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# Are amplifier ment only to linearly amplify small AC signals?

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#### alphacat

##### New Member
Regarding "amplify small AC signal":
The amplification of an amplifier depends on its operating point.
Therefore, if we insert different DC signals, which differ from each other by a significant portion, then we might damage the amplification.
So as i see it, we must set a permament operating point and allow only small AC signals as an input, small enough to keep the operating point unchanged, is it correct?

Regarding "linearly amplify small AC signal":
When the transistor (in the amplifier) is in cutoff, then all the current thats drawn from the Vcc source will go to the output, so allegedly thats the way to receive the maximum current amplification (if its a current amplifier), since we wont be able to get larger current than that in the output.
But I guess that (in CE current amplifier for example) we want the BJT to be forward biased to have a linear amplification, in order to have the output signal a duplication of the input signal, only with different amplitude?

Thank you.

Regarding "amplify small AC signal":
The amplification of an amplifier depends on its operating point.
Therefore, if we insert different DC signals, which differ from each other by a significant portion, then we might damage the amplification.
So as i see it, we must set a permament operating point and allow only small AC signals as an input, small enough to keep the operating point unchanged, is it correct?

Sounds like you have the correct thinking even though the way you worded it isn't quite right. You are linearizing the non-linear response of the transistor around the bias point. A small signal is a signal whose maximum amplitudes are still within the limits that this linear approximation is still valid.

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Are just you talking abou the CE amplifier or any amplifier?

Sorry but your question is pretty meaningless without relating it to a specific amplifier.

Thank you.

In order to have the output signal not riding a DC level, are coupling capacitors being connected in series with the output?

I ask it since I read about a current amplifier in a book and it's said there that we would want iOUT (= IOUT + iout) to be equal only iout, but they didnt mention there any coupling capacitor, and in all the simulations i ran, without using a coupling capacitor, I always received a DC level at the output (which the ac signal rides on).

So, is there any other way to eliminate the DC level at the output without coupling capacitors?

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Are just you talking abou the CE amplifier or any amplifier?

Sorry but your question is pretty meaningless without relating it to a specific amplifier.

I was talking generally.
I mentioned the CE amplifier since its the amplifier i know best and i have more experience with BJTs.

If the amplifier is DC coupled and its minimum input and voltage includes 0V there is no need for coupling capacitors.

yeah, well its the same thing to have internal coupling capacitors or to add them by yourself if they arent built-in.
I assume that if the amplifier is DC-coupled at its input, then it is biased by its VCC (through a voltage divider perhaps).

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Normally AC small signal amplifiers have series capacitors at their inputs and output to block DC and pass the AC.

If it's designed as a DC coupled amplifier i.e. the input and output can operate near or at ground, for example an op amp, then you don't need coupling capacitors to pass an AC signal.

alphacat,
This is yet another theoretical discussion in this Chat forum. It might be more at home in the theory forum. Although I suppose anything goes in a Chat forum like this, this was where I and perhaps a few others came to relax without third order differential equations. I Welcome your thoughts.
Mike

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