Fundamentals of Operational Amplifier

An operational amplifier (Opamp) is a versatile device which can be used to amplify AC as well as DC signals. It is manufactured in the form of integrated circuit package. It can perform different mathematical operations like addition, subtraction, integration, taking the derivative, comparison, etc. Hence it is known as operational amplifier. The most important section of Opamp is the differential amplifier. It is a circuit of two transistors which has very high gain and it rejects noise to produce high quality i.e. High Fidelity output with amplification.

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Prof. Dattaraj Vidyasagar (Series Author)

Fundamentals of Amplifier

Amplifier is a device or circuit, which increases strength of input signal in terms of either voltage or current. The process of increasing strength is called amplification. It works on external battery supply, takes up power from it, and then amplifies input signal. However, in general circuits of amplifier, external battery supply is not shown and it is understood that it is there!

There are two types of amplifiers: inverting amplifier and non-inverting amplifier. In inverting amplifier, input and output signals are out-of-phase. In non-inverting amplifier, they are in-phase.

The most important factor of amplifier is its gain. The gain of an amplifier is the ratio of output voltage to input voltage. It is denoted by A_v. It is always greater than unity.

Amplifier is used in all types of electronic instruments like CRO, function generator, voltage regulators, tape recorder, VCR, VCD player, MP3 player, TV, computers and number of others.

Concept of Feedback

Feedback means connecting a fraction of output quantity back to input of a circuit. Feedback is always used in amplifier circuits. There are two types of feedbacks, positive feedback and negative feedback. When feedback is used in inverting amplifier, it is called negative feedback and when it is used in non-inverting amplifier it is positive feedback.

Input-Output Resistance of Amplifier

When current flows into input section of amplifier, it experiences opposition. This opposition is termed as input resistance R_i of amplifier. In the same way, when amplifier gives out signal, in the form of current, after amplification, it flows into its output circuit and experiences opposition. This opposition is termed as output resistance R_o of the amplifier. Mathematically –

R_i = V_i / I_i  and  R_o = V_o / I_o

RC & DC Amplifier

In multistage amplification, when one stage is connected to next through capacitors and resistors it is called as RC amplifier. And when one stage is connected to next directly, it is called as direct-coupled DC amplifier.

The RC coupled amplifier is some-what better than Direct coupled amplifier. However, when quick and cheap amplifier circuit is required, then direct-coupled amplifier is used. But this amplifier has some drawbacks.

Drawbacks

Following are the main drawbacks of RC and DC types of amplifiers.

In RC coupled amplifier the capacitive reactance is high at low frequencies. So gain is low. At high frequencies, the gain becomes frequency dependent and so high frequency response is not uniform. It cannot amplify DC signal due to capacitors used in it.

The direct coupled amplifier has high gain. But it picks up more noise. The drift is also observed in it. When AC signal is amplified, it mixes with DC signal. So humming i.e. hissing sound is produced at the output.

In the next topic we shall see details of differential amplifier.