mstechca said:
The source is my superregen receiver detector.
You must know its output impedance (I can't remember it) in order to design an amplifier with a high enough input impedance so that the receiver's output isn't loaded-down.
I am totally lost here. How does impedance matching apply with capacitor coupling?
A capacitor passes frequencies
and impedances. If you connect a circuit (amplifier) with a 1k input impedance to a circuit with a 10k output impedance you get a 1/11 reduction due to "loading-down" by the action of the resulting voltage divider.
Is there an equation to calculate gain based on the resistors?
If a common emitter amplifier stage has a low impedance source and a high impedance load, its gain is simply the value of its collector resistor divided by the value of its unbypassed emitter resistor. The emitter resistor provides negative feedback and allows the transistor's base to be biased easily and securely with a simple voltage divider. The unbypassed emitter resistor vastly increases the transistor's input impedance.
This is about the only equation I know in terms of transistor amplifiers.
You can't just simply multiply the individual gains of transistor stages without investigating the effects of inputs and outputs being "loaded-down".
So is output impedance equal to RC where R is the resistor from collector to +ve and the capacitor is connected to collector? and is the input inpedance equal to RC where R is the resistor between base and +ve and the capacitor is the capacitor connected to base?
You shouldn't simply pick a coupling capacitor's value blindfolded. You might end up with no low frequencies or too much rumble.
The value of a coupling capacitor is determined by the frequency you want to be 3dB down (0.707 of the signal's AC voltage) and is calculated with its "R" as the sum of the source and load impedances.
The output impedance of a transistor stage without current feedback is roughly equal to the value of its collector resistor because a transistor's collector is a fairly high impedance current sink/source.
The input impedance of a transistor stage includes the biasing resistors in parallel and also in parallel with the input impedance of the transistor itself.
Doesn't that distort the radio signal if I just connect the resistor from base to +ve?, or does the emitter resistor cancel all distortion? and how high should the emitter resistor be?
You must bias the base of a transistor so that the output can swing equally positive and negative from its idling voltage. The easiest way is by using an emitter resistor to raise the emitter voltage (and therefore also raise the base voltage) to a known amount, determined by how much the supply voltage and temperature changes, usually 10% of the supply voltage. If unbypassed by a capacitor, the emitter resistor vastly reduces the stage's gain but the distortion (ordinary distortion, not clipping distortion) is reduced by a similar amount. Since the DC voltage required at the transistor's emitter and base are known, the base voltage is established with a simple voltage divider of the supply voltage.
I want to make an audio amplifier with the best gain and least distortion possible.
Doesn't everyone?
Then use many transistor stages with high input impedances and low output impedances, use current sinks and sources instead of collector resistors, use differential stages to cancel even harmonic distortion and use lots of negative feedback. Like an opamp or IC power amplifier.
1) Try it with a 6V supply.
2) Choose a 3.9k resistor for the collector resistor.
3) At 10% of the supply, the emitter's idling voltage will be 0.6V. The collector will swing from 6V to about 1.2V.
4) Therefore the collector's idling voltage will be half the swing which is 3.6V.
5) Therefore the collector and emitter current is 615uA (Ohm's Law).
6) Therefore the emitter resistor is 1k (Ohm's Law again).
7) The base voltage will be about 0.65V higher than the emitter voltage (from the transistor's datasheet) so the base voltage will be about 1.25V.
8) Most transistors have an hFE of about 200 so the base current is about 3uA.
9) Choose a base to ground current of 5 times the base current which is 15uA. Therefore the resistor is about 82k.
10) The 1.25V base to 6V supply resistor will have 18uA, therefore is 264k, use 270k.