Output Stage of an Op Amp and its Voltage Transfer Characteristics:

The output stage of an op amp is another requirement which should have very small output impedance and provide the external load current so that large output voltage swing is obtained.

The output of an op-amp is the voltage measured at the output of this final stage which is usually a push-pull complementary amplifier, as shown in Fig. 20.64.

When positive cycle of input voltage V_in arrives, Q₁ behaves as current supplier to load R_L and Q₂ is off.

When negative cycle of V_in arrives, Q₁ is off, Q₂ acts as a sink to remove current from the load, (it decreases the load current).

But the circuit bears crossover distortion when V_in = 0 (when voltage swings from positive to negative). To reduce this problem of getting V_in = V_BE = 0.7 V when it should be zero, we provide a constant dc bias voltage between the two bases so that small current exists in quiescent condition. For this, two diodes D₁ and D₂ are connected, as shown in Fig. 20.64 (b).

The transfer characteristics shows that if V_in = 0, V_out ≠ 0. Here, V_in is obtained from level shifter, we can have V_out = 0 with V_in = 0 by making quiescent value of V_in = -V_BE2.

Figure 20.66 can also reduce the crossover distortion. The output voltage is about 1.1 V between two bases. Here both transistors Q₁ and Q₂ conduct slightly under quiescent condition.