Open-loop gain, operational amplifiers

A typical response for an operational amplifier is given in Figure 6.1(b). The output potential Vb must have a value between Vg+ and Vs-. For an ideal operational amplifier, the open-loop gain Aop is very large (ideally infinite), such that... 

Estimate the maximum error in the term V — ref if the operational amplifiers used in Figure 6.8 had an open-loop gain of 10 and power source of 10 V. 

In previous considerations of operational amplifier analysis, we have assumed that the device was ideal in the sense that it can respond instantly to any change in response at the inputs. This is equivalent to assuming that the open loop gain is independent of frequency. The characteristic dependence of open loop gain for a real operational amplifier is shown in Fig. 11.8. The roll-off frequency determines the time constant for the potentiostat, Ti, through the relationship... 

During the late 1970s, instrumentation was improved by the availability of operational amplifiers of increasingly better quality (high open loop gain, wide frequency bandwidth. 

Thus, the output voltage v is the input voltage I i multiplied by the ratio of two resistors, Rf/R, and of opposite polarity. If the two resistors are precision resistors, the amplifier closed-loop gain. RflRf, can be made quite accurate. For example, if Rf were 100 kfl and R, were 10 kfl, the gain would be 10 and v = -10 x v,. Note that the accuracy of the gain depends on how accurately the two resistances are known and not on the open-loop gain. A, of the operational amplifier. 

An operational amplifier with an open-loop gain of l.O X 10 and an input resistance of 1.0 X 10 " Ls used in the voltage follower circutt of Figure 3-5. 

An operational amplifier to be used in a current follower has an open-loop gain A of 2 X 10 and an input bias current of 2.5 nA. 

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