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Capacitors in amplifiers: polarity or not?


autir
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I have been studying the principles of audio microphone preamps
(example: http://www.ee.washington.edu/circuit_archive/circuits/micamp.html
and transistor amplifiers in general.

Often I see the same circuit in two different sites, in one site the capacitors are not polarised and in the other site they are.

-> When do we need our capacitor to be polarised and when not?

-> How do we determine polarity?

I am talking especially for the "from previous stage" and "to next stage" capacitors, but not just for them.

Thank you in advance.

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When do we need our capacitor to be polarised and when not?

Non-polarized film capacitors have lower distortion than polarized electolytics. Film capacitors above about 1uF are very expensive so frequently electrolytics are used. You can buy non-polarized electrolytics which are two polarized ones in series and connected with opposing polarities, like C1 and C3 in the Micamp circuit you posted. A polarized cap should not be used when its polarity in a circuit could be in either direction.

How do we determine polarity?

Power supply bypass caps have an obvious polarity like C3 in the circuit you posted. The author was too careful by using the non-polarized series connection of C1 and C3 because the electret mic draws only about 500uA and the voltage at its connection to the cap will be a high voltage, while the cap's connection to the transistor's base will be only 0.6V.

I am talking especially for the "from previous stage" and "to next stage" capacitors

Coupling capacitors between stages having about the same output and input DC voltages are non-polarized film or electrolytic types in high quality audio circuits I have worked with and designed.
I have seen cheaper circuits use polarized electolytics instead. Frequently when power is applied or turned off, the output of the preceding stage reaches its operating voltage or ground first, putting both polarities across a coupling capacitor. ;D 
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A fine answer indeed!  ;D

What I've understood is that:

-> We do not need polarised capacitors. We use electrolytic capacitors not because of the polarity they posess but because of their high capacitance values. Electrolytics in this case can be installed in either direction, their polarity is unimportant. The proper thing to do, regarding quality, is to use expensive film capacitors of the desired values.

-> The only case we really need a polarised capacitor is when blocking DC or connecting stages whose voltages differ by far.

Questions:

Why do electrolytic capacitors produce higher distortion than film ones? Is it a symptom of the polarisation of the electrolytic capacitor, or does it depend on the different nature of the two species?

Why do two polarised capacitors connected in series produce a non polarised capacitor? Does it matter whether the + or - ends are the ones connected?

To construct a non polarised capacitor by connecting two electrolytics is exactly the same with bying a non polarised electrolytic?

Can't we block DC or voltage differences in general with a non polarised capacitor? Is polarisation really needed?

What do you mean by that?

The author was too careful by using the non-polarized series connection of C1 and C3 because the electret mic draws only about 500uA and the voltage at its connection to the cap will be a high voltage, while the cap's connection to the transistor's base will be only 0.6V.


What do you mean by that?
putting both polarities across a coupling capacitor


Thank you  :D
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What I've understood is that:

-> We do not need polarised capacitors. We use electrolytic capacitors not because of the polarity they posess but because of their high capacitance values. Electrolytics in this case can be installed in either direction, their polarity is unimportant. The proper thing to do, regarding quality, is to use expensive film capacitors of the desired values.

No. Polarity is extremely important with polarized capacitors, that is why it is marked. If they are installed backwards they blowup like a bomb if the charging current is high enough! Carefully try it.
With a low charging current a backwards polarized capacitor leaks a lot of DC current and its capacitance decreases over time.

-> The only case we really need a polarised capacitor is when blocking DC or connecting stages whose voltages differ by far.

You never need a polarized capacitor. Polarized electrolytics are cheaper and have high values.

Why do electrolytic capacitors produce higher distortion than film ones? Is it a symptom of the polarisation of the electrolytic capacitor, or does it depend on the different nature of the two species?

They just do have higher distortion. Maybe it is because their capacitance value changes with DC voltage, resulting in 2nd harmonic distortion. Their value isn't accurate anyway.

Why do two polarised capacitors connected in series produce a non polarised capacitor? Does it matter whether the + or - ends are the ones connected?

Since they are in series then their combined value is half of each one.
One will have the correct polarity and therefore have very low DC leakage current and the "backwards" one won't have any DC voltage across it.
It doesn't matter whether their "-" leads or "+" leads are joined.

To construct a non polarised capacitor by connecting two electrolytics is exactly the same with buying a non polarised electrolytic?

Yes, but yours will have the total value half of each one.

Can't we block DC or voltage differences in general with a non polarised capacitor?

Of course.

Is polarisation really needed?

Never, it is just a cheaper, fairly small capacitor.
Just think how many fairly expensive 1uF non-polarized capacitors it would take to make a fairly cheap 10F polarized "super capacitor". Ten million of the things! ;D

What do you mean by that?

A single polarized capacitor could be used since its polarity in this circuit is known.

What do you mean by that?

One polarity when the power is applied because the output DC voltage of the preceding stage comes up quickly.
The opposite polarity when the power is turned off because the output DC voltage of the preceding stage drops to ground quickly. ;D
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No. Polarity is extremely important with polarized capacitors, that is why it is marked. If they are installed backwards they blowup like a bomb if the charging current is high enough! Carefully try it.
With a low charging current a backwards polarized capacitor leaks a lot of DC current and its capacitance decreases over time.


I know all this. What I asked is ...hmmmm... take for example the circuit of the preamp. Why do input C3 and output C4 have the polarity they do?
When we design the next stage of this circuit, is the input capacitor of this stage supposed to have its "-" facing C4's "-", so as to form a non-polarised cap? Wouldn't the proper thing be to replace C4 with a non-polarised series of two electrolytic caps, so as not to pay attention to the polarity of the input cap at the next stage?

Another question: If we don't need a polarised capacitor to block DC, why do we use elctrolytics? Why not just use an inexpensive 100nF ceramic, for example?
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Why do input C3 and output C4 have the polarity they do?

Input C3 (I changed it to C2) has the opposite polarity of C1 to make a non-polarized pair. Input C3 (C2) isn't needed since the electret mic end is very positive and the base of the transistor is much less positive.
The polarity of C3 for a dynamic mic is shown correctly since the mic is a DC short to ground and the transistor's base voltage is positive.

When we design the next stage of this circuit, is the input capacitor of this stage supposed to have its "-" facing C4's "-", so as to form a non-polarised cap? Wouldn't the proper thing be to replace C4 with a non-polarised series of two electrolytic caps, so as not to pay attention to the polarity of the input cap at the next stage?

The collector of the transistor will be positive a few volts. If it feeds the base of another similar transistor circuit with its emitter grounded then just a single polarized coupling cap is needed as C4 and its polarity is shown correctly.

If we don't need a polarised capacitor to block DC, why do we use electrolytics? Why not just use an inexpensive 100nF ceramic, for example?

You don't want low frequencies? The impedance of the base of a transistor circuit like this one with its emitter grounded is only about 2k ohms. A 0.1uF coupling cap will cutoff frequencies below 800Hz. The 10uF cap shown passes frequencies down to 8 Hz.

post-1706-14279142395522_thumb.gif

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