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4.5: Summary

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  • In this chapter we have explored a variety of basic op amp circuits and learned a few analysis shortcuts. The basic assumptions are that the error voltage (differential input voltage) is zero and that the op amp's input current is zero. In all circuits, the gain or transfer parameter is a function of just one or two resistors. Circuits can be made that produce voltage gain, current gain, voltage-to-current conversion, or current-to-voltage conversion. The most popular op amp circuits are the noninverting voltage amplifier and the inverting voltage amplifier. These are based on SP and PP negative feedback, respectively. The noninverting type shows an ideally infinite input impedance, whereas the inverting type has its input impedance set by one of the feedback resistors. A variation of the inverting voltage amplifier is the summing amplifier. This adds its several input channels together in order to arrive at its single output signal. The input node is at virtual ground. The differential amplifier is basically the simultaneous use of both the inverting and noninverting voltage amplifier forms.

    The voltage-to-current transducer is based on SS feedback. Its transconductance is set by a single feedback resistor. In a similar manner, the current-to-voltage transducer is based on PP feedback and has a single feedback resistor to set its transresistance. The current amplifier is based on PS feedback.

    Although op amps are designed to run off bipolar power supplies, they can be used with unipolar supplies. Extra circuitry is needed for the proper bias. There is no restriction on AC gain; however, DC gain must be set to unity. Because lead networks are introduced, the system gain cannot be flat down to zero Hertz.

    Finally, if higher output current requirements need to be met, it is possible to boost the op amp's capabilities with a discrete output stage. This stage is typically a class B or class AB push-pull follower. In order to lower system distortion, the follower is kept within the op amp's feedback loop.

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