Biasing, not why, but how!

[MattL]

I've been trying to do research on biasing transistors because the instructions in these projects say that things should be biased so there's a drain of 4.8, for example. But I can't find anyone to tell me how to use the DMM to obtain this information. What do I touch the red side to? How about the black? I've ascertained that the "drain" is the emitter, so I assume I'm touching that with one side? I've found tons of heavy theory and the science of how things work, but nobody ever mentions the actual procedure, that I can find anyway. I'm about to build the Carmin Ghia pedal, so I need to know. Thanks!

[eniacmike]

set your dmm to measure DC voltage
black test lead connected to ground
red lead connected to the leg you want to measure.

on page 4 of the rump roast it shows you which legs to measure while you adjust the trimpots for bias.

[madbean]

Bias is a very important concept to grasp, obviously, and will help you greatly in understanding transistor fundamentals. It's a bit broad to try to explain in this setting, and frankly, there are a lot of learning materials out there that would do a better job than me!

In any case, the very basic thing to understand is that most transistors have an ideal range of supply and/or reference voltage where they like to operate. The different technologies of the transistors determine how that voltage should be applied to achieve this ideal range.

Determining if you have the correct bias is a matter of knowing where and how to measure. What Mike posted is the "how". The "where" is simply a matter of knowing what type of transistor you a measuring.

For FET and Mosfets, you generally have a drain, gate and source pin corresponding to the three legs of the transistor. The input is supplied at the gate, and is amplified in relation to the total resistance used at the source pin, provided that the correct supply voltage is applied to the drain (note this is for ONE particular way these transistors can be used---there are others).

Anyway, in the case of the Rump Roast, our J201's bias properly when their drains measure about half the supply voltage: 4.5v. This is where you will find the best sound, i.e. the most gain and the least amount of fizz or gate (not transistor gate---but sound gate) in the decay of the notes. So, we use our DMM to measure the drains on these two fellas to see that we have about 4.5v on them. Our trimpots let us adjust that supply voltage accordingly to get the correct measurement. In the case of a J201...about 22k on the drain usually works.

Knowing which pin is what matters, too, and fortunately this information can be found easily. Just google "J201 datasheet" and you will find any number of links from different manufactures illustrating the pinout of that transistor. A lot of the time, the arrangement of pins in one transistor are consistent across different manufacturers...but not always! So, reading the label on the transistor usually gives you a clue of who you need to look up.

(This is a lot more info than what you are asking for, I know, but I'm putting it here because it might help other members)

[MattL]

Thanks guys; between the info you both provided I now feel confident to try it. Your explanations brought me the most straight ahead answers to my questions that I've found!