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Opamp introduction

Opamp is a short form of operational amplifier which is capable of performing mathematical operations and signal conditioning(opamp is the heart of Instrumentation amplifier which is the front end signal conditioner of all data acquisition systems). Opamp is a DC coupled amplifier which amplifies even DC signals. Signetics μa741 series Opamp is one of the most successful opamp series. The schematic of opamp is

If input is given to inverting terminal with non inverting terminal grounded output will be exactly 180 degrees out of phase with input, hence the name inverting terminal. Similarly if input is given to Non inverting terminal with inverting terminal grounded, output will be exactly in phase with input. Above arguments are true provided they are no storage elements in the circuit with opamp.

Block diagram of opamp

The typical block diagram of opamp is shown below 

Opamp has the following three main stages

 a) Differential amplifier stage: Differential amplifier stage provides the following main functions

(1)High input impedance: Generally differential amplifier stage is employed with constant current source in the emitter circuit; this ensures irrespective of input voltage fluctuations emitter current remains constant and provides high resistance to any fluctuations in the collector current(alternatively you can thought of a transistor biased at some quiescent point now if input voltage at base changes small signal input resistance is given by ∆ Vbe/∆ Ib where ∆ Ib  is approximately zero as we have biased emitter current at a fixed current dictated by constant current source so input resistance will be very high). Even multiple differential stages are employed.

(2)High Common Mode Rejection Ratio: This is one of most desirable future essential to filter out the noise (noise will be common to both input terminals). By employing high resistance in the emitter circuit as CMMR is inversely related to emitter circuit resistance (constant current source provides high resistance when connected in series) and by carefully matching the transistors(very difficult even with highly advanced technology) high CMMRs can be achieved. After rejecting a lot of noise in the input signal is amplified in the next stage.

b) Gain stages: This intermediate stage will be provided with number of gain modules which are responsible for high gain of opamp. These modules are generally common emitter stages .Often level shifter module is also used to force the output voltage to ground potential i.e reference ground.

c) Output stage: This is generally a push pull amplifier (Power amplifier stage to drive more current or in turn more power) or common collector stage (which acts as buffer) which provides unit voltage gain with very low output resistance and high current gain.

Equivalent circuit of Opamp


Where V1 , V2 are the input voltages at inverting and non inverting terminals of an opamp; Av is voltage gain of opamp,

Vo is output voltage

And Vi = V1 – V2

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