Hi,
this all makes no sense.
at which frequency?
where can we see this?
Why?
We can´t see a "3.5 minute transient" (in the time domain).
Transients are usually fast (below microseconds), but "minutes" are so slow that it is not of interest when one talks about "audio".
Is the plotted gain:
* open loop
* or closed loop?
We can´t see what OPAMP you used.
The values for C2, C3 and R3, R5 make no sense (in my eyes) for an audio circuit.
Why the JFET input at all?
Klaus
Klaus,
Q) this all makes no sense.
=> my apologies. let me attempt round two.
Q) at which frequency?
=> At 1kHz. Basically I inject 10mV-rms 1kHz since wave at the JFET input and observe Vout = 500mV-rms at the output. Gain being = 1+Rf/Rs = 50V/V = 34.15dB.
Upon power up, the output is ~420mV-rms and slowly the output amplitude starts increasing over ~3.5minutes, until the output reaches ~504mV-rms. This is when the amplifier goes unstable. The output turns into 1.3MHz, -2V to 2V triangular shape wave.
Q) where can we see this?
=> I have annotated the original diagram with where the input and outputs are.
Q) Is the plotted gain:
* open loop
* or closed loop?
=> what's plotted is Loop-gain = feedback_factor_beta * Open_loop as a function of frequency. This was my attempt. In Cadence spectre, there exists a stability probe used with STB analysis. In LTSPICE, I am using the same approach using the Tian method (instead of Middlebrook) for loop-gain analysis for stability assessment.
Q) We can´t see what OPAMP you used.
AD743. I am using the malromodel provided by ADI.
Q) The values for C2, C3 and R3, R5 make no sense (in my eyes) for an audio circuit.
=> original concept is inspired by :
https://www.planetanalog.com/amplify-small-signals-in-low-noise-circuit-with-discrete-jfet/
Q) Why the JFET input at all?
I have a MEMS capacitive transducer that generates signals. The input-referred capacitance attenuates the signal. This JFET has least amount of capacitance, from it's PN-junction.
I have added transients (manually drawn) for illustrations below.