: With so many data sheet archives online, I could have hit the wrong one. Since the TL026 is a currently-produced part by TI, there's no need to sift through online archives. Just go to http://www.ti.com/ and look up the part number. What remains is the large current consumption - typ. 22 mA. For the SL612 the typ. value is 3.5 mA. But what didn't fail to get my attention was the remark "Other Characteristics Similar to NE592 and uA733". Fair enough. Of course, the TL026 *isn't* either of those parts, and the statement suggests to me that the TL026 has input and output characteristics similar to those non-AGC parts - relatively easy to use. Nothing at all about internal architecture. Perhaps you're reading something that isn't in the datasheet at all. Given how cheap the TL026 is, perhaps you might try one instead of an SL612 and see if your assumption is true. Cost is a nonissue as is production. Another 30 mA just for the IF amplifier of a noise blanker is an issue though. But, seriously, you're quite passionate about the SL612 and that's a wonderful thing. Cheers, Dana K6JQ No, for the noise blanker it's the best solution. The IF minus impulse noise will be processed by another no longer available IC, the TCA440. Its mixer and IF combined have an AGC range of ~100 dB with max input to the mixer, 500 mV rms, current consumption ~11mA. With the 30 dB AGC range of the 2nd mixer, 120 dB AGC range. That mixer will convert to 25 KHz, a nice value for razor-sharp notch filters and ideally working (synchronous) demodulators. Why be satisfied with less when it's so easy to design a radio that functions well with input signals between 0.1 uV and 100 mV and yet has a low power consumption? ;-) Jan |
Wes Hayward hybrid cascode amplifiers from 2007 QST.
On Tuesday, November 6, 2012 5:18:14 AM UTC-5, clifford wright wrote:
Has anyone else tried these circuits? I have built 2 examples, a 9 MHZ AGC controlled amplifier and a 20 dB wideband amp for a noise cancelling system. BIG problems! The diode string in the source of the FET always biases the J310 OFF. According to the spec sheet curves: The j310 gets turned off with a bias of 3.7vdc, Quiescent of 3ma that is called for gets set at 3.3vdc bias. Meaning the resistor in series with the diodes is going to drop .54vdc at 3ma. so it is going to fly on the Vdrop of the diodes Vdrop that changes with temp also? Might work in a spice sim but not in real life. Result, no gain or a loss!!! It took me an hour or two to twig what was going on. The trouble is that I cannot see how it can possibly work when the gate is referenced to ground. It is equivalent biasing off a valve! Not only that, but the AGC circuit published at the same time had extreme instability in the front end of the AGC amplifier. I am a VERY experienced electronics person (60 years) and was unable to find any obvious reason for this. Yeah this is a cascode amplifier, great for eliminating miller effect, and gain that is the gain of the bipolar times the gain of the fet. Another reason they swamp the gate with 2200 ohms with all that gain. well except in your case. SO Now I have actually began to lose faith in one of my favourite engineers. Or, have I done something really stupid in my old age. FYI the circuits were built "dead bug" style with extensive shielding and full decoupling. Lastly Since they are using diode drops for bias I would have used a TL431 precision and temp compensated regulator, would be alot easier to set that 3ma quiescent current. 73 OM de n8zu |
Wes Hayward hybrid cascode amplifiers from 2007 QST.
On Friday, December 14, 2012 4:08:58 PM UTC-8, raypsi wrote:
On Tuesday, November 6, 2012 5:18:14 AM UTC-5, clifford wright wrote: Has anyone else tried these circuits? I have built 2 examples, a 9 MHZ AGC controlled amplifier and a 20 dB wideband amp for a noise cancelling system. BIG problems! The diode string in the source of the FET always biases the J310 OFF. According to the spec sheet curves: The j310 gets turned off with a bias of 3.7vdc, The datasheet for the J310 specifies a pinch-off voltage range of -2.0 to -6.5V. Completely normal production variation may be quite different than the spec sheet curve. So it sounds like Clifford has J310s that have a relatively small pinch-off voltage, perhaps even out of spec, and the ~2V drop of the diode chain is biasing the FET off. Data sheet for the Fairchild parts: http://www.fairchildsemi.com/ds/J3/J310.pdf Dana K6JQ |
Wes Hayward hybrid cascode amplifiers from 2007 QST.
I had the same problem and did not understood why.
I replaced the J310 with J309 and it works properly. Regards |
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