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Old 19th Apr 2021, 7:40 pm   #12
G0HZU_JMR
Dekatron
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: Buffer Amplifier for Digital Frequency Counter

I've not played with a 2N2222A in many years but I think this old RF/switching transistor won't perform that well in your circuit.

Looking at the datasheet the feedback capacitance at a collector voltage of about 3V is about 4pF. If I assume 2mA collector current the final stage probably has a (low frequency) voltage gain of about Av = -70. So the Miller capacitance is going to be (Av +1) x 4pF which is nearly 300pF. This stage alone will restrict the bandwidth a lot because of this. The previous 2N2222A stage will also have a fair bit of voltage gain (at low frequencies) and so this will generate a fairly significant Miller capacitance.

I also expect the input JFET stage to produce a lot of negative resistance despite the series 1k resistor at the input. This isn't a great circuit.

Maybe the best thing to do is list what the requirements are for your buffer circuit and then make a circuit that meets those requirements. What you end up with might still be able to be squeezed onto your existing PCB.
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