coupling capacitor - Electronic Circuits Projects Diagrams Free

sniper007 · 9th August 2008, 11:12 AM

Hi!

Please explain me why exactly use coupling capacitor on the input/outputs at amplifiers ? only for remove DC component ?

Sniper

Mikebits · 9th August 2008, 11:27 AM

That is actually a good question and I find the answer not so easy to give, but I will try.

In circuits where AC signals are involved, they are often amplified, digified (New word), compared or what have you. Let's take the amp as our example. The amplifier is designed to operate at a set levels (Such as DC voltage) These set levels establish the operating region of the amp (Bias). So let us say we want to amplify a tiny signal from a thingymajig circuit. The amplifier is set to operate with signals from 0 to 1 volt.

Now consider the thingymajig outputs at 2vdc with the AC signal catching a ride on top. What we have is a little AC signal that starts at 2vdc yet our amp only works to 1vdc. This is a problem. Solution, put a cap between thingymajig and amp. This removes the 2vdc from the composite AC/DC signal and allows the small signal to enter the amp at the right level.

I hope that made sense

ericgibbs · 9th August 2008, 12:19 PM

Quote:

Originally Posted by sniper007

Hi!

Please explain me why exactly use coupling capacitor on the input/outputs at amplifiers ? only for remove DC component ?

Sniper

hi,
Basically they are used to interconnect 'ac signals' to voltage points at different 'dc' potentials.

They block the 'dc' and pass the 'ac'.

sniper007 · 9th August 2008, 12:21 PM

Quote:

Originally Posted by Mikebits

Now consider the thingymajig outputs at 2vdc with the AC signal catching a ride on top. What we have is a little AC signal that starts at 2vdc yet our amp only works to 1vdc. This is a problem. Solution, put a cap between thingymajig and amp. This removes the 2vdc from the composite AC/DC signal and allows the small signal to enter the amp at the right level.

Is this offset voltage what you are talking about?

Quote:

Originally Posted by Mikebits

I hope that made sense

absolutely, tnx for replay

Mikebits · 9th August 2008, 12:22 PM

Glad to be of service

sniper007 · 9th August 2008, 12:26 PM

Quote:

Originally Posted by ericgibbs

hi,
Basically they are used to connect 'ac signals' to voltage points at different 'dc' potentials.

So base is 'dc' potential and input signal is 'ac'. To connect it together use coupling cap. ok but why then output cap ?

ericgibbs · 9th August 2008, 12:31 PM

Quote:

Originally Posted by sniper007

So base is 'dc' potential and input signal is 'ac'. To connect it together use coupling cap. ok but why then output cap ?

The same reason, If the output of the amp was say at +12Vdc, if you connected it to a 3Ω speaker, 4amps would flow in the speaker coil,, not a good idea.

The output coupling cap would block the +12Vdc and only allow the 'ac music' signal thru to the speaker.

silvarblade · 9th August 2008, 02:46 PM

this is my first post in which i am not the questioneer!

the main purpose of coupling capacitor at the o/p is to block the dc from going in to the device that you are driving.(it produces noise to say the least and it can destroy the device by increasing the amount of power being delivered) the capacitor at the i/p in paralell with the biasing resistors is used to build a filter for you ac signal normally its high pass given by the eq: f=1/(2*pi*r*c).

Correct me if i am wrong.

audioguru · 10th August 2008, 03:54 AM

Yes.
A coupling capacitor and its load resistance is a highpass filter. It blocks DC and very low frequencies and allows higher AC frequencies to pass through the capacitor.

rag's · 11th August 2008, 01:48 PM

Hi
so hw does this coupling capacitor differ from an ordinary capacitor??Does'nt an ordinary cap too block an ac n send only the dc??

ericgibbs · 11th August 2008, 01:54 PM

Quote:

Originally Posted by rag's

Hi
so hw does this coupling capacitor differ from an ordinary capacitor??Does'nt an ordinary cap too block an ac n send only the dc??

hi,
Yes, all caps block dc.

The OP asked why 'coupling caps' are used.

Bob Scott · 11th August 2008, 11:15 PM

Quote:

Originally Posted by rag's

Hi
so hw does this coupling capacitor differ from an ordinary capacitor??Does'nt an ordinary cap too block an ac n send only the dc??

