The function of the last stage in an op-amp is to supply the load current and provide a low impedance output.a simple output stage is an emitter follower with complementary transistors.When Vi is positive,transistor Q1 is on and supplies current to load RL .If Vi is negative,Q1 is cutoff and Q2 acts as a sink to remove current from the load RL .There is however,a limitation in this circuit.The output voltage VO remains zero volt until the input Viexceeds VBE(cut in)=0.5V.This is called cross over distortion.It can be eliminated by applying a bias voltage V greater than QVBE(cut in)=1V between the two bases so that a small current flows in the transistor even in the quiescent state. read more

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There are two good reasons for using a level shifter in an IC(op-amp).As we want an op-amp to operate down to dc,no coupling capacitor is used.Because of direct coupling,the dc level rises from stage to stage.The increase in dc level tend to shift the operating point of next stage.This,in turn,limits the output voltage swing and even distort the output signal.It,therefore becomes essential to shift the quiescent voltage before it is applied to the next stage.Another requirement to be satisfied is that the output should have voltage level of 0V for input signal. he simplest type of a level shifter is shown in fig.1 given below.This is basically an emitter follower.Hence the level shifter also acts as buffer to isolate the high gain stages from output stage.The amount of shift read more

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Input Stage of an op-amp The input stage is a dual input,balanced output differential amplifier.It has 2 inputs Vin1 and Vin2 which are applied at the bases B1 and B2 of transistors Q1 and Q2.The output V0 ois measured between the two collectors C1 and C2 which are at the same dc potential.Because of the equal dc potential at the two collectors with respect to ground,the output is referred to as balanced output. Intermediate Stage The next stage is dual input,unbalanced output difference amplifier.Here two input signals are used however the output is measured at only one of the two collectors with reference to ground.The output is referred to as an unbalanced output because the collector at which the output voltage is measured is at some finite dc potential with reference to ground.In read more

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