Which signal (input) should be considered to determine the slew rate
You need to know how fast the amplifier can "move" its output voltage. If you put a steep sided (zero-time step) into the amplifier then it will attempt to follow that. The difference between the ideal (a step) and a sloping rise to the final voltage is the slew rate. So if it took a second to reach the steady state output and we will say that this might be one volt so it's easier to visualize, the slew rate would be one volt per second. If it was a 9 volt output in a second then it would be 9v/S and so on. In practice it will be far faster of course. If you read the notes at the end of a typical data sheet for an operational amplifier (start with the older ones like the 741 or even the 709) they will tell you how the measurement was made. You do need to be a bit careful since in a practical circuit you may tend to limit the performance by the way you build the circuit and you are now measuring the complete amplifier circuit, not the amplifier component itself.
The answers of Ronald and Smat are okay but i want to stress some facts. From the conceptual point of view you measure the parameter as you define it. The slew rate is the maximum rate of rise of the op amp output voltage upon application of a unit step voltage at the input of the the op amp provided that the op amp is connected as a voltage follower. Then as a property given in a data sheet and to compare the different op amps one have to follow the measurement instructions. The most appropriate input is unit step input because its rate of rise is infinity and therefore the rate of rise of the output will be completely defined by the op amp.
Thank you.
Why slew rate is important? Is the rise/full time enough?The rise time can tell only about the maximum frequency. Slew rate helps us to identify what is the maximum input frequency and amplitude applicable to the amplifier such that the output is not distorted.
thankyou - Abdelhalim Zekry, i thought of same i.e, the rate of rise for square wave is infinity and therefore the rate of rise of the output will be completely defined by the op amp. (i am convinced now)
@smat - i think its not difficult to find slew rate for sine wave (we can find dv for small time interval dt, this will give us the slew rate)
Thank you Sandeep, thanl you Mohammed Atef for hinting the importance of the slew rate. The slew rate is a performance parameter indicator for the the speed of the response of the operational amplifier to large signal signal variation with time. The small signal speed of the op amp is limited by the unity gain bandwidth fT.
In case of a sine wave input and assuming a sinusoidal output for linear op amp the rate of change of the output voltage with time will be:
dV/dt = w Vm cos wt where Vm is the amplitude of the output sine wave.
The maximum value of dV/dt will be = w Vm since the maximum value for the cosine is one. If w Vm gets larger than the slew rate the output sine wave will be distorted and it tends to be triangular and the rate of rise of the output voltage will never exceed the slew rate. Under this condition the rate of rise of the output voltage at the zero crossings will be equal to the slew rate.
My best regards.
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