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Old 21st Apr 2021, 10:29 am   #28
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 2,435
Default Re: Buffer Amplifier for Digital Frequency Counter

I'm not sure what the original claims are about the circuit design but at first glance one could be forgiven for thinking the following:

JFET source follower = unity gain

BJT1 provides some voltage gain followed by lots of voltage gain in the second BJT. One could be forgiven for assuming that because the BJTs are rated for VHF amplifier service then the circuit can be assumed to work well up into VHF.

However, I think reality kicks in if you work backwards through the circuit. The output BJT looks to be biased at just under 3mA and this means the collector voltage is only about 2V or so. It probably has about 4pF feedback capacitance (collector to base). It also has a 10k resistor providing negative feedback. The low frequency voltage gain is probably about 100 at Ic = 3mA and the input impedance will be dominated by the resistive feedback at low frequencies and by the Miller Effect at high frequencies. A few simple fag packet estimates put the input Rp at 100R due to the resistive feedback. The Miller Effect will put ((Av+1)*4pF) in series with about 1000/(Av+1) across this 100R so you end up with a very low input impedance to this stage. This will limit the gain of the previous BJT stage. This stage will also load the previous JFET stage quite a bit.

So the net result is a very loaded/lossy JFET buffer feeding a BJT that has limited gain that feeds another BJT that is going to have gain that drops by 20dB/decade above a few MHz.

It does look like it will droop to provide unity gain at maybe 40-50MHz but when you add coax cable and a counter to the output I think this will spoil the system gain even more at HF/VHF.

The input JFET will probably generate a lot of negative resistance up to a few MHz. I'm not sure if this would show up as a problem in typical use but I think the circuit can go unstable if probing networks that look like an inductor in this frequency range.
Regards, Jeremy G0HZU
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