Finding the bandwidth overall amplifier system

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

Mar 28, 2010
89
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 New Member

Dec 20, 2007
9,411
896
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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