A coupling capacitor is an ordinary capacitor. It's just that for coupling purposes (and also power filter purposes), the actual capacitance value is not critical. ie: If the value of the capacitor is 50% larger than specified, it should not be a problem.

BeeBop · 12th August 2008, 03:05 PM

Quote:

Originally Posted by rag's

Does'nt an ordinary cap too block an ac n send only the dc??

The other way around: it blocks DC and allows AC to pass.

I'm surprised neither of you two sorted him out here.

b_reagle · 12th August 2008, 03:53 PM

A cap to common will block AC; A cap in series will block DC.

dknguyen · 12th August 2008, 05:27 PM

Yeah, a capacitor blocks DC but passes AC. A capacitor is two plates. If you apply a voltage on one, a charge is produced and the opposite charge is produced on the opposite plate until they cancel each other out. In DC, they cancel each other out and stay that way. In AC, the voltage keeps changing so the charge on the opposite plate also keeps changing thus passing AC, while blocking DC.

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9th August 2008, 11:12 AM	(permalink)
sniper007	coupling capacitor Hi! Please explain me why exactly use coupling capacitor on the input/outputs at amplifiers ? only for remove DC component ? Sniper __________________ There are two major products that came out of Berkeley: LSD and BSD. We don't believe this to be a coincidence. —Jeremy S. Anderson

9th August 2008, 11:27 AM	(permalink)
Mikebits	That is actually a good question and I find the answer not so easy to give, but I will try. In circuits where AC signals are involved, they are often amplified, digified (New word), compared or what have you. Let's take the amp as our example. The amplifier is designed to operate at a set levels (Such as DC voltage) These set levels establish the operating region of the amp (Bias). So let us say we want to amplify a tiny signal from a thingymajig circuit. The amplifier is set to operate with signals from 0 to 1 volt. Now consider the thingymajig outputs at 2vdc with the AC signal catching a ride on top. What we have is a little AC signal that starts at 2vdc yet our amp only works to 1vdc. This is a problem. Solution, put a cap between thingymajig and amp. This removes the 2vdc from the composite AC/DC signal and allows the small signal to enter the amp at the right level. I hope that made sense __________________ Forget injuries, never forget kindnesses. Last edited by Mikebits; 9th August 2008 at 11:32 AM.

9th August 2008, 12:22 PM	(permalink)
Mikebits	Glad to be of service __________________ Forget injuries, never forget kindnesses. Last edited by Mikebits; 9th August 2008 at 12:23 PM.

9th August 2008, 02:46 PM	(permalink)
silvarblade	this is my first post in which i am not the questioneer! the main purpose of coupling capacitor at the o/p is to block the dc from going in to the device that you are driving.(it produces noise to say the least and it can destroy the device by increasing the amount of power being delivered) the capacitor at the i/p in paralell with the biasing resistors is used to build a filter for you ac signal normally its high pass given by the eq: f=1/(2pir*c). Correct me if i am wrong. __________________ i would explain it to you but it would make your brain explode!

10th August 2008, 03:54 AM	(permalink)
audioguru	Yes. A coupling capacitor and its load resistance is a highpass filter. It blocks DC and very low frequencies and allows higher AC frequencies to pass through the capacitor. __________________ Uncle $crooge

11th August 2008, 01:48 PM	(permalink)
rag's	Hi so hw does this coupling capacitor differ from an ordinary capacitor??Does'nt an ordinary cap too block an ac n send only the dc??

12th August 2008, 03:53 PM	(permalink)
b_reagle	A cap to common will block AC; A cap in series will block DC. __________________ The thought is in the question; The information is in the anwser. Last edited by b_reagle; 12th August 2008 at 04:01 PM.

12th August 2008, 05:27 PM	(permalink)
dknguyen	Yeah, a capacitor blocks DC but passes AC. A capacitor is two plates. If you apply a voltage on one, a charge is produced and the opposite charge is produced on the opposite plate until they cancel each other out. In DC, they cancel each other out and stay that way. In AC, the voltage keeps changing so the charge on the opposite plate also keeps changing thus passing AC, while blocking DC. __________________ NO, that picture isn't me so don't bother asking if we can be friends.