Question 12.15: Effect of Slew Rate Limit on an Amplifier Determine the effe...

Known Quantities: Slew rate limit, S_{0}; amplitude and frequency of sinusoidal input voltage; amplifier closed-loop gain.

Find: Sketch the theoretically correct output and the actual output of the amplifier in the same graph.

Schematics, Diagrams, Circuits, and Given Data: S_{0}=1 \mathrm{~V} / \mu \mathrm{S};v_{S}(t)=\sin \left(2 \pi \times 10^{5} t\right) ; A_{V}=10.

Assumptions: Assume slew-rate-limited (otherwise ideal) op-amp.

Analysis: Given the closed-loop voltage gain of 10 , we compute the theoretical output voltage to be:

v_{\text {out }}(t)=-10 \sin \left(2 \pi \times 10^{5} t\right)

The maximum slope of the output voltage is then computed as follows:

\left|\frac{d v_{\text {out }}(t)}{d t}\right|_{\max }=A \omega=10 \times 2 \pi \times 10^{5}=6.28 \quad \frac{V}{\mu s}

Clearly, the value calculated above far exceeds the slew rate limit. Figure 12.54 depicts the approximate appearance of the waveforms that one would measure in an experiment.

Comments: Note that in this example the slew rate limit has been exceeded severely, and the output waveform is visibly distorted, to the point that it has effectively become a triangular wave. The effect of the slew rate limit is not always necessarily so dramatic and visible; thus one needs to pay attention to the specifications of a given op-amp. The slew rate limit is listed in the device data sheets (see the data sheets in the accompanying CDROM, or in the device templates in the Electronics Workbench { }^{\mathrm{TM}} libraries, for examples). Typical values can range from 13 \mathrm{~V} / \mu \mathrm{s}, for the TLO81, to around 0.5 \mathrm{~V} / \mu s for a low-cost general-purpose amplifier (e.g., \mu \mathrm{A} 741c).