COMPUTING

Chapter 5 Operational AmplifiersBy substitution of (5.58) into (5.56) we obtainv out=R----- f( vR in2 – v in1 )1(5.59)Next, we will derive the input resistance for the differential input op amp circuit of Figure 5.63.For convenience in (5.58) we let R 2 = R 1 . Then R 3 = R fand with these simplifications the circuitof Figure 5.63 is as shown in Figure 5.66.v in2+– v i in1−R 1R 1R f−+R f+v out−Figure 5.66. Differential input op amp for derivation of the input resistanceApplication of KVL around the input circuit starting at the minus (−) input terminal and goingcounterclockwise, and observing that there is a virtual short between the inverting and non−inverting inputs, we obtainR 1 i+ R 1 i – ( v in2 – v in1 ) = 0Also, by definitionand from (5.60 and (5.61)v in2– v in1 = 2R 1 ivR in2 – v in1in = -------------------------iR in = 2R 1(5.60)(5.61)(5.62)Relation (5.59) reveals that for a large differential gain, we must make the feedback resistor R faslarge as possible and the resistance R 1 as small as possible. But with small R 1 the input impedancewill also become small as we can see from (5.62).5.16 Instrumentation AmplifiersHigh input resistance differential input amplifiers are suitable for use in differential measurementapplications and the associated circuits are referred to as instrumentation amplifiers such as thatshown in Figure 5.67.5−42Electronic Devices and Amplifier Circuits with MATLAB Computing, Second EditionCopyright © Orchard Publications