# signal in series with base bias volt.

Discussion in 'General Electronics Chat' started by hobbyist, Dec 16, 2008.

1. ### hobbyist Thread Starter Distinguished Member

Aug 10, 2008
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When we analyze a simple one transistor amp, we solve for the thevinin equivalents.

Then we put the signal source in series with the thevinin volt. and res.
So we can use superposition to analyze the signal effect on the amp.

Where the signal adds to the bias volt. and subtracts from the bias volt.

However in actuality we hook up the signal source in parrallel with the bias voltage on the base term.

So how did they come up with putting it in series for correct analysis of the amp?

2. ### mik3 Senior Member

Feb 4, 2008
4,846
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They are not actually in parallel because you couple the bias voltage and the source voltage with a capacitor. In this way only AC passes through the capacitor which is added on the DC bias voltage.

3. ### KL7AJ Senior Member

Nov 4, 2008
2,013
273

Back when transformer coupled audio amps were a lot more common, you'd often see the bias voltage applied through the secondary of a coupling tranformer. In this case the bias voltage and the signal voltage were indeed, in series.

Don't see that configgeration much these days, though. ")

4. ### steveb Senior Member

Jul 3, 2008
2,433
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Another way to look at is to consider that the AC source is a more "ideal" voltage source than the DC bias voltage. In other words, the DC bias voltage is going through a relatively large source impedance and can be pulled away from its average value by the AC source with low source impedance.

If you were to put an ideal AC source in parallel (with capacitive coupling) with an ideal DC voltage source (with zero source impedance), then it wouldn't work. The AC signal would be zero since it is shorted to ground through the DC source.

Last edited: Dec 19, 2008
5. ### hobbyist Thread Starter Distinguished Member

Aug 10, 2008
764
56
Yeh, elenco electronics AM/FM 14 trans. radio kit uses that configuration in there IF stages. The sec. is part of the base bias, on the next stage.

6. ### hobbyist Thread Starter Distinguished Member

Aug 10, 2008
764
56
So it's safe to say that even though the signal is coupled through cap. in parrallel with the base bias volt. due to the sources not being ideal, then the sources have the effect as if they were in series for analysis purposes.

Thanks everyone.

7. ### KL7AJ Senior Member

Nov 4, 2008
2,013
273
Another approach to this is to use Kirchoff's Current Law. The input signal through the coupling capacitor is just another current source into the node.

Eric