Servo Offset Biasing for JFETs

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MoonWatcher
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Post by MoonWatcher »

coldcraft wrote:the OD-3 uses whats called auto-biasing, where the Gate goes through a resistor to ground, and you cross your fingers that the JFET characteristics let it bias correctly. Sending the gate to a voltage other than ground is called offset biasing, and that's what the servo is doing.

The od-3 also has feedback clippers, and I would guess that they limit the signal before the bias has a chance to effect the clipping.
Yeah, I knew it was an auto-bias setup, just was hoping something might be applicable to offset biasing.

And I knew the clippers in the loop could potentially negate things as well (with regards to servo biasing).

Was just wondering if some part of that could be circuit-bent to offset-biasing instead of auto-biasing.

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misterg
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Post by misterg »

coldcraft wrote:I tried it out with a MPF102 and a J201. I'm not sure its working the way I intended. If the drain is at 4.5v, then the opamp will output 0v (without negative feedback). So if the JFET is capable of biasing up with the gate at 0v,then it will work. the J201 I had wouldn't work.
I believe your circuit should work, and is quite neat. I think that the reason the J201 didn't work is that the AD712 output cannot swing to the supply rails: On a +/-5V supply ('cos it's on the datasheet), it is restricted to roughly +4V / -3V.

If you replace the AD712 with an op-amp which can swing its output to the negative supply rail, you should be able to get anything to auto-bias as well as you could set it up with a trim pot. The op-amp doesn't have to be a 'rail to rail' device: an LM258 can swing it's output to the -ve supply rail and (I think) should work just fine.

HTH

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coldcraft
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Post by coldcraft »

misterg wrote:
I believe your circuit should work, and is quite neat. I think that the reason the J201 didn't work is that the AD712 output cannot swing to the supply rails: On a +/-5V supply ('cos it's on the datasheet), it is restricted to roughly +4V / -3V.

If you replace the AD712 with an op-amp which can swing its output to the negative supply rail, you should be able to get anything to auto-bias as well as you could set it up with a trim pot. The op-amp doesn't have to be a 'rail to rail' device: an LM258 can swing it's output to the -ve supply rail and (I think) should work just fine.

HTH
thanks, I actually used a TLC2262 and a LM833. I haven't given up on this yet.
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misterg
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Post by misterg »

TLC2262 should work, LM833 will suffer from the same problem as the AD712.

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Post by misterg »

Actually, thinking about it some more, the output of the op-amp would need to swing either side of the bias voltage because you're feeding the AC signal through it - probably better to filter out the AC signal *before* the op-amp (capacitor to ground from pin 2?) and just use the opamp for DC. You might need to limit the gain a bit to avoid oscillation (which could also be part of your current problem).

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Post by coldcraft »

the original schematic in post #1 has a filter for the AC signal for that very reason. Oscillation could very well be a problem though.
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Post by coldcraft »

I have a new idea, inspired by the way the FET in the orange squeezer is biased. Instead of changing the bias voltage on the Gate, I think we should fix the gate at zero, and use the servo to add additional voltage drop across the source resistor. I've simulated it and it checks out, but i'm not sure how realistic the jfet model is in the simulation.

I haven't bread boarded it yet, but it might require an additional low pass filter between the drain and R1. R14 and R15 could be a pot to vary the bias from say 4V to 7V.

I should note that its not really "Servo Offset Biasing" anymore.
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Post by commathe »

Bump. Did you breadboard this? Did anyone? I am hungering for a way to bias jfets automatically. Also, it's fascinating to think about this stuff from a technical stand point. Really interesting design!

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Post by DrNomis »

Have you ever thought of trying the Shunt-Regulated Push-Pull circuit with J Fets Coldcraft?....... :hmmm:
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Post by coldcraft »

DrNomis wrote:Have you ever thought of trying the Shunt-Regulated Push-Pull circuit with J Fets Coldcraft?....... :hmmm:

SRPP? Yeah I've used that. I prefer the sound of single JFETS though.
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