Page 39 - Op Amps Design, Applications, and Troubleshooting

22 BASIC CONCEPTS OF THE INTEGRATED OPERATIONAL AMPLIFIER

To further appreciate this characteristic, consider a square wave input to an
op amp. In the case of an ideal op amp, the output also will be a square wave. In
the case of a real op amp, however, the rise and fall times will be limited by the
slew rate of the op amp. In the extreme case, a square wave input can produce a
triangle output if the slew rate is so low that the output is not given adequate time
to fully change states during a given alternation of the input cycle.
The 741 op amp has a slew rate of 0.5 volts per microsecond. Other op amps
have significantly higher rates.
The slew rate (in conjunction with the output amplitude) limits the highest
usable frequency of the op amp. The highest sinewave frequency that can be
amplified without slew rate distortion is given by Equation (1.12).

where z? 0(max) is the maximum peak-to-peak output voltage swing. In the case of
a 741 op amp, for example, the 0.5 volts-per-microsecond slew rate limits the use-
able frequency range for a ±10 volt output swing to

1.4.5 Input Impedance
The input impedance of an op amp is the impedance that is seen by the driving
device. The lower the input impedance of the op amp, the greater is the amount of
current that must be supplied by the signal source. You will recall that we consid-
ered an ideal op amp to have an infinite input impedance, and therefore, drew no
current from the source.
A real op amp does require a certain amount of input current to operate but
the value is generally quite low compared to the other operating currents in the cir-
cuit. You may wish to reexamine Figure 1.2 and notice that the current for the input
terminals is essentially providing base current for the differential amplifier transis-
tors. Since the transistors have a constant current source in the emitter circuit, the
input impedance is very high. A typical op amp will have an input impedance in
excess of 1 megohm with several megohms being reasonable. If this is still not high
enough, then an op amp with a field-effect transistor input may be selected.
Appendix 1 shows the data sheet for a 741 op amp. If you look under the
heading of Input Resistance you will find that these devices have a minimum
input resistance rating of 0.3 megohms and a typical value of 2.0 megohms. Fur-
ther, the input impedance is not constant. It varies with both input frequency and
operating temperature. In many applications, we can ignore the nonideal effects
of input impedance. As we study the applications in this text, we will learn when
and how to consider the effects of less than ideal input impedances.