FET Bias

[txbluesboy]

When setting the bias on the FET, are you supposed to be adjusting the voltage that is dropped by the 10K resistor between the drain of the FET and 150K resistor tied to B+? So at the junction of the 10K and 150K, you should have about 20 volts, Then at the drain you should have 10 volts?

[markr14850]

Yes, the supply should idle at about 20V, and the FET drain should idle at about half the supply.

As you say, the supply voltage should be controlled by modifying the 150k/10k voltage divider off of B+5.

However, the voltage at the drain is set by changing the FET source resistor. I've seen schem's here that show it as 8k-12k ohm, but with the FETs I use, it seems to bias better around 1k.

Edit: I would suggest that you not change the 10k drain resistor, unless you have reason to modify the gain of the circuit.

--Mark

[ChrisM]

Want half of Vcc on the drain. I'd use a 100K trimpot, set the bias, then take it out, measure it, replace with resistor of fixed value.

Can play with source resistor to set gain.

[markr14850]

Want half of Vcc on the drain. I'd use a 100K trimpot, set the bias, then take it out, measure it, replace with resistor of fixed value.

Yep. But I'd suggest a 10k pot.

Can play with source resistor to set gain.

I'm under the impression that it's more standard to set the gain with the drain resistor - like a plate resistor in a tube.

[txbluesboy]

Thanks for the quick replys! I ended up changing the 8K4 resistor between the 150K and ground to 20K to get my source voltage up to 16 volts (originally I only had about 8 volts here). Then I used a 10K pot on the source resistor to adjust my drain voltage to 8.5 volts. Is this close enough or should I try to get the supply voltage up to 20-22 volts? It sounds pretty good and I don't think I need any more gain. Terry

[markr14850]

Is this close enough or should I try to get the supply voltage up to 20-22 volts?

That's probably fine. You can go a bit higher - which could give you more output before clipping at the rail, but you have to be sure not violate the specified Drain-Source max voltage. What FET did you use?

It sounds pretty good and I don't think I need any more gain.

I think you have your answer.

[heisthl]

Thanks for the quick replys! I ended up changing the 8K4 resistor between the 150K and ground to 20K to get my source voltage up to 16 volts (originally I only had about 8 volts here). Then I used a 10K pot on the source resistor to adjust my drain voltage to 8.5 volts. Is this close enough or should I try to get the supply voltage up to 20-22 volts? It sounds pretty good and I don't think I need any more gain. Terry

A 150k/27K has been the values I ended up using.

[txbluesboy]

I used the NTE 452 FET. Thanks, I think I'm good to go. Terry

[David Root]

I had only 12.5V at the junction of the 150K and 10K so I changed the 8K2 from the 150K to ground to 15K.

Now I have 22.3V at the junction of the 150K and 10K, but I have 19V at the other end of the 10K and 2.6V from source to ground. Using NTE 452.

Any suggestions? This is the first FET input I have done and I'm not at all knowledgeable about how to adjust it.

[txbluesboy]

Now you adjust the bias by changing the value of the resistor between source and ground ( in parallel with the 4.7 uf capacitor, I think this is equivalent to a cathode resistor and bypass cap, someone correct me if I'm wrong ) I used a 25K trimpot here to get the bias right, then replaced it with a resistor of the correct value.

[David Root]

OK, thanx muchly, I'll do that. So I need to increase that resistor some.

[martin manning]

You'll have to reduce the value of the source resistor, which will increase the drain current and the voltage drop across the 10k. It will also increase the voltage drop across the 150k, so you might need to make another iteration, increasing the ground leg resistance of the voltage divider further to get the supply voltage back up again.

MPM

[David Root]

OK, I now have 22K at the junction of the 150K and the 10K and I'm getting 20.7V, with 12.3V at the other end of the 10K, and 2.3V on the source resistor which is now 2K7.

My problem now is very low signal out, no serious volume available even with the 10K pot cranked.

I checked my wiring of the FET stereo jack and it appears to be correct as per the #124 layout I have been working from.

Any suggestions?

[martin manning]

Voltages sound like it should be working. Do you mean you now have a 22k for the lower leg of the voltage divider? You could go down to 2k2 on the source resistor to get Vd down closer to half of Vcc (150k-10k junction). What is the B+5 (top of 150k)? Can you lift the input and output wires and feed signal directly to the FET?

MPM

[David Root]

Thanx Martin, B+5 is about 285V and yes the 22K is the lower leg, in parallel with the 100uF cap.

I could pull the input and output leads and connect a signal generator, if that's what you are suggesting.