Page 83 - Op Amps Design, Applications, and Troubleshooting

66 AMPLIFIERS

drop because of the voltage divider action described in the previous section. Finally,
if the load resistance is reduced all the way to 0 ohms, the output current will be
limited to the short circuit value. This value can be found in the data sheet
(Appendix 1) and is 20 milliamperes for the 741 device.
As the load resistance varies from infinity (open) to 0 (short), the output cur-
rent from the op amp varies from z' F to 20 milliamperes. The limiting factor is the
amount of reduction that can be tolerated on the output voltage.
On an unprotected op amp, the value of load current plus the value of feed-
back current must be kept below the stated output current rating. If this value is
not supplied in the data sheet, it can be estimated by using the maximum power
dissipation data; recall that power - voltage x current.

Bandwidth. The discussion of bandwidth presented for the inverting amplifier
circuit is also applicable to the noninverting configuration. That is, as long as the
circuit has no reactive components, the frequency response will extend all the way
down to DC on the low-frequency end. We can estimate the high-frequency end of
the frequency response by applying Equation (2.22):

Recall that the open-loop gain of the op amp falls off rapidly as the input fre-
quency is increased above a few hertz. As the open-loop gain value approaches
the computed closed-loop gain value, the actual circuit gain also begins to drop.
Thus, we begin to experience increased errors in our gain calculations as the fre-
quency is increased.
For these equations to be valid, it is important that the op amp output volt-
age swing be small enough to avoid the effects of slew rate limiting. The highest
amplitude that can be amplified at a given frequency without the effects of slew
rate limiting is given as

The slew rate is determined by the particular amplifier,/is the frequency of
interest, and z? 0(max) is the highest peak-to-peak amplitude in the output before
slew rate limiting begins to distort the signal. In the present case, if we try to oper-
ate at the upper cutoff frequency (155 kHz), we have to keep the output voltage
below the value computed: