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Help with Analog signal conditioner in FrequencyCounter

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fobyellow

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I found most of the Analog section in FreqCounter design use a FET as input impedance converter and amplitude amplfier, why not use some High-input-impedance OP? like opa655 which might be easier to adjust
 

design using OPA655
 

Figure 1.jpg is not complete. Left part is missing.
Reasons of using discrete components amplifier are available supply voltages, current consumption and it's max output voltage swing.
 

sorry for figure1
thank you for your reply
the attachment is input circuit of HP-5386A Universal counter.
the input buffer is built with a 2N4416, a 400MHz fet.
slew-rate, input impedance seem not superior than an OP??
 

It is usual input stage for the time it was designed.
 

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