OPERATIONAL AMPLIFIERS

The characteristics of an ideal operational amplifier are described first, and the characteristics and performance limitations of a practical operational amplifier are described next. There is a section on classification of operational amplifiers and some notes on how to select an operational amplifier for an application. 1.1 IDEAL OPERATIONAL AMPLIFIER

1.1.1 Properties of An Ideal Operational Amplifier

The characteristics or the properties of an ideal operational amplifier are: i. Infinite Open Loop Gain,

ii. Infinite Input Impedance,

iii. Zero Output Impedance,

iv. Infinite Bandwidth,

v. Zero Output Offset, and

vi. Zero Noise Contribution.

The opamp, an abbreviation for the operational amplifier, is the most important linear IC. The circuit symbol of an opamp shown in Fig. 1.1. The three terminals are: the non-inverting input terminal, the inverting input terminal and the output terminal. The details of power supply are not shown in a circuit symbol. 1.1.2 Infinite Open Loop Gain

From Fig.1.1, it is found that vo = - Ao × vi, where `Ao' is known as the open-loop gain of the opamp. Let vo be -10 Volts, and Ao be 105. Then vi is 100 :V. Here 2

the input voltage is very small compared to the output voltage. If Ao is very large, vi is negligibly small for a finite vo. For the ideal opamp, Ao is taken to be infinite in value. That means, for an ideal opamp vi = 0 for a finite vo. Typical values of Ao range from 20,000 in low-grade consumer audio-range opamps to more than 2,000,000 in premium grade opamps ( typically 200,000 to 300,000). The first property of an ideal opamp: Open Loop Gain Ao = infinity. 1.1.3 Infinite Input Impedance and Zero Output Impedance

An ideal opamp has an infinite input impedance and zero output impedance. The sketch in Fig. 1.2 is used to illustrate these properties. From Fig. 1.2, it can be seen that iin is zero if Rin is equal to infinity.

The second property of an ideal opamp: Rin = infinity or iin =0. From Fig. 1.2, we get that

If the output resistance Ro is very small, there is no drop in output voltage due to the output resistance of an opamp.

The third property of an ideal opamp: Ro = 0.

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1.1.4 Infinite Bandwidth

An ideal opamp has an infinite bandwidth. A practical opamp has a limited bandwidth, which falls far short of the ideal value. The variation of gain with frequency has been shown in Fig. 1.3, which is obtained by modelling the opamp with a single dominant pole, whereas the practical opamp may have more than a single pole.

The asymptotic log-magnitude plot in Fig. 1.3 can be expressed by a first-order equation shown below.

It is seen that two frequencies, wH and wT, have been marked in the frequency response plot in Fig. 1.3.. Here wT is the frequency at which the gain A(jw) is equal to unity. If A(jwT) is to be equal to unity,

Since Ao is very large, it means that wT = Ao * wH .

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1.1.5 Zero Noise Contribution and Zero Output Offset

A practical opamp generates noise signals, like any other device, whereas an ideal opamp produces no noise. Premium opamps are available which contribute very low noise to the rest of circuits. These devices are usually called as premium lownoise types.

The output offset voltage of any amplifier is the output voltage that exists when it should be zero. In an ideal opamp, this offset voltage is zero. 1.2 PRACTICAL OPERATIONAL AMPLIFIERS

This section describes the properties of practical opamps and relates these characteristics to design of analog electronic circuits. A practical operational amplifier has limitations to its performance. It is necessary to understand these limitations in order to select the correct opamp for an application and design the circuit properly.

Like any other semiconductor device, a practical opamp also has a code number. For example, let us take the code LM 741CP. The first two letters, LM here, denote the manufacturer. The next three digits,...