# Capacitive coupling

Discussion in 'Homework Help' started by epsilonjon, Jan 2, 2012.

1. ### epsilonjon Thread Starter Member

Feb 15, 2011
65
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Hi. The book i'm reading is just going over the basics of a BJT amplifier using the circuit below. It says the ac voltage is superimposed onto the dc bias voltage by capacitive coupling, but it doesn't seem correct to me.

I've read some bits on the internet about what capacitive coupling is and I think I understand it. I also read the AAC info on biasing techniques http://www.allaboutcircuits.com/vol_3/chpt_4/9.html which has a similar circuit to the one below, but which makes sense to me.

For this circuit below, I don't get how $V_{in}$ can be the ac voltage shown. Using superposition $V_{in}$ is just $V_{BB}$ isn't it? Also, I don't really get the purpose of capacitive coupling here. Why not just put the ac and dc sources in series with one another and $R_{B}$, and leave out the capacitor altogether?

Could someone explain it to me please?

Thanks!

2. ### ceidas Member

Dec 26, 2011
50
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The first thing we do when we work with transistors is biasing them in the linear region. This is what VBB do. And if we hadnt placed the capacitor the dc current from VBB would go in the Vs. Thats a waste we dont want.

When we are sure that the transistor is correctly biased in the linear region, then we do what we call small signal analysis by making the AC equivalent circuit. Thats the dc circuit but with the dc voltages short circuited and the transistor replaced with the forward active model.

Dec 26, 2010
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This is a confusing circuit, and as shown literally would not work.
Effectively, RB should be in series with VBB, so that it is possible for Vs to affect the base voltage.

If this circuit is on a forum page you might think of sending some feedback about it.

4. ### ceidas Member

Dec 26, 2011
50
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True, thats why we implement this kind of VBB with two resistors, one connected in CB and an other in BE.

Dec 26, 2010
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There are a number of reasons for using capacitive coupling. Amongst these are the facts that it may be undesirable to amplify a possibly variable DC voltage, and that it often is not convenient or even possible to put things in series.

A signal source may have a connection to ground, or to some other potential which cannot be removed - for example the signal may come from the collector of another transistor. The DC voltage at the signal source may also not be very stable.

If only AC signals are of interest, as in the case of an audio amplifier, it may be sensible to "divorce" the AC content from the unwanted DC, and replace the latter with a stable bias voltage.

Whether or not the signal source has any DC level on it to start with, the coupling capacitor also prevents the bias from being applied to the signal source, which in some cases would be adversely affected by it.

6. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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But the model is still wrong...regardless.

Dec 26, 2010
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Quite true.

@ Epsilonjon: Wherever that schematic came from, on this forum or elsewhere, some feedback might be in order. What was the source?

8. ### epsilonjon Thread Starter Member

Feb 15, 2011
65
1
Thanks for all the replies to confirm what I thought about the circuit and explain a bit more about why capacitve coupling is used.

I could forgive the circuit a bit more if it came from a forum or some other free source, but it actually comes from the (rather expensive) book Electronic Devices by Thomas L. Floyd. This circuit comes from the 9th edition of the book, but interestingly I managed to download the seventh edition and in that edition they had the circuit as this:

Dec 26, 2010
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I think I can see what may have happened. This form of equivalent circuit does not include the coupling capacitor, and shows the bias voltage as actually in series with the signal, which is uncommon in practice.

Perhaps it was suggested that the circuit was changed into a more realistic form, but a mistake was made when this was done. Maybe they will fix it in a future edition?

10. ### epsilonjon Thread Starter Member

Feb 15, 2011
65
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Yeah you're probably right. I'd have thought after 9 editions at £50 a pop they might have got it sorted by now, but nevermind. At least it prompts the reader to think about it for themself a bit?