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Opamp Circuit Configurations

We have seen that the opamp gain is very large, typically 10⁴ - 10⁶. Think about this. Suppose we have an opamp with a $\pm 15$ V power supply, and gain 10⁶. To avoid clipping, we need $\vert v_{out} \vert < 15$ V. What is the corresponding difference between the inputs? This is simply

$$\vert v_+ - v_- \vert < \frac{15}{10^6} = 15.0 \ \mu{\rm V} ,$$

which is quite small. So it doesn't take much difference in inputs to staturate the amplifier.

This is useful in some applications, but in general we need to shape the gain of an opamp amplifier circuit using feedback.

Negative feedback is useful in applications like audio amplifiers, where we want nice, clean stable   amplification without clipping and insensitivity to device parameter variations.

Positive feedback is also useful. For example, in oscillator circuits one wants to generate an oscillating waveform, and this is (marginally) unstable. Oscillators are used widely in analog and digital electronics for creating a variety of waveforms, e.g.

• In your PC, crystal oscillators are used to produce the clock signal that regulates the behaviour of the computer.

• Sound waveforms in electronic synthesizers.

• Timing waveforms in TV sets.

A detailed analysis of feedback and oscillators is beyond the scope of this course, but we will be taking a look at an example of a flashing bike light circuit in Hardware Lab 3. For the remainder of this chapter we will look at opamps in negative feedback configurations. Example circuits are given in Section 2.5.

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