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I am trying to fully understand the following circuit and have some questions about it. Some preliminary info is that this is an audio voltage amplifier specifically for guitar use. Now overall, it looks like a run of the mill common source amp but it has a variable source resistor and also the gate is biased via a divider being fed the drain voltage. The DC drain voltage of course changes as the source resistance is changed thus changing the gate bias voltage.

What is the benefit of biasing the gate like this? Wouldn't feeding the gate voltage divider the 9V supply to keep the gate at a constant 4.5V work just as well? It seems the FET would still remain in saturation this way.

schematic

simulate this circuit – Schematic created using CircuitLab

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It's called negative feedback and the biasing attempts to keep the gate at "just the right amount of voltage" - should the drain voltage droop a bit low, the DC voltage applied to the gate reduces and, this in turn, turns the fet off slightly and, in turn, this makes the drain voltage rise a bit. Thus there is a measure of stability! It's proper negative feedback.

R6 (the pot) in the source connection is only 100 ohm max and this will alter things a little bit but it's not a showstopper at one end of its travel or at the other. Overall, AC gain will be a tad limited when R6 is at 100 ohms but it won't affect things too much.

So, in short, feeding the gate half the supply doesn't necessarily keep the FET biased as well as the "negative feedback" method AND, a source resistor of 0 to 100 ohms isn't going to change things that much. Think about how much voltage the gate needs (with respect to source) to adequately "turn on" the FET - if it is (say) 2V then the drain has to be at 4V (due to R3 and R4) - if it needs 3V then the drain has to be at 6V. Because we can't be as sure about a FET (compared to a BJT) this is a sensible way of biasing it. Having said that I've seen plenty of BJT circuits that use EXACTLY the same method for biasing the base i.e. collector feedback.

On a slightly off-question point - opamps work precisely this way.

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  • \$\begingroup\$ Thanks for the reply Andy. I actually messed up and the variable resistor is a 5k so it does alter the gain quite a bit but your explanation cleared up a lot for me. You say that the biasing arrangement provides stability through negative feedback. Now it's apparent the source resistor here is used as a gain control but could one say that a source resistor also provides stability? Meaning that, gate-source voltage will decrease as drain current increases (causing source voltage to increase)? \$\endgroup\$ – disorder Mar 13 '15 at 15:17
  • \$\begingroup\$ Absolutely - it's another form of negative feedback used a lot to stabilize BJT circuits but works pretty good for MOSFETs too. \$\endgroup\$ – Andy aka Mar 13 '15 at 15:36

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