Operational Amplifiers
John A. Watson · 1986
Although the transistor amplifiers we looked at in Chapter 10 are useful for all sorts of applications, designers of amplifier systems are turning more and more to integrated circuit designs. The operational amplifier (the name goes back to the days when analog computers were more widely used, but that’s another story …) is perhaps the basic building-block of linear electronic systems. The ‘op-amp’ (a commonly used abbreviation) is designed to be a close approximation to a perfect amplifier. Here is a specification for such a perfect device: 1. Gain. This should be infinitely high. Obviously an amplifier with infinite gain would be useless, as the smallest input would result in full output! As we shall see, a very high or even infinite gain can be controlled by suitable feedback. 2. Input resistance. Ideally, this should also be infinite, so that there is no loading of the input source at all. 3. Output resistance. Ideally, this should be zero. With a zero output resistance, the amplifier can be connected to a load of any resistance without the output voltage being affected. 4. Bandwidth. This should be infinite, which means that the amplifier should be able to amplify (infinitely!) any frequency from zero (direct voltage) to light! 5. Common mode rejection ratio. This, too, should be infinite, but an explanation is needed. An operational amplifier has one output, but two inputs, an inverting input and a non-inverting input. Figure 12.1 shows an op-amp system, with the two inputs clearly marked, ‘-’ for inverting and ‘+’ for non-inverting. A positive voltage applied to the inverting input makes the output swing negative and a positive voltage applied to the non-inverting input makes the output swing positive. It is vital to note that the inputs are relative to each other and not to either of the supply lines. Thus, if both inputs (common mode) are Open image in new window Figure 12.1 fig 12.1 circuit symbol for an operational amplifier Open image in new window Figure 12.2 fig 12.2 showing the output of the op-amp for various types of input made more positive or negative, there should be no output. This is illustrated in Figure 12.2. 6. Supply voltage. The amplifier should be unaffected by reasonable variations in its power supply voltage.