Please explain Capacitor coupling

RichardF

Since the output of a microcontroller is a 0 to 5 positive volt swing, how is it that a capacitor·can couple the output to another device, such as the LM386 audio amp? It works, but I don't understand how without a negative and positive voltage swing about ground. I thought I understood capacitors, I guess I don't. Will someone please explain?
Thanks,
Richard

Ale

The capacitor loads itself with a difference in potential, positive and negative are just conventions. If a difference in potential exists the capacitor loads itself and when the potential across the plates reaches the external potential equilibrium is reached. When the loaded capacitor is conected to a lower potential (or none) across the plates, it releases its charge, "conducting". A pulsing wave, any kind will make the capacitor "conduct", actually it will load it and make it release its load.

RichardF

Ale, thanks
Richard

Tracy Allen

One analogy to think of a coupling capacitor is as a tube with a highly elastic diaphragm across inside, and current is a fluid moving in the tube in response to differences in pressure. Motion of fluid stretches the diaphragm and stores energy, and the stored energy can at times move the fluid back out the other direction.

In the case of coupling to a high impedance at the output, think of it as a closed tube at the outlet with a pressure sensor, and the pressure on the inlet side is transferred directly to the outlet through the membrane, and the membrane doesn't even have to move or store any energy. In the case of coupling into a short circuit, think of the output ending in a big lake so the pressure is there constant. Fluid is transfered from inlet to outlet until the difference between the inlet and outlet pressure is exactly counterbalanced by the stretch in the diaphragm. That means in analogy that the capacitor is charged up to the difference voltage. Resistance in series with the capacitor on the input or output is like a length of a smaller diameter tube with fricative (dissipative) forces.

The LM386 audio amp is pretty close to presenting a high impedance, so it is like that pressure measuring device. But you can't make the capacitor too small, because the LM386 does have a finite input impedance, and too low a capacitance value will couple through a low, distorted signal.


With a high frequency sinusoidal input, relative to 1/R*C, the stretch in the diaphragm (voltage across the capacitor) will be greatest when fluid flow (current) passes through zero. Then the voltage and the current waveforms are 90 degrees out of phase.

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Tracy Allen
www.emesystems.com