# Finding the bandwidth overall amplifier system

Discussion in 'Homework Help' started by Digit0001, May 31, 2012.

1. ### Digit0001 Thread Starter Member

Mar 28, 2010
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0
Hi

Can someone explain to me why the bandwidth of overall system is 20kHZ for the following question on the attachment:
View attachment tut_5_qns3.pdf

Hope someone can answer my question

P.S

2. ### jegues Well-Known Member

Sep 13, 2010
735
43
The bandwidth is found as follows,

$f_{3dB} = \frac{f_{unity}}{A_{f}}$

Where, $A_{f} = \frac{A}{1 + A\beta}$

Typically the loop gain $(A\beta)$ is large (i.e. much larger than 1) and thus $A_{f} \rightarrow \frac{1}{\beta}$

Clearly the if the bandwidth of the 2nd stage is 20kHz, the overall bandwidth is limited to 20kHz.

Who cares if you can pass frequencies of 20kHz+ in the first stage if they are cut off in the second stage, correct?

3. ### Audioguru AAC Fanatic!

Dec 20, 2007
9,418
903
Look at the frequency response curve of an old 741 opamp that has a unity-gain bandwidth of about 1MHz. With a voltage gain of 50 its cutoff frequency is about 20kHz.
Its very poor slew rate cuts maximum output above only 9kHz.

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Mar 28, 2010
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