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Why does capacitor polarity matter or maybe it does not? and when it comes to capacitors in a filter circuit, what is the difference for picking one type to another, namely, electrolytic or ceramic.

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For instance, the classical multiviberator circuit as shown in the diagram above, there are 2 capacitors: C1 and C2. Why are they polarized? I am sure that ceramic caps works here fine because I have tested it.

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Another commonly seen mic amplifier as shown above, C1 sometimes is electrolytic and sometimes ceramic, generally having the value from the range of 0.1 uf to 10 uf. Let's say both are 0.1 uf, is there a difference for using electrolytic than ceramic capacitor for C1? and I am very confused, why does C1 have a range from 0.1 uf to 10 uf? which value should I use for C1?

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  • \$\begingroup\$ Being polarised is the nature of some capacitors. You can always replace a polarised capacitor with a non-polarised one it it is otherwise equivalent. Depending on the circuit you may not be able to do this the other way round. Polarised capacitors are often used because they are smaller or cheaper than the polarised version for the same capacitance and voltage rating. \$\endgroup\$ – Warren Hill Aug 19 '17 at 12:37
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Not all capacitors are polarized. Electrolytics and tantalums have polarity but ceramic and film capacitors for example don't. This is because of the chemistry of the electrolyte materials used - something I don't understand sufficiently to delve further into. Film and ceramic capacitors don't use electrolytes so they don't have polarity requirements.

So why should we use polarized types when we can use non-polarized? Because both tantalums and electrolytic capacitors have more capacity for their size and can have much larger capacitance values without taking up too much PCB real estate.

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Of course your circuit works on ceramic caps .When the Astable was designed with transistors there were no large value small size ceramic caps cheaply available .Even in 1988 I would use electrolytics above one microfarad .The Astable is older than the BJT .Valove versions did not call for huge caps .Nowdays 10 microfarad 50VDC ceramic is normal and 100 microfarad ceramic can be had at 6.3VDC .When you use normal Si BJTs in the astable the supply volts should not go above the VEB rating of the BJT which is about 6 to 8 VDC .So the Astable is fine on ceramics .Now there are some snags when when using them to substitute electros as rail bypass because the much lower ESR means higher Q which can mean ringing issues if precautions are not taken .Also there can be snags when you want a linear ramp or low distortion because the small size big C ceramic caps change their capacitance with voltage .I use metal film caps for the critical analog stuff .The film caps are bigger ,cost more and are hard to find in SMD.

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