# coupling capacitor

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#### sniper007

##### New Member
Hi!

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

Sniper

#### Mikebits

##### Well-Known Member
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.

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#### ericgibbs

##### Well-Known Member
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'.

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#### sniper007

##### New Member
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?

absolutely, tnx for replay

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#### sniper007

##### New Member
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

##### Well-Known Member
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.

##### Member
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

##### Well-Known Member
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

##### New Member
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

##### Well-Known Member
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

##### New Member
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

##### Active Member

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

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

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#### dknguyen

##### Well-Known Member
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.

#### DragonForce

##### New Member
Just to confuse things, and sorry for bumping this old thread.

If a voltage does not dip below zero volts, it isn't AC

So, how does one obtain an AC audio signal (centred on zero volts) from a circuit powered on a DC voltage?

How can you have an AC signal riding on a DC voltage? It's not AC if that voltage does not go below zero volts - it's simply a pulsating DC voltage.

The question I have, is that is the signal output (via a capacitor) from a simple transistor amplifier a true AC signal (as in, voltage is centred at zero volts) or is it that the lowest value in the waveform is actually zero volts, the waveform itself being at or above zero volts, never below.

It is due to this question that I found this thread and your forum.

#### audioguru

##### Well-Known Member
If a voltage does not dip below zero volts, it isn't AC
WRONG!
Of course it is AC but it also has a DC voltage. A coupling capacitor can block the DC but pass the AC.
The average DC output voltage can be ANY voltage, not just 0VDC.

So, how does one obtain an AC audio signal (centred on zero volts) from a circuit powered on a DC voltage?
Simply use an output coupling capacitor feeding a resistor connected to zero volts.

How can you have an AC signal riding on a DC voltage? It's not AC if that voltage does not go below zero volts - it's simply a pulsating DC voltage.
Of course AC can also have a DC voltage. It does not need to go positive and negative. It just needs to alternate up and down.

The question I have, is that is the signal output (via a capacitor) from a simple transistor amplifier a true AC signal (as in, voltage is centred at zero volts) or is it that the lowest value in the waveform is actually zero volts, the waveform itself being at or above zero volts, never below.
You are mixed up.
An AC signal does not need to go positive and negative. Its voltage simply needs to swing up and down. Maybe the signal goes from +90V to +100V and back to +90V over and over. It is AC (but it also has a DC voltage).
The AC output of a transistor that does not have an output coupling capacitor could be an AC output that swings from +2V to +12V over and over.
An AC output could also swing from -3V to -15V over and over.

Some AC signals pass through 0V but not all of them.

#### DragonForce

##### New Member
Imagine a water pipe with a diaphragm placed in the middle. Water pressure from (say) the left hand side distorts the diaphragm to the right - water on the right hand side of the diaphragm gets displaced and flows. There is no direct connection, both sides are completely isolated from each other, water flows due to pressure on the diaphragm. When the pressure from the left stays constant, the diaphragm reaches a stable state and no further displacement takes place. Increase the pressure slightly and the diaphragm again distorts and pushes more water out the pipe. The water pipe can now said to be "charged".

Lower the pressure, the diaphragm returns to its normal state, causing a drop of pressure on the right hand side. Water then flows back into the pipe in the opposite direction - a negative flow if you like.

In this way, a capacitor "conducts" an AC signal and blocks a DC signal. pulsating DC is DC, unless the polarity changes, it is DC - whether it is positive DC or negative DC. It is NOT AC, and there is no AC on a signal that does not change polarity - just changes of pressure, which the capacitor reacts to and allows to pass. This is why you WILL get a current flow from a DC source through a capacitor whilst it is charging. Once charged, current flow stops.

This can be proven - wire up a 555 timer to give a square wave DC signal - a capacitor will pass that signal.

Take a capacitor and charge it with a positive DC source - then send a positive pulse down the wire - the capacitor will pass that pulse, even though it is DC. The output of the capacitor is AC, that is true, the polarity alternates because of the changes in pressure causing current to flow back into the capacitor as described above.

#### ericgibbs

##### Well-Known Member
hi DF.

Look at this LTSpice simulation, it clearly shows the C1 current changes direction.

An Alternating Current [AC] is a current which reverses its direction.

E.

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