13.1 INTRODUCTION

Operational amplifier or OPAMP is one of the most important building blocks in all aspects of analog circuitry. It becomes the universal analog IC because of its wide range of application in analog signals processing. It can function as a line drive, comparator (one bit A/D), amplifier, level shifter, oscillator, filter, signal conditioner, actuator driver, current source, voltage source, and many other applications.

The OPAMP is a multistage two-input differential amplifier that is designed to be an almost ideal control device, specifically a voltage-controlled voltage source. Basically, it is an abstraction of ideal voltage amplifier with very high gain. We will employ the OPAMP to construct more complex circuits using its simple, abstract model. The equivalent circuit of OPAMP is shown in Figure 13.1.

The integrated circuit OPAMP usually consists of a cascade of three stages:

• a) Input stage: May contain one or two differential amplifier stages

• b) Intermediate stage: Emitter follower and DC level shifter.

• c) Output stage: Driver stage (Class B amplifier)

Internally, the OPAMP itself is a moderately complicated circuit and its design is beyond the scope of this book. The heart of the OPAMP is its input stage acting as differential amplifier using BJT/MOSFET. This differential input stage gives the OPAMP its high input resistance and a high gain with very little noise. It also converts the differential input voltage to a single-ended output. A typical OPAMP also has a second stage similar to the first stage, which provides additional amplification and level shifts the output voltage to zero when both inputs are equal. OPAMPs may also have an output stage similar to the buffer, due to which OPAMP exhibits low output impedance.

Symbolic representation of the operational amplifier shown in Figure 13.2, suggests that it as a two-port device. The two ports are an input and an output port. A + V voltage (for example, 15 V) is applied at the ‘plus’ power port and a –V voltage (for example, −15 V) is applied at the ‘minus’ power port. An input voltage (the control) applied across the non-inverting and inverting input terminals of the OPAMP is amplified by a large amount and appears at the output port.