Slew Rate

Slew Rate:

Another important frequency related parameter of an op-amp is the slew rate. (Slew rate is the maximum rate of change of output voltage with respect to time. Specified in V/μs).

Reason for Slew rate:

There is usually a capacitor within 0, outside an op-amp oscillation. It is this capacitor which prevents the o/p voltage from fast changing input. The rate at which the volt across the capacitor increases is given by

dV_c/dt = I/C --------(1)

I -> Maximum amount furnished by the op-amp to capacitor C. Op-amp should have the either a higher current or small compensating capacitors.

For 741 IC, the maximum internal capacitor charging current is limited to about 15μA. So the slew rate of 741 IC is

SR = dV_c/dt |_max = Imax/C .

 For a sine wave input, the effect of slew rate can be calculated as consider volt follower -> The input is large amp, high frequency sine wave .

If V_s = V_mSinwt then output V₀ = V_msinwt . The rate of change of output is given

dV₀/dt = V_m w coswt.

The max rate of change of output across when coswt =1

(i.e) SR = dV₀/dt |max = wV_m.

SR = 2∏fV_m

V/s = 2∏fV_m v/ms.

Thus the maximum frequency f_max at which we can obtain an undistorted output volt of peak value V_m is given by

f_max (Hz) = Slew rate/6.28 * V_m .

called the full power response. It is maximum frequency of a large amplitude sine wave with which op-amp can have without distortion.