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Schematic for Low Capacitance Fet Buffer

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June 12th 06, 10:11 PM posted to rec.radio.amateur.homebrew

Roy Lewallen

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Schematic for Low Capacitance Fet Buffer

wrote:
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.

The "totem pole" follower (a FET source follower with a matched FET as
the source "resistor") was widely used by Tektronix. It provides flat,
near unity gain over a very wide bandwidth, with high input resistance
and low input capacitance. It would be very difficult to make a cascode
amplifier with response that flat.

The 1 meg ohm resistor is to establish the DC and low frequency
impedance. The unavoidable shunt C from the FETs and other sources makes
it immaterial at high frequencies. I agree that 1 pF is optimistic -- it
would be very difficult to achieve. You would probably have to do some
bootstrapping to reduce the C to that level, if you can do it at all,
and bootstrapping becomes more difficult as frequency increases.

. . .

Roy Lewallen, W7EL

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