formula for determining correct biasing resistor value

[powerplayj]

Rangemaster biasing question:

If my collector voltage is 5.6V and the bias resistor value is 68K can I determine the correct resistor value needed to get the collector voltage up to 7.0V?  Here's the possible catch.... I don't know the gain/leakage of my PNP transistor.  I could throw in a 100k trim pot but, I was curious if a formula would give a good estimate as well.

The schematic is at the link below..

Would there be a need for other values (Qb, Qe, etc?)

http://www.generalguitargadgets.com/diagrams/geb_rm_sc_pp.gif

[niftydog]

it's been a while... so I don't have specifics for you, sadly.

You can calculate the quiescent current entering the base, and from this estimate the quiescent collector current. This may get you close to working out a value.

have a hunt around some basics tutorials and look for the re value which is the dynamic resistance of the base-emitter junction.

If you're really stuck, post a schemo and I'll have a bash at it. It'll be good excercise to fire up the old brain cells!

[petemoore]

I could throw in a 100k trim pot but
 A Rangemaster board is quick and easy, I built one with sockets and mostly trimpots where fixed resistors were, and was able to find bias with many different transistors.

[R.G.]

Yes and no.

Yes, a formula exists that will give you a very good if not perfect estimate of the necessary resistance.

No, it's not practical for someone not already skilled in the art.

The "formula" is obtained by doing a circuit analysis on the circuit, taking into account all of the parts including the transistor parameters, then solving the resulting set of simultaneous equations for the part you want to estimate.  This is a useful teaching exercise, and it's taught this way in EE circuits courses. Or it was 30 years ago, don't know about today.

The resulting "formula" is applicable to that and only that circuit.

The reason you get suggestions to put in a pot and try is that the pot method requires no particular training and will get  you to an answer quickly. It's only disadvantage is that it teaches you nothing.

[davebungo]

:oops: Sorry - I posted garbage so I've removed it so as not to confuse anyone.
There are some observations to be made though and someone will no doubt correct me if I'm mistaken:

With your 68K resistor for R2, VB is -1.13V.

Assuming negligible base current, IE = IC = ( -5.6V - -9 ) / 10K = 340uA.
VE = 0 - IE * R4 = 340uA * 3900 = -1.326V.

Therefore the base emitter pn junction is reverse biased so the collector current in this case must be leakage, is this right?  Perhaps a typical but not a good example of a Ge transistor for this circuit?