5.3 PRACTICAL PROPERTIES OF OPERATIONAL AMPLIFIERS 239−
+RS IBnRS IBpFigure 5.3.7Illustration of unequal bias currents, which will cause
a differential input voltage even with equal source resistances.Power-Supply Rejection Ratio (PSRR)
An op amp’s ability to disregard changes in power-supply voltage is measured by the power-
supply rejection ratio (PSRR), which is specified by the change in offset voltageVOSfor a 1-V
change in dc power supply and is usually expressed inμV/V. A typical value is 15μV/V.
Maximum Differential Input Voltage
This is the maximum value of differential input voltagevp−vnthat can be applied without
damaging the op amp.
Maximum Common-Mode Input Voltage
This is the maximum voltage that the two inputs can be raised above ground potential before the
op amp becomes nonlinear.
Output Voltage Swing
Ideally this is equal to the difference between the two supply rail voltages, although in practice it
is a few volts less.
Internal Frequency Compensation
Some op amps, such as the 741, have internalRCnetworks which are intentionally designed to
reduce gain at high frequency. The result (shown in Figure 5.2.4) is that the open-loop gain begins
to fall at a few hertz and then has a characteristic falling at 20 dB/decade or 6 dB/octave (i.e.,
gain∝1/f) until eventually the gain becomes unity (0 dB) at about 1 MHz.
External Frequency Compensation
For applications requiring a more extended high-frequency response, there are op amps with no
internal compensation, and external frequency-compensation terminals are provided on these op
amps (for example, the 709) so that the frequency response can be tailored to avoid instability
without the heavy degrading of frequency response due to internal compensation. A typical
frequency-compensation circuit for theμA 709 is illustrated in Figure 5.3.8. Op-amp manufac-
turers supply plots of open- and closed-loop responses for various values of components connected
to the frequency-compensation terminals.
Slew Rate
Slew (or slewing) rate is a measure of how fast the output voltage can change. It is given by the
maximum value ofdvo/dt, which is normally measured in response to a large input voltage step