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Old 20th Apr 2021, 3:23 am   #17
ortek_service
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Join Date: May 2018
Location: Northampton, Northamptonshire, UK.
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Default Re: Buffer Amplifier for Digital Frequency Counter

I wonder if anyone's tried simulating the circuit in the free LTspice ?
- Or maybe the Micro-Cap Simulator software (made available free > 1yr ago for apparently a limited time, but still there), as mentioned recently here: https://www.vintage-radio.net/forum/...56#post1364956 (Where this Preamp circuit also appears)

And you could then try a variety of different loads on it.

I've done this a lot for OpAmp circuits, but not tried a transistor one yet, so would be nice to have a go.

If the MPF102 (Obsolete?) / 2N2222 (quite commonly used in quite a few HF & VHF circuits, even though much higher frequency ones are are available. But would be perfectly-adequate if being used as an emitter-follower) transistor are not in their libraries, then could probably find something reasonably-close.


I do quite like the CM Howes circuit with an RF dual-gate MOSFET, as can vary gain by altering the DC-biased gate voltage.
And it does appear they've tried to ensure a proper 50R output match, using an emitter follower but also a matching transformer (that could limit B/W & not sure what the spec. of that was if you had to wind your own)

You could probably also do this with an IC, like the old Plessey RF amp ones, or a Video OpAmp. And Analog Devices do DC-150MHz precision differential amps with accurate gain control, but maybe less liable to have these more custom parts lying around.

Last edited by ortek_service; 20th Apr 2021 at 3:49 am.
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