Predicting Op Amp Slew Rate Limi

　　The following analysis of sine and step voltage responses applies to all single dominant pole op amps such as the LM101A， LM107， LM108A， LM112， LM118 and the LM741. Each of these op amps has an open loop response curve with a shape similar to the one shown in Figure 1. The distinguishing feature of this curve is the single low frequency turnover from a flat response to a uniform b20 dB per decade of frequency （b6 dB/octave） drop in gain， at least until the curve passes through the 0 dB line. Closing the loop to 40 dB （X100） as shown with a dotted line on Figure 1 does not change the shape of the curve， but it does move the turnover to a higher frequency. These open loop and closed loop response curves determine the gain applied to small signal inputs. The logical question then arises as to when a signal can no longer be treated as a small signal and the amplifier response begins to deviate from this curve

　　FIGURE 1. Open and Closed Loop Frequency Response The answer lies in the slew rate limit of the op amp. The slew rate limit is the maximum rate of change of the amplifier’s output voltage and is due to the fact that the compensation capacitor inside the amplifier only has finite currents1 available for charging and discharging. A sinusoidal output signal will cease being a small signal when its maximum rate of change equals the slew rate limit Sr of the amplifier. The maximum rate of change for a sine wave occurs at the zero crossing and may be derived as follows：