Electronics Tutorial: The Operational Amplifier

The operational amplifier is one of the basic building blocks in analog circuits. It is used in many electronic circuits and applications. This article assumes the reader has a basic knowledge of electrical theory and transistors. It introduces the reader to the operational amplifier.

A equivalent transistorized circuit for the operational amplifier is shown in figure one. It contains a differential amplifier and a common emitter amplifier. The differential amplifier comprises R1, R2, Q1, Q2 and a constant current source Is. The plus input is the input to Q2. Note that the total current from the plus side of the voltage source to ground cannot be more than the current source (Is) output.

Hence if the voltage on the base if Q1 increases and consequently the current from collector to emitter of Q1 increases, the current from collector to emitter of Q2 decreases causing the voltage at the base of Q3 to increase. The voltage at the collector of Q3 decreases. Hence the input to Q1 is the inverting input of the transistorized operational amplifier.

If the voltage on the base of Q2 increases, the current through Q2 increases, the voltage at the collector of Q2 decreases causing the voltage at the base of Q3 to decrease and the voltage at the collector of Q3 to increase. Hence, the input to Q2 is the non-inverting input.

The inverting input is referred to as the minus input and the non inverting input is referred to as the plus input.

The symbol for the operational amplifier is shown in figure two. It also has two inputs and one output.

The operational amplifier has the following characteristics:

A high input impedance

A low output impedance

Extremely high voltage gain

A virtual short exists between the two input terminals of the operational amplifier. Another words, the input voltage is very close to zero. This could be easily realized when considering the following formula:

The operational amplifier voltage gain (Av) is equal to the output voltage Vo divided by the input voltage Vi.

The gain (Av) is extremely high. The output voltage (Vo) is finite and dependent on the power output capabilities. Hence the input voltage (Vi) must be very close to zero to yield such a high voltage gain.

An operational amplifier circuit is shown in Figure Three. To calculate the gain of this circuit, we utilize the virtual short concept. This means that the inverting input (or minus input) must be at the same voltage level as the non-inverting input (or plus input.) Hence the minus input of the operational amplifier must be at ground or zero volts.

This means that the input voltage is the voltage across R1 and the output voltage is the voltage across R2.

The voltage gain of the circuit is equal to the output voltage divided by the input voltage.

Vo/Vi = Av = R2/R1

The second operational amplifier circuit is shown in Figure Four.

We once gain assume the virtual input concept. This means that the input voltage is across R1. The

output voltage is the voltage across R1 and R2. Hence the voltage gain (Av) is

Av = Vo/Vi = (R1 + R2)/R1

This concludes the introduction to the operational amplifier.

References:

I have a Bachelor of Science In Electrical Engineering and worked as an Electronics Technician.

Electronic Principles: Third Edition
ISBN 0-07-039912-3