Biasing transistors for gain.

[ReeceAblaze]

Which resistor in an amplification stage can be changed to allow for more distortion? Is it the one from power to collector? and if this is the case. When you reverse the transistor and the Emitter and collector are swapped, does this still apply?

[antonis]

CE amp Gain is roughly R_collector / R_emitter..
(It doesn't matter if it's a n-p-n or p-n-p transistor)

Any of resistors altering value results in altering gain but also in mis-bias..
(you should have distortion but in an asymmetrical mode..)

It should be better to set in parallel with Emitter resistor a cap in series with a trimpot..

Or cascade another stage..
(for reasons beyond your original query..)

[ReeceAblaze]

CE amp Gain is roughly R_collector / R_emitter..
(It doesn't matter if it's a n-p-n or p-n-p transistor)

Any of resistors altering value results in altering gain but also in mis-bias..
(you should have distortion but in an asymmetrical mode..)

It should be better to set in parallel with Emitter resistor a cap in series with a trimpot..

By reversed i meant putting an npn transistor in its slot backwards so that C goes to where E should be and vice versa. So was the suggestion to set gain on the emitter side and keep the collector at a constant value? What value is required at the collector side?

[R.G.]

To get to the answers to your questions, you have to understand what the transistor and power supply are doing. It's not as simple as "which resistor do I change?"
A current into the base (for NPN, to keep this simple) lets through a proportionate and larger current from collector and the sum of the base current and collector current goes out through the emitter. The current gain, hfe, or "beta" is the ratio of the collector current to the base current, and is generally so big that we ignore the base current for simple cases.
For this to happen, the power supply has to be the correct polarity to set the junctions up (collector-base reverse biased, base-emitter forward biased) and have enough voltage to run the junctions, and enough current to let this all happen.
The collector acts like a current sink, sucking down a multiple of the base current. The collector resistor can be anything at all, from a short circuit to an open circuit. If it's a short circuit, the collector voltage is simply the power supply voltage and no voltage gain happens at all. An open circuit lets no current flow, so the collector really can't do much. In between, a real collector resistor causes a voltage drop of the collector current times the resistance.
>>So to get voltage gain, there must be an intermediate value of resistor in the collector to power supply.<<
One of your questions was what size is this resistor. It depends on a lot of things, but mostly on how much current you want the transistor stage to use. The collector resistor (plus emitter resistor, later) limits the total current that the NPN stage can possibly let through. If you decide you want to let through one milliampere and you have a 9V supply, the total of the collector resistor and emitter resistors must be 9V/0.001A = 9000 ohms. That's with the transistor fully on. With the transistor fully off, no current flows, and the voltage at the collector is the 9V of the power supply. Doing the same math with a microamp, an ampere, or 10A changes only the value of the resistance, not any "gain".
>>The maximum gain of the stage is equal to the ratio of the collector to the emitter resistors.<<
This is obvious if you think about it the right way. The base is tied to the emitter by the base emitter junction being forward biased. If it's not, nothing works. Any change in the base voltage is tied directly to a change in the emitter voltage. The current at the emitter must be the emitter voltage divided by the emitter resistor. Actually, the current gain of the transistor forces enough current to flow through the emitter to make this true. So the emitter current equals the emitter voltage divided by the emitter resistor. And since the emitter current must be the same as the collector current, minus the trivial and negligible base current, the same current flows in the collector resistor. So by Ohm's law, the voltage change across the collector resistor must be the ratio of the collector resistor to the emitter resistor.
>>AC gain is not equal to DC gain.<<
You can increase AC gain by using a capacitor to bypass the emitter resistor. The capacitor acts like a "short circuit" for AC signals and so the emitter resistor no longer limits gain. There is a tiny resistance inside the base emitter junction of the transistor that acts as the only limit on gain in this case. You can put a resistor in series with a capacitor to bypass the emitter resistor and get a higher but still well defined AC gain as well.
>>Gain is not equal to distortion.<<
You can get more distortion by changing the DC bias point or the power supply so that the current swing bangs into either the power supply or saturation at lower signal levels. This adds distortion without changing the AC gain. In fact, getting more distortion by increasing gain really amounts to changing the gain so that the signal bangs into the power supply or saturation as well. Gain isn't distortion, but you can increase gain until you get distortion.
>>Reverse operation (swapping collector for emitter) is possible, but way different than you think.<<
Reverse operation means that the current gain/hfe/beta now happens as a "feature" of the base-collector junction, not the base-emitter junction. This fundamentally changes the gain of the device. An NPN with a forward gain of 100-200 may have a reverse current gain of one to ten. There are certain situations where the side effects of doing this may be useful, but it's not generally a useful tool in the way you have described.
Questions?