Op Amps for Everyone - The Repeater Builder's Technical ...

Input Common-Mode Range
V CC
V Bias
V BE
IN –
IN +
GND
Figure 18–3. Input Circuit of a NonRRI Op Amp
An op amp with a NPN input stage works in a similar way around the positive supply rail.
It can sense voltages close to V CC and maybe slightly above V CC , but it won’t work when
it is within 1.5 V of ground. The solution for this problem is to include parallel input circuits
as shown in Figure 18–4.
Bipolar Input
CMOS Input
V CC
V CC
IN+
IN+
IN–
IN–
GND
GND
Figure 18–4. Input Circuit of an RRI Op Amp
The RRI op amps have parallel input stages. There are both PNP and NPN differential
amplifiers used in the input stages of the RRI op amp, thus the RRI op amp can operate
above and below the power supply voltage. As Figure 18–4 shows, the parallel input
stages can be made in bipolar or MOS technology.
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