# Why do we need a coupling capacitor?

1. Aug 26, 2010

### Peon666

Okay, I get that we need it to block DC while allowing the AC to pass. But if we have to block DC, why apply it in the first place? Is it because we apply it to a certain level and then we want to keep it to that level so we block any further interference by DC?

Thanks.

2. Aug 26, 2010

### waht

In transistor amplifiers there will be DC voltage present at the input of the amplifier.

So if were you hook up some AC source, DC voltage from the amplifier would be applied to the AC source. And if the AC source loads the DC voltage too much, the operating point of the amplifier will change, hence affecting the gain and its impedance.

3. Aug 26, 2010

### Peon666

So we can't apply AC source only?

4. Aug 26, 2010

### skeptic2

An amplifier could be designed with a dual supply in which both the input and output would have a DC bias at zero volts similar to an opamp. However this would make the amplifier much more complicated than it needs to be. Also what would you do if the AC source had a DC bias?

In RF amplifiers, often the DC blocking or coupling capacitor may serve other purposes such as matching or transforming impedances between stages or filtering the signal.

5. Aug 26, 2010

### dlgoff

And if you need to know what a small ac signal riding on a high dc voltage looks like, say with a oscilloscope, you'd want a small cap between your probe and the dc.

6. Aug 26, 2010

### Peon666

"Also what would you do if the AC source had a DC bias?"

What does this mean? Do we have to bias the AC source at some point?

7. Aug 26, 2010

### dlgoff

If you are interested in just having the DC bias information, you'll need a circuit to filter/integrate out the AC. Look how a power supply rectifies then filters/smooths the ripple (using capacitors). You probably wouldn't need a "coupling capacitor" though.

8. Aug 26, 2010

### skeptic2

Suppose you want to use your amplifier with an electret microphone. These microphones have to be DC biased to work.