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Arty 21st September 2012 02:31 PM

amplifier design question
Actualy i did never design any amplifier.
Maybe played with some component values for few chipamps, but that's about it.

I seen opamps used o tame DC offset, and this led me to a question i would like to find an answer to. Do notice, I em honestly clueless if what got into my mind can be done, and even if it can be, is it a good idea or not.

Let's assume a power amplifier, something realy generic.
Is it possible to use an opamp to amplify the same input signal as the one the generic poweramp uses, compare the 2 signals, take the difference and feed it back to the generic poweramp to.. sorthof force it to make an ouput verry similar to the opamp ?
Even if it is possible, can it improve a generic poweramp?

demeterart 21st September 2012 02:36 PM

Hello Arty

what you describe indirectly thru your question is the negative feedback, the underlying control mechanism for any tamed amplifier. the op-amp would have to control the overall loop gain and thus be an integral part of the power amp design. you can not really take two completely unrelated amps and "marry" them like this without access to their internals.

Arty 21st September 2012 02:43 PM

okay, make the generic amp be say.. my beloved tda1557q,
and the opamp TL081 for example.
How would this have to be wired up?
And will it make a difference?
To what degree could the opamp correct the poweramp?

demeterart 21st September 2012 03:02 PM

from what i gather from web pics the tda1557 is a complete amp with no access to internals.
can not do :-(

johnr66 21st September 2012 08:24 PM


Originally Posted by demeterart (
from what i gather from web pics the tda1557 is a complete amp with no access to internals.
can not do :-(

Sounds like the OP wants global negative feedback. Take the output of the power amp and feed it back to the inverting input of the opamp through the resistor divider needed to set the proper gain. This global NF arrangement now dictates the total gain of the amplifier and corrects non linearity. If the power amp IC stage was nonlinear, the opamp would be feeding the corrective signal to its input.

I don't see why this is necessary as the power amp IC has its NF loop. The ones with fixed gain have it internal.

AndrewT 21st September 2012 08:27 PM

Maybe he should read Jung, composite opamps.

demeterart 21st September 2012 08:51 PM

The proposed configuration by johnr66 has many potential pitfalls given the lack of access to the internal input terminals of the power amp and needed input DC bias terminals. Arty has asked for a specific forced marriage and that calls for special conditions.

Arty 21st September 2012 10:13 PM

it does not have to be a tda1557q ic, it was just one that popped up my mind.
say.. lm 1875 can be a target too.

has to admit i have no clue why this idea bugs me for least 30 hours now.

Arty 21st September 2012 10:56 PM

1 Attachment(s)
ah, managed a crude block diagram that more or less describes what i had in mind.

surely this sketch is unable to work. but shows the basic idea behind my question.

AndrewT 22nd September 2012 07:22 AM

Have you read Jung yet?

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