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This is the basic common emitter amplifier.I have read that polarized capacitors should have proper polarity connected to terminals of DC voltage source and they may blow off if they are connected in a wrong way and the polarized capacitors cannot be used with AC voltage source.

But I have been asking myself how does a polarized capacitor can be used in this below circuit because we have an AC signal as input which is to be amplified and this AC signal passes through C1 capacitor.During the positive cycle the polarities are matched but when the AC signal is negative cycle the polarities doesnt match and capacitor should blow off but that doesnt happen. I was trying to figure it out the reason.Could anyone help me with this Thanks.I know this must be a basic question to understand.

I have been said that Capacitors are used to block DC and allows only AC.We block DC so that the operating point does not shift but Iam unable to understand where the DC signal comes from. Does an AC signal has dc components in it? or may be some other reason? if so can anyone provide me some good link.Thanks


Polaized capacitors must always have 0 or greater volts on them from the positive terminal to the negative. However, that does not mean there can't be a AC component to the voltage, only that the lowest peaks don't go negative. Put another way, you are fine as long as the cap has a DC bias at least as large as the negative peak of the AC.

In the circuit you show, the right side of C1 will be at some positive bias, with the left side having a small AC signal centered at 0. If C1 were polarized, the right side would need to be the positive terminal.

Things are not as bad with C1 as you think. First, just because there is a AC voltage applied at one end doesn't mean that voltage appears accross the cap. In fact, the point of C1 is to block DC and pass AC, so at least for the frequencies of interest to this circuit, the signal will largely not appear accross C1. Ideally, the whole AC signal is superimposed on the bias point and appears on the base of Q1. If it does, it can't be also accross C1.

Second, the AC signal level at node 1 is clearly meant to be "small". If it were too large, then the circuit would clip and act non-linearly, which is not usually desired from a "amplifier".

Third, in this case the voltages are small enough and the required capacitance small enough that C1 would generally not need to be polarized in the first place.

The other capacitors have clear DC biases on them and can't be driven negative. Again, depending on the impedances, frequencies, and voltages, they may not need to be polarized at all. You generally don't want to use polarized capacitors in signal paths.


It's pretty straight forward, there is an AC voltage which rides on top of the DC voltage. Ideally the combined voltages should never go negative, which means that for the most negative portion of the AC voltage it will still have a lower magnitude than the DC component. SO the fact that the capacitors block DC voltage is a good thing as that supports the biasing of the capacitor into a safe regime.

Looking at yoru diagram you can see that CE and CL are easily understood to have a DC voltage on them. It is only really C1 which might be questionable, simply because Vs is not defined here.

Using polarized caps in these applications is a good way to get high capacitance in either a small package or at a lower cost. but there are down sides as they are far from ideal in their performance characteristics.


What makes electrolytic capacitors polarized is that they rely on voltage of the correct polarity to maintain a layer of aluminum oxide which insulates the layers of the capacitor. A voltage of the opposite polarity will actively eat away this layer, and soon the layers of the capacitor will short, converting your capacitor a small electric heater. As the electrolytes of the capacitor rapidly heat, they vaporize and expand, causing either the safety vent on the top to rupture (boring) or shooting the can enclosing the guts of the capacitor violently across the room with a loud pop, leaving bits of goo and insulating paper all over your circuit (exciting).

I encourage you to try this at least once (with safety glasses and a safe distance) if you have never experienced it. Narrow, tall capacitors, cheaply manufactured by unknown brands in China with no safety vent (looks like embossing on the top) seem to work best.

Anyway, short excursions (milliseconds) to the opposite polarity aren't a problem, because they aren't long enough to destroy the insulating layer. The important thing is that the average voltage (that is, the DC bias) is the correct polarity.


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