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#1
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Schematic for Low Capacitance Fet Buffer
Hi,
I am looking for a circuit for a low capacitance FET circuit to act as a Buffer/Amp for VHF signals (ie. to probe Oscillators etc). (Homebrew Active Probe). I have searched quite a bit on www but cannot locate a suitable design. Thanks in advance Regards David |
#2
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Schematic for Low Capacitance Fet Buffer
I am looking for a circuit for a low capacitance FET circuit to act as a
Buffer/Amp for VHF signals (ie. to probe Oscillators etc). (Homebrew Active Probe). I have searched quite a bit on www but cannot locate a suitable design. One starting point might be a homebrew active-FET oscilloscope probe design shown by Robert Pease in "Troubleshooting Analog Circuits" (page 16). As shown, it has a bandwidth of 90 MHz, slew rate of 300 V/usec, input Z of 10^11 ohms in parallel with 0.29 pF. Construction is "ugly" style, with components soldered together in midair to minimize parasitic capacitance to ground. It uses a cascoded pair of 2N5486 or 2N5485 JFETs, driving a 2N3904/2N3906 buffer, and takes +/-15 VDC supply voltage. I suspect that its bandwidth might be improved by using faster bipolar-buffer transistors (maybe MPSH-something-or-other) and possibly J310 or U310 JFETs for the front end. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#3
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Schematic for Low Capacitance Fet Buffer
Dave,
Thanks for the tip. I don't want to purchase an entire book for one circuit though. I do have some BF998 Dual Gate Mosfets, maybe I can make up a circuit based on these. I need around 1pF input capacitance and 1 Meg Ohm, Frequency up to a couple of hundred MHz. Regards David Dave Platt wrote: I am looking for a circuit for a low capacitance FET circuit to act as a Buffer/Amp for VHF signals (ie. to probe Oscillators etc). (Homebrew Active Probe). I have searched quite a bit on www but cannot locate a suitable design. One starting point might be a homebrew active-FET oscilloscope probe design shown by Robert Pease in "Troubleshooting Analog Circuits" (page 16). As shown, it has a bandwidth of 90 MHz, slew rate of 300 V/usec, input Z of 10^11 ohms in parallel with 0.29 pF. Construction is "ugly" style, with components soldered together in midair to minimize parasitic capacitance to ground. It uses a cascoded pair of 2N5486 or 2N5485 JFETs, driving a 2N3904/2N3906 buffer, and takes +/-15 VDC supply voltage. I suspect that its bandwidth might be improved by using faster bipolar-buffer transistors (maybe MPSH-something-or-other) and possibly J310 or U310 JFETs for the front end. |
#4
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Schematic for Low Capacitance Fet Buffer
David wrote:
I need around 1pF input capacitance and 1 Meg Ohm, Frequency up to a couple of hundred MHz. These specs are probably rather optimistic, in my opinion. I would probably consider something like a large resistor in series with the input of a cascode amplifier (cascode chosen to reduce effective input capacitance due to feedback). Or perhaps a resistive voltage divider, rather than just a series resistor, in an attempt to get flat response over frequency. But there really is no such thing as a 1 Meg resistor at VHF. The stray capacitance across resistors limits the maximum achievable impedance to perhaps tens of kilohms. However, I'm not expert on such circuits. You might try posting the question to sci.electronics.design. There are some very clever electronic designers there (along with a number who get their jollies from insulting others). Steve VE3SMA |
#5
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Schematic for Low Capacitance Fet Buffer
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