BJT and MOS Class AB Output Stages Comparison

BJT and MOS Class AB Output Stages: Comparison and Contrast Mohsen Mesbah

Motivation �So far we examined various classes of output stages based on bjt transistors. However, another useful output stage configuration is based on MOS transistors. The aim of this presentation is to develop a good understanding of this class. �At first, we take a brief review on MOS operation in the following diagram.

Overdrive Voltage Increases Brief Review on MOS operation

Class AB Output Stages BJT Vs. MOS Class

Let’s Start Step-by-Step Analysis! Diode Connected Transistors!

Step-by-Step Analysis contd. In case of constant bias current from M 3, the LHS is constant, so increasing Vgs 1 decreases Vsg 2 and vice versa. Hence, output bias current is under controlled by diode connected transistors!

Derivation of maximum output swing � Despite being similar to the bjt counterpart relation, it bears some difficulties: I. The gate-source voltage includes a threshold component that is absent in the base-emitter voltage. II. It suffers from the body effect since Vt changes by further changes in Vo. III. Overdrive voltage rises rapidly with increasing current than bjt counterpart. Because overdrive voltage is proportional to the square root of the current but the base-emitter voltage is proportional

�So, with respect to limitations illustrated above, is there any alternatives to enhance the performance of the circuit?

Common-Source Configuration with Error Amplifier

Small Signal Equivalent Circuit

Step-by-Step Analysis

Step-by-Step Analysis contd. So, as depicted in the above relation, the output swing is increased in comparison with single class AB MOS configuration.