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why emiter feedback why can't collector feedback

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electronics_kumar

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collector feedback configuration

question(1)
for stability we are giving emiiter resistance as feedback from emitter to base..
what we will if feedback from collector to base was given by a resistance
question(2)
can we achieve more if both collector as well as emitter feeback were given
 

palosian

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(1 )feed back is given from emitter to the base in in the comman emitter configuration.
2. by this u will disturb the bising condition of transistor
 

evehits

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Actually this is impossible with CE configuration because the input signal path is between base and emitter so any resistor at the collector will have no influence on the feedback subcircuit (at least from this point of view !) but considering CC configuration yeah the collector resistance will be in the feedback loop and it will enhance the stability but urgent attention and good design methodalogy should be used for this kind of config.
 

electronics_kumar

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evehits said:
Actually this is impossible with CE configuration because the input signal path is between base and emitter so any resistor at the collector will have no influence on the feedback subcircuit (at least from this point of view !) but considering CC configuration yeah the collector resistance will be in the feedback loop and it will enhance the stability but urgent attention and good design methodalogy should be used for this kind of config.
how come "evehits" u are saying that input signal path is from base to emitter for CE config....whose current gain depends upon the ratio between collector and base current
 

brahma

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for 1.
generally collector current increases with temperature or change in β value, by using emitter biasing current through this resistor increases and voltage drop across the resistor increases reducing the voltage difference between base and emitter (i.e Vbe). Due to this base current decreases which in turn decrease collector current, Thus stability increases..
 

Eric Best

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electronics_kumar, don't trust in everything you read above:
A feedback from collector in SE configuration IS POSSIBLE of course, and also used, see the attached figure "se.png".

While the feedback inserted by an emitter resistor is a negative current feedback, the feedback from collector is a negative voltage feedback. Study general feedback features to find out its influence on circuit properties (input impedance, gain, output impedance, distortion, etc.).

Best Regards
Eric
 

Kral

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electronics_kumar,
As Eric mentioned, both emitter and collector feedback can be used. Both will reduce the voltage gain, and both will enhance bias stability and gain stability. There is one big difference, however:
~
Emitter feedback increases the input impedance of the amplifier.
Collector feedback decreases the input impedance of the amplifier.
Regards,
Kral
 

unkarc

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Hi electronics_kumar,

Let me show you some links as an addition to Eric and Kral very good contribution.

See http://www.allaboutcircuits.com/vol_3/chpt_4/10.html in general theory

Some info for the audio frequency range:

http://www.dself.dsl.pipex.com/ampins/discrete/singleq.htm

but in case of using both the collector and emitter feedback you can benefit in controlling input and output impedance, wide bandwidth etc, see

http://www.electronics-tutorials.com/amplifiers/broad-band-amplifiers.htm

It is also worth seeing interesting combinations for shunt/series feedback at the bottom of this page: http://www.du.edu/~etuttle/electron/elect8.htm

Here is finally another interesting link to show what can be done by applying correctly negative feedback (this is again an audio example):

http://www.mitedu.freeserve.co.uk/Design/lazarnfb.htm

Regards and Merry Xmas

unkarc
 

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