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24.6 Operational Amplifiers
The shunt-injected feedback circuit (Fig. 19.22) was treated in Sec.
19.17 as a current amplifier. In practice a signal current source takes
the form of a voltage source in series with such a high impedance that
the current is determined almost entirely by it rather than by
whatever circuit it works into. If this high impedance takes the form
of a resistance that is a permanent part of the amplifier, the basis is
laid for an interesting and important class of feedback voltage
amplifiers.
Fig. 24.13 is essentially Fig. 19.22 plus this resistance, R
x
. If the
OUT
Fig. 24.13 The simplest form of operational amplifie ...