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Why are capacitors use in transistor cicuits? What does it do with biasing operations? I always see capacitors on transistors being used as amplifiers.

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up vote 6 down vote accepted

A capacitor blocks DC, so it can be used to pass a signal (e.g. audio, etc) without it's DC level interfering with the DC bias of a transistor. This way the DC offset of the input signal can be at any level and the transistor amplifier will treat it the same way.

For example, if you have one transistor with it's collector output at a DC level of 5V, and the next transistor stage has it's base biased around 1V, directly connecting them would turn the second stage full on all the time as the input voltage will always be too high.
If we add a capacitor in between the stages, one side can be at the 5V DC level, and the other side can be at the 1V level, and only the AC variations pass through. This way the second stage operates correctly.

Take this simple circuit as an example:

Transistor Capacitor Coupling

Here are the waveforms at the various points:

Transistor Coupling Sim

Notice how the input voltage is a 1kHz 10mV signal with a 10VDC offset. After the input capacitor the DC level is now ~870mV. The same can be seen at the output cap. The gain is around 20 (200mV / 10mV = 20)

Now if we remove the caps:

No caps

And look at the input/output waveforms:

No caps sim

Things have changed drastically - the transistor base is now at 10VDC, which means the emitter will be at around 8-9V (the usual 0.7V drop will be higher due to excessive current) and the emitter resistor current will be ~8.5V / 100 = ~90mA. The output is stuck at around 20mV higher than the emitter. The circuit has no gain.
This is an extreme example, but even a few tens of millivolts either side of ideal bias point will have quite an effect on this circuit.

Apart from coupling, capacitors are also used for things like emitter bypass (as Vlad mentions) Here is the first circuit again with an emitter bypass capacitor added:

Emitter Bypass

And the simulation:

Emitter bypass simulation

Notice the gain has increase to around 100 from the first circuit (1V / 10mV = 100) This is because the emitter resistor provides negative feedback and controls the gain in the first circuit. With the capacitor added, the DC is unaffected but the AC now sees a lower impedance path to ground (the capacitor) so the AC gain is increased. So the AC is "bypassed" to ground.

There are many other uses for a capacitor, but these are the main uses in a typical audio amplifier circuit (apart from power rail filtering of course)

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Just a side note, there are also those used in parallel across emitter's or collector's resistors, they introduce poles/zeros for bandwidth alteration, according to specifications. – Vlad Sep 28 '12 at 11:11
@Vlad - More added, hopefully this gives an basic introduction to their use. – Oli Glaser Sep 28 '12 at 11:57
Truth be told, I only said that because I couldn't figure out which ones the OP was referring to :) Nice post! – Vlad Sep 29 '12 at 7:03

The input and the external capacitors are used to block the dc supply. The emitter bypass capacitor is used to increase the AC gain of the amplifier by shunting the emitter resistance for AC . We should include the emitter resistance in the circuit because it gives better DC stability. Keeping the emitter resistance reduces the DC gain of the amplifier. so to increase the gain of the amplifier a capacitor is placed across emitter resistance.

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