Page 131 - Op Amps Design, Applications, and Troubleshooting
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114 AMPLIFIERS
In our case, the minimum load current is computed as
The highest load current is found in a similar manner as
z' L(max) = j'Xmax) x A l
= 50 juA x 57
= 2.85 mA
Range of Acceptable Input Currents. In order for the circuit in Figure 2.30
to operate as a current source whose value is proportional to input current it is
essential mat the output voltage (v 0) be less than the saturation voltage in either
polarity. This, then, is the factor that restricts the range of acceptable input cur-
rents. The output voltage can be expressed as a sum of two voltages by using
Kirchhoff's Voltage Law.
Substituting current and resistance values (i.e., V = IR) produces
To calculate the amount of input current needed to drive the output to satu-
ration, we transpose this equation to find i/ and substitute the value of V SAT for v 0.
For our example, let us assume that the saturation voltage is determined from the
manufacturer's data sheet to be 13 volts. The maximum input current, then, is
computed as
More specifically, for our present circuit we have
