# Finding the bandwidth overall amplifier system

#### Digit0001

Joined Mar 28, 2010
100
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

#### jegues

Joined Sep 13, 2010
733
Hi

Can someone explain to me why the bandwidth of overall system is 20kHZ for the following question on the attachment:
View attachment 43280

Hope someone can answer my question

P.S
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?

#### Audioguru

Joined Dec 20, 2007
11,248
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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#### Digit0001

Joined Mar 28, 2010
100