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