Help with split rail FET circuit

R

rossbalch

Well-known member
Hey folks, I've been designing this Blackface-ish inspired preamp circuit. The thing is it inverts the output which I want to avoid. For stubborn reason I've been trying to avoid using another op amp for the task of re-inverting the signal, adding a bit of gain, and buffering the output. I've been using this circuit which works well enough but one half of the waveform buts up against the 12v rail. Given that I have -12v available as well I'm sure there must be a way to leverage that to make biasing easier. Honestly maybe the solution is just to use an op-amp, but either way I would like to learn about using discrete circuits with split rail.

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Well have you tried just putting the source and emitter resistors and J-FET's biasing 1M to -12V?

But more to the point, have you checked what kind of voltages you're currently getting on the J-FET and transistor?
 
Scruffie said:
Well have you tried just putting the source and emitter resistors and J-FET's biasing 1M to -12V?

But more to the point, have you checked what kind of voltages you're currently getting on the J-FET and transistor?
Click to expand...

Hmm, now the whole thing just acts as a buffer. Without -12v on the emitter of Q1 there is zero voltage on the output. At least before I was getting a gain of ~3 which is what I had wanted in the first place.

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Blackboarcult said:
On the top circuit: you are missing a resistor to ground at the positive input of the first opamp. Won't work well as is.

Bottom circuit: as Scruffie said :D
Click to expand...

Thanks, I think I will just scrap the first JFET buffer, it doesn't seem necessary really.
 
What are your bias voltages, the transistor buffer is relying on the J-FET in this situation for optimal bias.

Poor bias, poor signal swing and unfortunately, the J-FET's biasing is also impacting its gain.
 
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