Someone please HELP!

  • Thread starter dave12
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Ok I have this question that Im finding hard to answer. I understand that you guys won't answer homework questions but all I ask is a little point in the right direction. Heres the question anyway.

An amplifier has an open-loop, gain of 10,000 and a bandwith of 2MHz. Its input and output impedances without feedback are 50Kohm and 500ohm respectively. If the gain is subsequently reduced to 50, by application of negative feedback determine
a) The new bandwidth
b)The feedback fraction Beta
c)input impedance
d) Output impedance.

I think I can work out the Feedback fraction I make it to be, 0.0199.

But Im not sure on the rest, my notes don't tell me any way of working out the new bandwith or impedances.

Please can someone help me? Any formulas or hints apprecited.

Thanks in advanced Everything helps!
 

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  • #2
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Originally posted by dave12
Ok I have this question that Im finding hard to answer. I understand that you guys won't answer homework questions but all I ask is a little point in the right direction. Heres the question anyway.

An amplifier has an open-loop, gain of 10,000 and a bandwith of 2MHz. Its input and output impedances without feedback are 50Kohm and 500ohm respectively. If the gain is subsequently reduced to 50, by application of negative feedback determine
a) The new bandwidth
b)The feedback fraction Beta
c)input impedance
d) Output impedance.

I think I can work out the Feedback fraction I make it to be, 0.0199.

But Im not sure on the rest, my notes don't tell me any way of working out the new bandwith or impedances.

Please can someone help me? Any formulas or hints apprecited.

Thanks in advanced Everything helps!

It would be ridiculous to give you this problem without covering feedback amplifiers first. Any decent circuit design text book will have a chapter on feedback (See Gray & Meyer Ch 8 for e.g). You can't determine the input & output impedances with the information you've given because the result depends on whether it is series-series, shunt-shunt, series-shunt or shunt-series feedback. That is you have to know how the 'input' signal is sensed & how the output signal is applied by the feedback circuit.

For a start, gain with feedback is a/(1+af) = 50 and a = 10000. Solve for f. For the rest, you need to read a chapter on feedback & then ask questions. I don't think anyone can explain it all to you here.
 

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