I've heard a lot of things about negative feedback but I don't understand how you can reduce noise more than you can reduce the signal. If you inject part of the output into the input again, you are reducing the noise, but at the same time, the signal. So when you crank the signal back up, isn't the noise there too? Where exactly does the noise come from?
For this amplifier I tried to build, I had an NPN/PNP pair with emitters grounded (first stage). The first stage NPN's collector was tied to the secondary stage's PNP's base whose emitter was on the + rail. The opposite was true for the negative rail. In other words, both stages were common emitter (inverting). The first stage was common to ground, the second stage, one the NPN was common to the - rail and the PNP was common to the + rail. Both stages combined formed a non-inverting amplifier, so how could I use NFB in a situation like this? How could I get global NFB without using op-amps?
NFB
You could put a NFB loop from the emitter of the last stage to the base of the input stage, but it is not strictly global.
To use global NFB you need another inverting stage, which is easy: base to collector of last stage, and give it equal emitter and collector resistors, so its gain is 1, but the collector has the inverted signal. That can be fed back to the input.
Jan Didden
You could put a NFB loop from the emitter of the last stage to the base of the input stage, but it is not strictly global.
To use global NFB you need another inverting stage, which is easy: base to collector of last stage, and give it equal emitter and collector resistors, so its gain is 1, but the collector has the inverted signal. That can be fed back to the input.
Jan Didden
Solid Snake said:
You can't reduce any noise, therefore it's important to make the first stage low noise and it must amplify so much so the amplified noise is less than the noise created in the secnd stage.
Feedback can't handle noise, sorry.
Hasn't Texas got a large document, very wellwritten about feedback?
Can't find it right now but here are lot's of info.
http://dspvillage.ti.com/docs/catal...eId=&toolTypeFlagId=&viewType=&applicationId=
Feedback isn't the medicine. Feedback can't make the noise go away but you can increase noise by poor design. You can also get more noise with bad parts. I got evetially very much unregular noise in my old LF357 based preamp. I had bad sockets but the chips had turned bad. Maybe the heat had something to do with this?Sch3mat1c said:
If your amp allways have been noisy it's one thing but has the amp become noisier by the time it's probely someting which has been worn out. Bad electrolythic caps, bad chips? Old chips can absorb pollutions and this can create bad behavior, irritating noise. Plastic package isn't totally tight against enviromental things.
I have got a very good file about feedback, 160 pages but you don't have to read it all.
SLOD006A.pdf at www.ti.com
Unfortunately I can't find it. Anyone else who can?
The file is 800k and is a very good peice and I think selected chapters can be very educational.
If noone can help you, drop me a note if you want the file emialed.
SLOD006A.pdf at www.ti.com
Unfortunately I can't find it. Anyone else who can?
The file is 800k and is a very good peice and I think selected chapters can be very educational.
If noone can help you, drop me a note if you want the file emialed.
Hi,
The dominant source of noise in an amplifier is usually the input stage as already mentioned. Overall NFB reduces the overall gain (closed loop gain) of an amplifier. Thus you can say because of the reduced overall gain of the amplifier due to the NFB the noise generated by the input stage is less amplified. The noise itself can’t be reduced. That’s all.
The dominant source of noise in an amplifier is usually the input stage as already mentioned. Overall NFB reduces the overall gain (closed loop gain) of an amplifier. Thus you can say because of the reduced overall gain of the amplifier due to the NFB the noise generated by the input stage is less amplified. The noise itself can’t be reduced. That’s all.
This is an explaination I found on a site:
I think what he's trying to say is that when you feedback part of the signal, the input signal fights the inverted signal, but there is nothing to counteract the noise in the NFB, so the inverted noise is much greater than the normal signal, greatly reducing it???
I think what he's trying to say is that when you feedback part of the signal, the input signal fights the inverted signal, but there is nothing to counteract the noise in the NFB, so the inverted noise is much greater than the normal signal, greatly reducing it???
Pjotr said:Hi,
The dominant source of noise in an amplifier is usually the input stage as already mentioned. Overall NFB reduces the overall gain (closed loop gain) of an amplifier. Thus you can say because of the reduced overall gain of the amplifier due to the NFB the noise generated by the input stage is less amplified. The noise itself can’t be reduced. That’s all.
I don't know where you guys all get the idea that noise can't be reduced, but that's wrong. Noise isn't very special, just something that is generated in amp stages just like harmonics due to THD. Same with DC offset, temperature drift, etc. NFB reduces it, IF it is generated inside the FB loop. NFB cannot reduce anything outside the loop, where it has no control, be it noise, THD, etc.
Jan Didden
Hi Jan,
Maybe I was not that clear in my post. I intended to say the noise of the input stage itself can’t be reduced.
In general when you have a chain of amplifiers the noise of the amplifier stage that is most amplified in the chain dominates. In most amplifier chains this is usually the noise of the input stage. On the OUTPUT the noise is reduced by the amount of overall feedback when applied to such a chain. And an audio amp is usually a chain of amplifier stages. The noise of the source itself is not reduced by NFB, only the effect of it at the output. Same holds for distortion in the chain.
It is just basic textbook electronics. Indeed, I think we are on the same line.
Cheers
Maybe I was not that clear in my post. I intended to say the noise of the input stage itself can’t be reduced.
In general when you have a chain of amplifiers the noise of the amplifier stage that is most amplified in the chain dominates. In most amplifier chains this is usually the noise of the input stage. On the OUTPUT the noise is reduced by the amount of overall feedback when applied to such a chain. And an audio amp is usually a chain of amplifier stages. The noise of the source itself is not reduced by NFB, only the effect of it at the output. Same holds for distortion in the chain.
It is just basic textbook electronics. Indeed, I think we are on the same line.
Cheers
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