Outline Common Source Amplifier Common Source Amplifier with resistor Rs in the source 2/19/2015 1
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Outline
Common Source Amplifier
Common Source Amplifier with resistor Rs in the source
2/19/2015 1
Recap
T-Model of amplifer
Characterstics Amplifiers
Common Source Amplifer
2/19/2015 2
Review: MOSFET Amplifier Design
• A MOSFET amplifier circuit should be designed to
1. ensure that the MOSFET operates in the saturation
region,
2. allow the desired level of DC current to flow, and
3. couple to a small-signal input source and to an output
“load”.
Proper “DC biasing” is required! (DC analysis using large-signal MOSFET model)
• Key amplifier parameters: (AC analysis using small-signal MOSFET model) – Voltage gain Av vout/vin – Input resistance Rin resistance seen between the
input node and ground (with output terminal floating) – Output resistance Rout resistance seen between the
output node and ground (with input terminal grounded)
Common Source(CS) Amplifier
Common Source(CS) Amplifier
Constant
Current
source for
biasing
Coupling
Capacitors
to block DC
AC Ground
RG should
be High
Value
DC Analysis
In order to analyze the small-signal model:
• Replace ideal constant ac voltage sources
with short circuits
• Replace ideal constant ac current sources
with open-circuits.
2/19/2015 6
DC Analysis
•Capacitors acts as open
during DC Analysis •Replace ideal constant ac voltage
sources with short circuits
Analyzing the Small Signal
In order to analyze the small-signal model:
• Replace ideal constant dc voltage sources
with short circuits
• Replace ideal constant dc current sources
with open-circuits.
2/19/2015 8
constant dc voltage
sources with short circuits
•Capacitors acts as short
during AC Analysis
constant dc current
sources with open-circuits
Common Source(CS) Amplifier
constant dc voltage
sources with short circuits
•Capacitors acts as short
during AC Analysis
constant dc current
sources with open-circuits
(AC)Small
Signal Analysis
Π Model
Common Source(CS) Amplifier
Current into
gate terminal
Input Resistance, Rin
Common Source(CS) Amplifier
Input Voltage, vi
Common Source(CS) Amplifier
Common Source(CS) Amplifier
Drain Current id
id = gmvgs
vd = vo =–gmvgsRD
We know that
Output Voltage V0 ?
Common Source(CS) Amplifier
vd = vo =–gmvgsRD
Common Source(CS) Amplifier
Common Source(CS) Amplifier
Common Source(CS) Amplifier
Common Source(CS) Amplifier
Output Resistance of MOSFET
Common Source(CS) Amplifier
Common Source(CS) Amplifier with source
Resistance
Common Source(CS) Amplifier with source
Resistance
Common Source(CS) Amplifier with source
Resistance
Common Source(CS) Amplifier with source
Resistance
(AC)Small
Signal Analysis T Model
Input Resistance, Rin
Input Voltage, vi
Common Source(CS) Amplifier with source
Resistance
Common Source(CS) Amplifier with source
Resistance
Gate to source voltage ,vgs
Common Source(CS) Amplifier with source
Resistance
Drain Current, id
id
Drain current
decreases
Common Source(CS) Amplifier with source
Resistance
Output voltage, vo
Common Source(CS) Amplifier with source
Resistance
Important
Observation
Common Source(CS) Amplifier with source
Resistance
Problem 2
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