Chapter 7 Building Blocks of Integrated Circuit Amplifiers ...faculty.weber.edu/snaik/ECE3120/01Ch7partB_CascodeAmps.pdf · Building Blocks of Integrated Circuit Amplifiers: Part

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ECE 3120 Microelectronics II Dr. Suketu Naik

Chapter 7

Building Blocks

of Integrated

Circuit Amplifiers:

Part B:

Cascode Amplifiers

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Discrete Amplifier Design: Gain Resistor

CS (Common Source) Amplifier

Resistor RD provides the gain

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IC Amplifier Design: Current Source

Constant current source to provide gain and bias

the transistor

CS (Common Source)

gain cell

CE (Common Emitter)

gain cell

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7.2 The Basic Gain Cell

Two types of basic gain cells on IC:

Common-source (CS)

Common-emitter (CE)

Both are loaded with constant-current source

Resistor-on-chip is difficult to fabricate (tolerance and area)

Current source provides higher output resistance than

discrete resistor and will increase the gain

Av=-gmro||RD for resistor-loaded CS amplifier

Av=-gmro||ro_currentsource for active-loaded CS amplifier

These circuits are referred to as current-source loaded or

active-loaded

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1) Learn how current source is implemented in IC using

an active load

2) Use these lessons to learn about Current Mirrors (CM)

both in IC and discrete design

Learning Objectives

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Current Source in IC

Current-source of the CS amplifier can be implemented

using a PMOS transistor biased in the saturation

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7.2.3 Effect of Active Load

The current source, which is implemented by the active load,

will decrease the output resistance and voltage gain when compared to

an ideal current source

Av=-gmro1||ro2

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7.3 Cascode Amplifier

How do you increase gain?

A: Increase output resistance

How do you increase output resistance?

A: Current buffer

What does a current buffer do?

A: it passes the current through and increases the

output resistance

How do you make a current buffer?

A: Place CG (CB) stage on top of the CS (CE) stage

Also called Cascoding

The gain of the basic gain cell can be increased by cascoding

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Cascoding: Improving gain and output resistance

Cascode Amplifier

with CS-CG

configuration

With an ideal current source load, total gain = -(gm ro)2

Cascode-stage gain

(1) CS amp gain, Avo=-gm1 Ro

where, Ro=total output resistance

=(gm2 ro2 ) ro1

CS amp gain, Avo=-gm1 (gm2 ro2 ) ro1

(2) If identical transistors then,

CS amp gain=

Avo=-(gm ro)2 …(eq7.27)

Total output resistance

Ro=(gm2 ro2 ) ro1=gm ro2 …(eq7.25)

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Cascode Amplifier with CS-CG configuration and Active load

Ro = Rop || Ron= ro3 || ((gm2 ro2 ) ro1)

Av = - gm1 Ro= - gm1 (ro3 || ((gm2 ro2 ) ro1))

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7.3 Cascode Amplifier

Cascode Amplifier with Cascode Current Source

Q3 (CG stage) raises the output resistance of

the current source Q4 (CS Stage)

Q2 (CG stage) raises the output resistance of

the amplifier Q1 (CS Stage)

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Double Cascoding

Q: What are the

limitations to

Cascoding?

With an ideal current source load,

total gain = ? total output resistance = ?

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What is the impact of active load on each circuit?

CS Amplifier

with Active-load

Cascode CS Amplifier

with Active-load

Cascode CS Amplifier

with Cascode Active-

loadCircuit

Rin

Ro

Av

Av (identical

transistors)

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Advantages

Disadvantages

Cascoding: What are the advantages and disadvantages?

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BJT Cascode

Current

Source or

Active Load

Amplifier

Stages

Limitation of BJT Cascode: max

possible Ro = β2 ro2

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In Class Practice Problems

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p7.24: MOS cascode amplifier

Documents

Chapter 7 Building Blocks of Integrated Circuit Amplifiers ...faculty.weber.edu/snaik/ECE3120/01Ch7partB_CascodeAmps.pdf · Building Blocks of Integrated Circuit Amplifiers: Part