Basic BJT biasing questions

[Jmariner]

I'm not sure what I am seeing with the biasing of BJT's in many circuits.
With this simple PNP fuzz face as an example, where is the base getting it's bias?

Is Q1 reliant on the input signal so it loses 90 degrees from each half cycle?
Is Q1 obtaining + reference through the collector, and negative biasing from the 100K resistor?

I see many designs both simple and elaborate, where BJT's are used and there's no + or - bias resistors at all, which leads me to conclude it's reliant on the input signal to switch it on and get charger carriers to flow. From a purists standpoint if I'm correct, wouldn't their be a significant loss of input signal and no real benefit by avoiding use of these resistors?
It seems if that is the case, using something from the FET family of transistors would be more practical.

[Clint Eastwood]

Q1 is getting its bias from the 100k resistor and, because it is a germanium transistor, from collector -base leakage. A germanium transistor can actually self bias with leakage current, just like grid biasing with tubes.

[antonis]

I see many designs both simple and elaborate, where BJT's are used and there's no + or - bias resistors at all

Bias resistor are like stars..
(we might can't see them but the're always there..)

[Rob Strand]

The way the bias is set up is like this:

Start at Q1.  Q1 needs base current to bias.  It can only come from the 100k base resistor.  And that means we need a voltage on Q2's emitter.

You can see how the bias voltages is developed from these steps:

- Suppose Q2 is biasing correctly at around 4.5V on the collector.  There will be 9V-4.5V = 4.5V across the collector resistor of Q2.  That means the Q2 collector current will be IC2 = 4.5V/(8.2k + 470 ohm) = 520uA. 

- The collector current and emitter current of Q2 is about equal, that's normal for a transistor in an amplifier.  The emitter current passes through the 1k emitter resistor (the Fuzz pot).  The voltage on the emitter of Q2 is approximately 520uA * 1k = 520mV = 0.52V.


The 0.52V on the emitter of Q2 is where Q1 gets its base bias from.  Since 0.52V greater than the 0.15V or so of Q1's VBE.    Q1's base current is roughly IB1 = (0.52V - 0.15V)/100k = 37uA.

The leakages change the numbers a bit but they don't change the idea of where Q1 gets it's bias from.

The motivation for pulling the bias for Q1 off Q2's emitter is it forms a DC feedback loop to keep the DC voltages stable.  The fuzz face is DC coupled (Q1 collector directly connects to Q2 base).  With such a high gain amplifier it's impossible to keep the DC biasing stable without DC feedback.

When you have stages which are separated by coupling caps (C2) like this one you can bias the transistors individually,

[antonis]

Just a little addition to Rob's excelent analysis, concerning DC feedback bias..
(IMHO, it should be more easy to understand in a negative ground supply circuit..)




If, for any reason, the quiescent current of Q2 is too high, the voltage on Q2 Emitter should also be high, turning Q1 a little more on (more Base current, hence more Collector current) and thus turning Q2 a little less on (by lowering its Base voltage due to higher voltage drop across Q1 Collector resistor) so reducing quiescent current..
(and vice versa..)

[Jmariner]

Thanks everyone, this is truly helpful. I should have found the NPN fuzz face for reference and ease of understanding but went with PNP since it's the popular. I had assumed it was possible to get bias through the D.C. sources being directly coupled by networks of resistors ultimately tying the bases to the DC through collectors and emitters, but lack experience with it. From a design standpoint, it certainly makes a lot more sense to reduce part count and use more math to achieve the desired current values. Initially it seemed disadvantageous to obtain base bias through a collector resistor, but I'm now grasping it's in fact the opposite.

In fact, it just clicked in that you only need to get some leakage current flowing with minority carriers to switch the transistor on, and use emitter and collector resistors more effectively for setting gain as well as providing the bias to get leakage current to flow.

[antonis]

you only need to get some leakage current flowing with minority carriers to switch the transistor on, and use emitter and collector resistors more effectively for setting gain as well as providing the bias to get leakage current to flow.

I can't get what you mean but, in general, leakage curent role in bias should be strictly avoided..
(stability factors quit and go to the nearest bar to get drunk..)

[Jmariner]

you only need to get some leakage current flowing with minority carriers to switch the transistor on, and use emitter and collector resistors more effectively for setting gain as well as providing the bias to get leakage current to flow.

I can't get what you mean but, in general, leakage curent role in bias should be strictly avoided..
(stability factors quit and go to the nearest bar to get drunk..)

]

lol, good one.
I'm sure you know what leakage current is, but for those that may not, this is something I still managed to remember well from school. This is the reverse current produced by the minority charge carriers in bipolar junction transistors, and required to thin the junction barrier so current can start flowing and switch the transistor on.

This is probably the main reason I have favored BJT's, aside from being so damn cheap, and the concept that this reverse current is giving us some natural and good quality compression. Since I employed that series common-emitter amplifier in my guitars, I have had this brilliantly tight, dense tone and was able to use a really steep low pass filter on the input and out, which perfectly addressed the insane twang and thinness I grew to despise in passive electronics, even if a ridiculous amount of money has been spent on purchasing them.
My pinch harmonics really took on density and richness after installing that active stage in guitars.

In fact, with many of you being more knowledgeable, I had a question about my theory on compression due to leakage current. That help with understanding the DC bias available through direct coupling was awesome of you folks. I know I covered it in school, but it was 11 years ago and my referring to it is largely to help get myself motivated and the help I get from others here is motivation.
I don't want to get into the details of it, but a woman I will for convenience regard as my "ex girlfriend", was likely the culprit behind killing a couple of my hard drives, and there was surely over a thousand pages of notes I took roughly a decade ago during school!

I had planned to do a complete review of all my physical school materials and maybe some additional notes, but now I have to rewrite all of them from square 1 which is really just an unwanted blessing in disguise.
I'm tolerably but still greatly disappointed in myself having let all that hard work deteriorate over the last 11 years. The 2nd time around should be a breeze, and I certainly got far enough that I can become competent again without returning to school.
I'm 45 and that's not a demand I am willing to make upon myself 

[antonis]

If you say so..