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Common-Source Amplifier: EE 105 Spring 2000 Week 10, Lecture 22

This document discusses the common-source amplifier circuit configuration. It provides three key points: 1) A common-source amplifier uses a MOSFET with its source terminal connected to a common ground or voltage supply. This sets the DC level of the output voltage, rather than being at zero volts. 2) Small-signal analysis models the load as a resistor RL, ignoring any DC current it may draw. The typical non-linear load acts as a small-signal resistance. 3) Biasing the common-source amplifier involves setting the drain-source voltage VDS to operate the MOSFET in its active region, while ignoring the small-signal input and load resistances for

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113 views11 pages

Common-Source Amplifier: EE 105 Spring 2000 Week 10, Lecture 22

This document discusses the common-source amplifier circuit configuration. It provides three key points: 1) A common-source amplifier uses a MOSFET with its source terminal connected to a common ground or voltage supply. This sets the DC level of the output voltage, rather than being at zero volts. 2) Small-signal analysis models the load as a resistor RL, ignoring any DC current it may draw. The typical non-linear load acts as a small-signal resistance. 3) Biasing the common-source amplifier involves setting the drain-source voltage VDS to operate the MOSFET in its active region, while ignoring the small-signal input and load resistances for

Uploaded by

sujit24chavada
Copyright
© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
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Common-Source Amplifier

* “Common” means “grounded” or more generally, “connected to a DC supply”

What is going on with the load resistor RL?


DC level of the output voltage is NOT zero ... but
A “typical load” does not draw much if any DC current ... because it is non-
linear and the load resistor is the load’s small-signal model!
What is a “typical load”?

EE 105 Spring 2000 Page 1 Week 10, Lecture 22


DC Bias Point of the Common-Source Amplifier

For biasing, we
1. ignore the small-signal source vs and its small-signal resistance: RS --> 0 Ω
2. ignore the load resistor (since it’s a small-signal resistance, too): RL --> inf. Ω

Where to set VOUT?

EE 105 Spring 2000 Page 2 Week 10, Lecture 22


Graphical “Load-Line” Analysis

The current through RD must equal the drain current.


V DD – V OUT
I D = -------------------------------- = I R
RD D

What does this equation mean?

VOUT

VBIAS

EE 105 Spring 2000 Page 3 Week 10, Lecture 22


Small-Signal Model of CS Amplifier

* Substitute parameters at operating point selected so that V OUT ≈ V DD ⁄ 2

* Find two-port parameters of this amplifier:


“natural” to use the transconductance form

Rin =

Rout =

Gm =

EE 105 Spring 2000 Page 4 Week 10, Lecture 22


Two-Port Model of Common-Source Amplifier

* Attach the source and load to find output current as a function of the source
voltage

Infinite input resistance is ideal for a voltage input

Output resistance increases with RD increasing, but DC drain current ID will


decrease and gm will decrease with ID1/2

EE 105 Spring 2000 Page 5 Week 10, Lecture 22


Current-Source Supplies

* A current source to supply current, rather than a resistor, allows a high DC


current for the device with a large incremental (small-signal) resistance

The plot of iSUP vs. vSUP is: (note that vSUP must be positive)

iSUP

vSUP

EE 105 Spring 2000 Page 6 Week 10, Lecture 22


Common-Source with Current Source Supply

* RD is replaced with idealized current source with internal resistance

* For DC bias analysis, the small-signal source (with RS) and the load resistor RL
are eliminated, along with the internal resistance roc of the current source

EE 105 Spring 2000 Page 7 Week 10, Lecture 22


Graphical Analysis of CS Amplifier with
Current-Source Supply

The region of input bias voltage VBIAS for which the current source and the
MOSFET are in their constant-current regions is extremely small ....
VOUT

VBIAS

EE 105 Spring 2000 Page 8 Week 10, Lecture 22


Common-Source/Current-Source Supply Models

* The small-signal model is identical to the resistor supply, except that the current
source’s internal resistance roc replaces RD

Tradeoffs are different from case of resistor load since ID is now decoupled from
the small-signal current supply resistance roc

EE 105 Spring 2000 Page 9 Week 10, Lecture 22


p-Channel Common-Source Amplifier

* Source of p-channel is tied to positive supply; current supply sinks ISUP to


ground or to lower supply

* DC bias:
Eliminate small-signal sources; control voltage is VSG = VDD - VBIAS

EE 105 Spring 2000 Page 10 Week 10, Lecture 22


p-Channel CS Small-Signal Model

* p-channel MOSFET small-signal model has the source at the top

Transform this into a circuit with vgs as the control voltage

EE 105 Spring 2000 Page 11 Week 10, Lecture 22

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