Page 91 - Op Amps Design, Applications, and Troubleshooting
P. 91
74 AMPLIFJERS
If we had used typical values for R OP, we would have gotten an even higher value
for Z IN. In any case, the value is so high that we can consider it as infinite for most
applications.
Input Current Requirement. The input current for the circuit in Figure 2.16
is only the bias current for the (+) input terminal. This is ideally 0 and for most
applications may be neglected. If more precision is desired, then the manufac-
turer's data sheet in Appendix 1 can be referenced. The data sheet indicates that
the input bias current will be no higher than 500 nanoamperes, with a more typ-
ical value listed as 80 nanoamperes. Even though this current is temperature
dependent, the absolute values are so small that they may be neglected in many
applications.
Maximum Output Voltage Swing. The maximum output voltage swing for
the follower circuit is determined in the same manner, Equation (2.10), as that
used with preceding amplifiers. That is,
If a more accurate value is desired, the manufacturer's data sheet can be used to
find a more precise value for the worst-case saturation voltage.
Slew-Rate Limiting Frequency. As with the amplifier configurations dis-
cussed previously, the highest frequency that can be amplified with a full output
voltage swing and no slew-rate limited distortion is computed as in Equation (2.11):
If it is known for certain that the actual output swing will never be required to
reach its limits, then the lower actual output swing can be used to compute the
slew-rate limiting frequency.
Maximum Input Voltage Swing. Since the amplifier has a voltage gain of 1,
the maximum input voltage swing is equal to the maximum output voltage
swing. Thus, in the case of Figure 2.16, we could have an input signal as large as
±13 volts without causing the amplifier to saturate. Again, if you plan to push the
amplifier to its limits, you should refer to the manufacturer's data sheet and select
the worst-case output saturation voltage at the worst-case temperature. The com-
putations, however, remain similar.
