Page 132 - Op Amps Design, Applications, and Troubleshooting

Current Amplifier 115

The lower range is computed in a similar manner by using the other saturation
limit. In most cases (i.e., balanced dual power supply circuits), the values of ±V SAT
will be the same. If this were true for the circuit in Figure 2.30, we could have a
range of input currents that extended from -57.3 microamps to +57.3 microamps—
the polarity, of course, telling us the direction of current flow.

Maximum Load Resistance. Another way to view the preceding calcula-
tions is to consider a known range of input currents and a variable value for R L.
Again, the output voltage must be kept from reaching the saturation limits. We
can transpose Equation (2.46) for z'j(irtaximum) to get the following result:

where z/ is the highest expected input current. In the case of Figure 2.30, we can
determine the maximum value for the load resistance as

Input Resistance. Although the input resistance of the circuit in Figure 2.30 i
ideally 0, there may be applications that require us to know a more accurate valu
for it. The following equation can be used to estimate the input resistance of tb
current amplifier in Figure 2.30:

where R x is the resistance of RI and R 2 in parallel (i.e., R 1JR 2/(R 1 + R 2)) and A v is the
open-loop gain of the op amp at a particular frequency.
In the case of Figure 2.30, let us compute the input resistance for DC condi-
tions. First, the open-loop gain at IX! can be found in the data sheet (Appendix 1)
to be at least 50,000. The value for R x is calculated as