# Bandwidth vs slew rate

I have some difficulties in understanding the difference between bandwidth and slew rate.

I got that we are talking about large or small signal but what if for example the input signal is a PWM one with a given frequency and a peak voltage which can be considered large signal? Which parameter do I have to look at in this case?

• Both are still applicable. – Andy aka Oct 19 '20 at 13:47
• How so? In this case I would use only the slew rate to know which is maximum frequency of the input signal for which the output signal follows it. How can I use the bandwidth as well? – Manzillo Oct 19 '20 at 14:41
• Frequency of PWM signal does not matter. The edge of a square wave is the important part. An infinitely fast edge has infinite slew rate, and thus needs infinite bandwidth. Frequency of PWM wave is just the repetition rate of infinitely fast edges, unless the slew rate is limited which limits the bandwidth. – Justme Oct 19 '20 at 16:04

## 2 Answers

To determine the maximum achievable closed loop gain at a given frequency use GBW but bear in mind that the output signal will be distorted if the frequency or output amplitude are so large that slewrate limiting comes into play.

For instance...

A 741 has a GBW of around 1MHz and so a maximum closed loop gain of 100 will be achievable at a frequency of 10kHz. Above this frequency the gain rolls off. (Actually down 3dB at 10kHz).

It's possible to determine the maximum allowable pk output amplitude at 10kHz which doesn't cause slewrate limiting using the following formula....

Vpk = Slewrate/(2 * pi * f)

The slewrate of a 741 is 0.5V/us and so the maximum allowable pk amplitude at 10kHz is 8V.

However, the same 741 configured for a closed loop gain of 10 is GBW limited to a bandwidth of 100kHz but at 100kHz the maximum possible pk amplitude which won't induce slewrate limiting is....

Slewrate/frequency = (0.5 x 10^6)/2pi100kHz = 0.8V

I have some difficulties in understanding the difference between bandwidth and slew rate.

Speaking about "bandwidth" you have to discriminate between "small-signal bandwidth" (related to the GBW) and "large-signal bandwidth". Only the latter one is related to the slew rate (as explained by James in his answer).