MPF-102 Transistor Bias

[soupbone]

I found a circuit that uses 1 MPF-102 tranny.It says on the build doc to "bias" the transistor for 4.5v.Anyone have any info on how to do this?

[smallbearelec]

I found a circuit that uses 1 MPF-102 tranny.It says on the build doc to "bias" the transistor for 4.5v.Anyone have any info on how to do this?

There should be a trimpot in the drain circuit of the transistor. With your meter connected from drain to ground, tweak that pot till you see 4.5 Volts.

[soupbone]

I found a circuit that uses 1 MPF-102 tranny.It says on the build doc to "bias" the transistor for 4.5v.Anyone have any info on how to do this?

There should be a trimpot in the drain circuit of the transistor. With your meter connected from drain to ground, tweak that pot till you see 4.5 Volts.

Cool!This layout doesn't use a trimpot.Should i adjust the resistor,If need be that is connected to the drain of the transistor?

[frank_p]

I found a circuit that uses 1 MPF-102 tranny.It says on the build doc to "bias" the transistor for 4.5v.Anyone have any info on how to do this?

There should be a trimpot in the drain circuit of the transistor. With your meter connected from drain to ground, tweak that pot till you see 4.5 Volts.

Cool!This layout doesn't use a trimpot.Should i adjust the resistor,If need be that is connected to the drain of the transistor?

Choose trimpot to higher value than the drain resistor on your shem.

Replace the resistor with trimpot like this:


adjust to 4.5v

Than if you have it, replace with close value resistor or leave trimpot.

Measure, try to play.

http://www.runoffgroove.com/fetzervalve.html
http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp6.htm

[soupbone]

I found a circuit that uses 1 MPF-102 tranny.It says on the build doc to "bias" the transistor for 4.5v.Anyone have any info on how to do this?

There should be a trimpot in the drain circuit of the transistor. With your meter connected from drain to ground, tweak that pot till you see 4.5 Volts.

Cool!This layout doesn't use a trimpot.Should i adjust the resistor,If need be that is connected to the drain of the transistor?

Oh ok,So you just replace the resistor with a trimpot instead.Excellent!Thanks!




Choose trimpot to higher value than the drain resistor on your shem.

Replace the resistor with trimpot like this:


adjust to 4.5v

Than if you have it, replace with close value resistor or leave trimpot.

Measure, try to play.

http://www.runoffgroove.com/fetzervalve.html
http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp6.htm

Oh ok,So you just replace the resistor with a trimpot instead.Excellent!Merci!

[soupbone]

I found a circuit that uses 1 MPF-102 tranny.It says on the build doc to "bias" the transistor for 4.5v.Anyone have any info on how to do this?

There should be a trimpot in the drain circuit of the transistor. With your meter connected from drain to ground, tweak that pot till you see 4.5 Volts.

Thanks Steve for the info!

[diydave]

If you don't want to use a trimpot for the drain, maybe these rules-off-thumb can help.
Vp/2 gives Idss/4.
Check out runoffgroove's fetzervalve to get the Idss (max current with gate=grnd) and Vp (pinch-off-voltage) off your fet.
So you can calculate a source-resistor Rs = (Vp/2)/(Idss/4).
To bias for 4.5 volts at the drain, you need a drain-resistor Rd = (4.5 volt / (Idss/4) ) - Rs.

I've written a calculator + Vp/Id curve as a visual aid to bias fets.
You can find it at www.diydave.be/tools/fetcalc/index_eng.html

[soupbone]

If you don't want to use a trimpot for the drain, maybe these rules-off-thumb can help.
Vp/2 gives Idss/4.
Check out runoffgroove's fetzervalve to get the Idss (max current with gate=grnd) and Vp (pinch-off-voltage) off your fet.
So you can calculate a source-resistor Rs = (Vp/2)/(Idss/4).
To bias for 4.5 volts at the drain, you need a drain-resistor Rd = (4.5 volt / (Idss/4) ) - Rs.

I've written a calculator + Vp/Id curve as a visual aid to bias fets.
You can find it at www.diydave.be/tools/fetcalc/index_eng.html

Wow Dave thats some great info!Thanks!