Download BJT and MOS Class AB Output Stages: Comparison and

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 M3, the
LHS is constant, so
increasing Vgs1
decreases Vsg2 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 to the logarithm of the current.
 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.