Download 15.4.4 GENERALIZATION ON INPUT RESISTANCE * It is obviously

15.4.4 G E N E R A L I Z A T I O N O N I N P U T R E S I S T A N C E *
It is obviously of some importance to the circuit designer to know whether
feedback is going to increase or decrease the effective input resistance of a circuit.
We can generalize from the two circuits we have examined to state that the effect
of feedback on input resistance depends on the circuit topology. If the source
current and the current through the feedback resistor and the current through
the Op Amp input resistor ri all sum at a common node as in Figure 15.12,
then the effective input resistance is very low, as shown in Equations 15.36 and
15.38. (Remember, here we are referring to Ri , the resistance of the Op Amp
circuit to the right of Rs .) Equation 15.36 is in fact a general result: The input
conductance for any feedback circuit with this input topology (neglecting ri ) is
the conductance without feedback, here 1/(Rf + rt ), multiplied by 1 + A.
If, on the other hand, the source and the Op Amp input resistor are in series,
forming a loop with the feedback resistor, as in Figure 15.14, the effective input
resistance of the circuit will be very high. In a word, if at the Op Amp input we
sum currents at a node, the circuit input resistance is low, if we sum voltages in
a loop, the input resistance is high.
855a