ELEC 244 Lab 2

A major characteristic of Op-Amps is their Slew Rate. The Slew Rate of an Op-Amp is the maximum rate of change of the output voltage with respect to time. I.e.

Slew Rate = max(

dV_out / dt

)

Why Slew Rate is Important

Slew Rate is an important parameter in circuit design. If your circuit operates at high frequencies, you need to use an Op-Amp whose Slew Rate is rated above the frequency and amplitude you plan to use. For sinusoidal signals, your signal must satisfy the following condition to avoid Sine Wave Distortion:

Slew Rate ≥ 2πfV_p

Where f is the maximum frequency you plan to work with and V_p is the Peak Voltage.

Why Op-Amps have a Slew Rate

Slew Rate can be understood by looking at the internal circuitry of the Op-Amp. If you look at the Functional Block Diagram of the LM741, you will notice that there is a 30pF capacitor (C1) within the circuit. This capacitor is known as the Compensation Capacitor.

TI LM741 Datasheet: https://www.ti.com/lit/ds/symlink/lm741.pdf

This capacitor affects the area around it, and is one of the main causes of Slewing. If you have not buried the trauma of ELEC 241 or PHYS 102, you’ll remember that the current of a capacitor is given by:

I = C

dV / dt

If we rearrange the equation, we get the Slew Rate equation we defined above in terms of Output Current and Capacitance:

dV / dt

=

I / C

This equation can be rearranged to:

dt =

C / I

dV

And when integrated, the result becomes:

Δt =

CΔV / I

Since C and ΔV are fixed, and I is the maximum current, there is a minimum time it takes for the Capacitor Voltage to change by ΔV. This is one of the main reasons as to why the change in the Output Voltage of the Op-Amp is limited as it is restricted by the time its internal circuitry can change voltage.
ELEC244_LAB2_SOLUTION_S26.pdf