Help with feedback topology.

chelleteng

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Hello,

I need some pointers on how to analyse this circuit as a feedback topology. One of the keys I was taught to analyse the input topology is to look out for the feedback resistor, but I'm not even sure which are the feedback resistors in this circuit. As for the output topology, I was taught to look out for the load resistor and determine from there what is the topology like. However, in this circuit there isn't any load resistor and it says to consider where the load would be connected to. Is it to be connected in parallel to R2?

Aside from that, I'm not too certain about how to analyse this circuit as the pnp transistor caused quite a confusion for me. As well as that, the presence of R1 // R6 (considering C2 would be short-circuited at mid-band frequency), are they to be considered as the emitter resistances of both Q1 and Q2? i.e: for emitter resistance of Q1: R1 // R6 whereas for emitter resistance of Q2: R2 + (R1 // R6)..

I hope some of my doubts could be cleared as I really need help to understand and get started off to analyse this feedback topology.
Thank you!

Mark_T

Looks like homework to me, so I'll give a hint. The first transistor can be thought of as like a opamp, the base is inverting input, emitter acts like non-inverting input, collector is output.

Another term for this topology is shunt feedback I believe.

LoopyByteloose

Elliot Sound Products has an information section with some designs of op amps made from individual transistors. Also, it has audio power amps that use feedback to stabilize the quality of output.

http://sound.westhost.com/

And he has quite a bit of material on using op amps for mixing and in multiple series where feedback is used to limit the gain so that the quality of the audio output has a wider range.

Feedback in principle is quite simple. Negative feedback is used to limit gain while increasing bandwidth because extreme gain causes the quality of the output to fall apart. So it is determined by a resistor from the output to the input.

Positive feedback is has uses, but not as many.

http://www.allaboutcircuits.com/vol_3/chpt_8/12.html

Audio pre-amps can get fancy and add capacitors in the feedback network to 'roll off' bass gain. And audio pre-amps for phonograph cartridges can have very fancy feedback networks to compensate for the actual performance of the various kinds of RIAA phono cartriges.

http://en.wikipedia.org/wiki/RIAA_equalization

If you can figure out how op amps exploit feedback, you have a good basis for just about anything. Higher frequency responses may require more engineering, but the general principles of feedback theory are well developed in audio analog.

Some months ago, Nuts and Volts had a wonderful article on building an 'automatic gain control'. Elliot Sound Products doesn't cover this rather advanced form of feedback that uses diodes. I also have not seen anything on the internet about it, but it is quite creative and copies work that was originally patented. The patents have expired and the application in Nuts and Volts for an automatic stereo volume control would be quite useful with listening to YOUtube and other stuff on computer. I never know what the volume will be when I click on a video. That is quite annoying.

If the above approach is too advanced, try studying the NEETS module on Amplification. These are US Navy documents that are provided free without copyright due to the U.S. Constitution deciding that government publications should not be controlled by copyright.

http://www.hep.uiuc.edu/home/sibert/NEETS/index.htm

http://www.hep.uiuc.edu/home/sibert/NEETS/Mod08.pdf