Using capacitors to filter DC

[Richardcavell]

If I use a capacitor to filter DC from an AC signal, is it necessary for me to use a bipolar capacitor rather than a unipolar one?

Richard

 

[Brevor]

Not sure what you mean by filtering DC from an AC signal. You need to rectify the AC first to get pulsating DC then filter it with the capacitor. It doesn't need to be bipolar.

 

[atferrari]

Not sure what you mean by filtering DC from an AC signal. You need to rectify the AC first to get pulsating DC then filter it with the capacitor. It doesn't need to be bipolar.

Are you sure it is what hee neds to do?

It seems he wants to block DC and let the signal (AC) to pass.

 

[Richardcavell]

Here's a schematic

Alright then, let me rephrase the question. In this circuit, the lecturer describes the output of the microphone as AC. These capacitors appear to be unipolar. Is that going to work, or do they need to be bipolar?

Richard

 

[Diver300]

For small signals, polarised capacitors would work.

The one on the left, between the microphone and the base of the transistor, would be the wrong way round.

However, the values would be so small that there would not be much point in using polarised capacitors.

 

[audioguru]

For small signals, polarised capacitors would work.

The one on the left, between the microphone and the base of the transistor, would be the wrong way round.

However, the values would be so small that there would not be much point in using polarised capacitors.

No.
The input impedance of a typical 2N3904 transistor is shown on a graph on its datasheet. Then you don't need to calculate it.
If the collector current is 10mA then the input impedance is only 500 ohms because the emitter resistor has a bypass capacitor. The base bias resistors have fairly high values so they reduce the input impedance of the transistor to maybe 480 ohms.

If you want a cutoff frequency of 50Hz then the coupling capacitor value is 6.7uF which is a polarized electrolytic type unless you can find a non-polar one.