I am just beginning to study analog integrated design.

Say I've designed a two stage operational amplifier with compensation. The slew rate would is given by:

$$\text{SR} = I_{\text{sat}}/C$$

For a sinusoidal waveform not to be subject to slew rate limitation, the slew rate must be

$$\text{SR} \geq 2\pi \times f \times V_m$$

  1. Doesn't this mean that at different signal frequencies, slew rate is different?

  2. Why do larger signals place a limit on bandwidth of opamp? If the signal amplitude is high, slew rate is also more. Am I correct?

I would also like to know why slew rate is called the large signal bandwidth.


In an op-amp, the slew rate is defined as the rate at wich the output voltage changes, for a step change at the input, with respect to time.

If you apply a step at the input, you obtain a ramp on the output, and the output voltage is changing at the maximum possible speed for that particular op-amp. So the slew rate represents the maximum slope in the V/T curve that you can obtain on the output.

If you are amplifying a sinusoid, the maximum slope that you encounter is at zero crossing, and its vaule is

$$\text2\pi \times f \times V_m$$

You cannot obtain a sinusoid on the output if that slope is greater than the op-amp slew rate.

The slew rate is not a characteristic of the waveform, but of the amplifier.

Since the slope of the sinusoid depends on the frequency AND the amplitude, the selw rate affects more larger signals than small ones.

Smaller signals are more affected by the gain-bandwidth product.

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The actual slew rate at the output of an amplifier is indeed dependent on the input, provided it is not in slew rate limit.

The larger the signal at the output (and therefore the input), the greater the total slew required in terms of volts / time - you even reference it in your equation above.

The slew rate is the fastest the output can possibly change state, and for a signal that would require a slew rate faster than that available, the output cannot follow and we are now in large signal territory; as this is the fastest the output can slew, this is the maximum large signal bandwidth available.

This paper from TI may be of help.


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