Help with Analog signal conditioner in FrequencyCounter

[fobyellow]

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

 

[fobyellow]

design using OPA655

 

[Borber]

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.

 

[fobyellow]

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??

 

[Borber]

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