Below is a section of the schematic of the front end of a Kenwood VT-181 AC Millivoltmeter.
It's a fairly conventional discrete FET differential amp acting as a voltage follower / impedance converter.
The output feeds a 0dB / 30dB attenuator selected by a relay.
I'm puzzled about the positive feedback path via R106.
As well as providing the small input current required by Q101 (the input is AC coupled) it appears to slightly lift the hf frequency response in conjunction with R103 and C104.
What I don't understand is that the hf lift will surely depend upon the impedance of whatever is connected to the input.
If it's connected to a high impedance source then it won't have much effect.
Am I missing something?
It's a fairly conventional discrete FET differential amp acting as a voltage follower / impedance converter.
The output feeds a 0dB / 30dB attenuator selected by a relay.
I'm puzzled about the positive feedback path via R106.
As well as providing the small input current required by Q101 (the input is AC coupled) it appears to slightly lift the hf frequency response in conjunction with R103 and C104.
What I don't understand is that the hf lift will surely depend upon the impedance of whatever is connected to the input.
If it's connected to a high impedance source then it won't have much effect.
Am I missing something?