The following circuit is a slow switching +-15V audio signal attenuator:
Why are the measured voltages (shown in white boxes) so different from the SPICE values (shown in the graph)?
Note that circuit works fine in practice because the J175 pinch off voltage is only 6V. I just want to understand:
- 1) where the voltage drops are coming from and
- 2) why LTSPICE didn't simulate them.
Specifically, if the voltage drop across R4 is 12.6 - 9.2 = 3.4 and we compute the current of 3.4 / 470K that's 7.2uV. I don't see where the 7.2uV could be going?
I thought the gate current of a JFET was picoamps? If I temporarily remove the capacitor the voltages don't change at all so it's not the cap leaking. I've reproduced this on a soldered board and then bread-boarded with completely different set of parts.
I got the J175 model from a popular fet library on the Interweb. Graph lines are control signal blue, NPN collector green and gates red.
What am I missing here?