Download MOSFET Common Source Amplifiers

Lecture 13 – MOSFET Common-Source Amplifiers
The MOSFET common-source (CS) amplifier is the most widely used amplifier
configuration. It gets the name from the fact that the source terminal is ‘common’
to both the input and output of the small signal equivalent circuit – it forms the a.c.
ground. It has the advantages of a reasonable gain, combined with a very high
input resistance. It has a relatively high output resistance which is a disadvantage if
it has to drive high currents into a small load resistor. When it is used as part of an
integrated circuit the high output impedance is not so much of a problem because
the ‘load’ is the next stage of the circuit which would typically be the high
impedance gate of a MOSFET.
When analyzing any amplifier circuit we need to be careful to distinguish between
three measures of the gain:
1) the open loop gain, Avo = the gain that would be measured for no load
(RL = ∞) using an ideal signal source (Rsig = 0)
2) the loaded gain, Av = the gain that would be measured for a finite load
(RL < ∞) using an ideal signal source (Rsig = 0)
3) the overall gain, Gv = the gain that would be measured for a finite load
(RL < ∞) using a ‘real’ signal source with a non zero output impedance
(Rsig > 0)
13.1) The Basic Structure of the MOSFET Common-Source Amplifier
EEE334 Circuits II – T J Thornton
Spring 2017 – Lecture 13
13.2) Small-signal Analysis of the CS Amplifier
Full circuit schematic. For the DC bias analysis the capacitors are replaced by
open circuits. For the final AC analysis we assume that CC1, CC2 and CS are large
enough that they can be considered as short circuits for any AC frequency.
EEE334 Circuits II – T J Thornton
Spring 2017 – Lecture 13
Circuit used to calculate the DC bias conditions. The capacitors are treated as
open circuits at DC.
EEE334 Circuits II – T J Thornton
Spring 2017 – Lecture 13
The circuit used for the small signal AC analysis. The capacitors have been
replaced by short circuits.
EEE334 Circuits II – T J Thornton
Spring 2017 – Lecture 13