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Op amp equations | op amp gain,slew rate

This page on op amp equations covers op amp gain equation and slew rate equation. It covers op amp gain for inverting and non inverting op amp type. It mentions CMRR and full power bandwidth equations.

Op amp gain

op amp gain

Figure-1 mentions op amp gain for inverting type and non inverting type operational amplifier(op amp). As mentioned, Op amp gain for inverting and non inverting type in terms of voltage is ratio of output by input. It is expressed as follows.
op amp gain = Vout/Vin
op amp gain (inverting type) = - Rf/Rin
op amp gain (non inverting type) = 1 + Rf/Rin

op amp gain types

Let us understand difference between various op amp gain types such as closed-loop gain, open-loop gain, signal gain, loop gain and noise gain. Signal gain is same as mentioned in above op amp equations. Noise gain for both inverting and non inverting op amp is expressed as follows.
Noise gain = 1 + Rf/Rin
Figure-2 mentions comparison between various op amp gain types.

op amp CMRR

The full form of CMRR is common mode rejection ratio. If the signal is applied with equal amount to both the inputs of op amp such that differential input voltage is un-affected then output should not be affected as well. In general changes in common mode voltage will produce change in the output. The op amp CMRR is the ratio of the common mode gain to differential mode gain.
CMRR = common mode gain/differential mode gain
It is expressed in dB.

op amp Full Power Bandwidth

It is the max. output frequency at which slew limiting happens. It is expressed as follows.
FPBW(Full Power Bandwidth) = Slew rate/(2*pi*Vp)

The op amp FPBW should be approx. 5 to 10 times than max. output frequency This is needed to obtain acceptable distortion performance using op amp circuit.

op amp Slew rate

The rate of change in the output voltage with respect to time for a step change at the input of op amp is referred as slew rate. It is expressed as V/s or V/µs
Typically op amps will have different slew rates during the negative and positive transitions.

op amp slew rate

Let us assume sine wave with amplitude of 2Vp and frequency f
Output voltage V(t) = Vp*sin(2*pi*f*t)

Maximum slew rate for this is expressed as
dv/dt(max.) = 2* pi *f*Vp

The slew rate is generally measured between 10% and 90% of the final value.


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