# Capacitors

Discussion in 'General Electronics Chat' started by Mazaag, Dec 2, 2004.

1. ### Mazaag Thread Starter Senior Member

Oct 23, 2004
255
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Hey guys ,

I want to know why do we need coupling capacitors?
I understand that its basic function is to transmit an ac voltage from one point to another, but... doesn't that occur anyway ? i don't quite get what that means, so if someone could just briefly explain what it does, and why do we need it .

Thanks

Jan 19, 2004
220
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aand block DC components present in the wave

3. ### Mazaag Thread Starter Senior Member

Oct 23, 2004
255
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Yeah i know it blocks DC components, but why do we need it ? i need some sort of explanation onto why we need it , and what happens if its not there .

4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
Hi,

Amplifiers are just as happy to work with DC as AC signal components. But if you need a large gain, and the DC is not part of the signal of interest, then a coupling capacitor eliminates it.

Take a chain of amplifiers with an overall gain of 10,000. If the first amp has an offset voltage of 5 millivolts, the final output would have pumped that 5 millivolts up to 50 volts. Without using chopper stabilization, each amp is going to have some DC offset component which will be amplified along with the rest of the signal. Using DC blocking caps eliminates this problem.

5. ### mozikluv AAC Fanatic!

Jan 22, 2004
1,437
1
hi

it also helps in preventing disturbance of the dc bias of a transistor

6. ### nanobyte Senior Member

May 26, 2004
118
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How exactly do the coupling capactiors block the DC components?

7. ### kimoni New Member

Dec 3, 2004
6
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This is just a guess, but could it be to do with the fact that i=C(dv/dt)
in DC the voltage is constant, hence it's differential will be 0,
i=C(0)=0
Conversely in an AC signal the voltage is not constant, but oscillating which means it will have differential values other than 0

but that's just my thoughts

8. ### Martin P Dembrowski Member

Dec 3, 2004
10
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Okay, I'm going to introduce myself by putting my two cents in on this one.

The calculus is fine if you understand it but what I think sometimes helps more is a visual. Ever take a cap apart? Do you know what the schematic representation of a capacitor is? Of course you do, two lines (wires) connecting to two other parallel lines (that don't touch), just like inside the device.

Isn't that kind of like an open switch (a switch in the off position)? A capacitor is just like an open switch to a direct current most of the time. I say most of the time because for a split second current will flow to charge the plates but then the current stops when the charges on the plates are significant enough to repel any further charging. It's almost like filling a bottle but not quite, you can't put any more in when it's full. This is how a direct current "sees" a capacitor, like an open circuit (within reason).

AC (alternating current), on the other hand, keeps filling and then emptying the plates of their positive and negative charges, and so because of the continual "change over time" (dV/dt) you will always have current flow although reversing direction every 1/2 the period of the waveform (that means a 60Hz, or house current, wave would charge and recharge the plates every 120th of a second.) AC flows but DC is "turned off" by the open circuit nature from DC's perspective.

This is useful to keep the bias (or working) voltages (for tubes) and currents (for transistors) at or near their "nominal" working values, else they will be destroyed by values that their structures aren't capable of tolerating. In tubes the coatings on the plates and cathodes can be permanently stripped from the metal and arcing can occur. In semiconductors "holes" will be punched through the semiconducting material, destroying it's properties. High enough voltages or currents could just plain melt or cause the devices to ignite.

Each device has it's specified range that it will perform optimally in, called the bias, and that's why we need DC blocking capacitors separating stages of amplifiers and other basic building blocks of electronic machines. Remember: "We gotta keep 'em separated" or they'll kill each other.

Of course, there are situations that stages are directly linked, but we won't get into that here.

Hi, guys, I don't know much but I try...