Input Impedance Booster Circuit Diagram


The input impedance of
a.c.-coupled op amp circuits depends almost entirely on the resistance
that sets the d.c. operating point. If CMOS op
amps are used, the input is high, in current op amps up to 10 MΩ. If a
higher value is needed, a bootstrap may be used, which enables the input
impedance to be boosted artificially to a very high value. In the
diagram, resistors R1 plus R2 form the resistance that sets the d.c.
operating point for opamp IC1. If no other actions were taken, the input
impedance would be about 20 MΩ. However, part of the input signal is
fed back in phase, so that the alternating current through R1 is
smaller. The input impedance, Zin, is then: Zin=(R2+R3)/R3)(R1+R2). With
component values as specified, Zin has a value of about 1GΩ. The
circuit draws a current of about 3 mA.


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