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Class A bias vs input signal
Class A bias vs input signal
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Old 15th June 2016, 06:51 PM   #1
elderjoe is offline elderjoe
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Default Class A bias vs input signal

I can not find an answer to this simple question :

I build the simplest class A stage with a 2N3055, speaker as load. Resistor divider for bias. How does the input signal ( very small -mV ? ) "overpower"
the bias ? Say output impedance of previous stage is 47k and the bias resistors
are low value - hundreds of ohms. AND, what if I lift the base voltage in relation to the ground by adding an emiter resistor ?

Sorry for such stupid question - I am a mechanical engineer.
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Old 15th June 2016, 07:45 PM   #2
DF96 is offline DF96  England
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Quote:
Originally Posted by elderjoe
How does the input signal ( very small -mV ? ) "overpower"
the bias ? Say output impedance of previous stage is 47k and the bias resistors
are low value - hundreds of ohms.
In that circuit the input signal would not "overpower" the bias. To get useful amplification you need low source impedance and high bias impedance.
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Old 15th June 2016, 07:48 PM   #3
Mooly is offline Mooly  United Kingdom
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Class A bias vs input signal
You would have to draw out what have configured but I think what you are getting at is this...

The bias isn't 'overpowered' by the input signal, but rather it is modulated by it. The effect of that is to alter the base current flowing in the transistor, either adding to it, or decreasing it depending on which way the signal is going. That change is amplified by the gain of the transistor and allows you to get a usable current flow in the collector of the transistor, which in turn drives the speaker.


Adding an emitter resistor 'degenerates' the transistor stage and does a couple of things. It makes the bias setting a bit more stable with regard to temperature change and also differences in individual transistors, and it also lower the gain of the stage.

Draw out what you have and we can take a look. For instance, I assume you have AC coupled both the speaker and the input source to the transistor.
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