OPERATIONAL AMPLIFEIR

INTRODUCTION

Op-Amp (operational amplifier) is basically an amplifier available in the IC form. The word “operational” is used because the amplifier can be used to perform a variety of mathematical operations such as addition, subtraction, integration, differentiation etc.

Figure 1 below shows the symbol of an Op-Amp.

It has two inputs and one output. The input marked “-“  is known as Inverting input and the input marked “+” is known as Non-inverting input.

• If a voltage V_i is applied at the inverting input ( keeping the non-inverting input at ground) as shown below.

The output voltage V_o= -AV_i is amplified but is out of phase with respect to the input signal by 180⁰.

• If a voltage V_i is fed at the non-inverting input ( Keeping the inverting input at ground) as shown below.

The output voltage V_o= AV_i is amplified and in-phase with the input signal.

• If two different voltages V₁ and V₂  are applied to an ideal Op-Amp as shown below.

The output voltage will be V_o = A(V₁ – V₂)

i.e the difference of the tow volatages is amplified. Hence an Op-Amp is also called as a High gain differential amplifier.

Note: Op-Amp is 8 pin IC ( named as μA 741) with pin details as shown.

Block Diagram of an Op-AMP

An Op-Amp consists of four blocks cascaded as shown above

Input stage: It consists of a dual input, balanced output differential amplifier. Its function is to amplify the difference between the two input signals. It provides high differential gain, high input impedance and low output impedance.

Intermediate stage: The overall gain requirement of an Op-Amp is very high. Since the input stage alone cannot provide such a high gain. Intermediate stage is used to provide the required additional voltage gain.

It consists of another differential amplifier with dual input, and unbalanced ( single ended) output

Buffer and Level shifting stage

As the Op-Amp amplifies D.C signals also, the small D.C. quiescent voltage level of previous stages may get amplified and get applied as the input to the next stage causing distortion the final output.

Hence the level shifting stage is used to bring down the D.C. level to ground potential, when no signal is applied at the input terminals. Buffer is usually an emitter follower used for impedance matching.

Output stage- It consists of a push-pull complementary amplifier which provides large A.C. output voltage swing and high current sourcing and sinking along with low output impedance.

Concept of Virtual ground

We know that , an ideal Op-Amp has perfect balance (ie output will be zero when input voltages are equal).

Hence when output voltage V_o = 0, we can say that both the input voltages are equal ie V₁ = V₂.

Since the input impedances of an ideal Op-Amp is infinite ( R_i = ). There is no current flow between the two terminals.

Hence when one terminal ( say V₂ ) is connected to ground (ie V₂ = 0) as shown.

Then because of virtual ground V₁ will also be zero.