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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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ECE 3120 Microelectronics II Dr. Suketu Naik
Discrete Amplifier Design: Gain Resistor
CS (Common Source) Amplifier
Resistor RD provides the gain
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ECE 3120 Microelectronics II Dr. Suketu Naik
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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ECE 3120 Microelectronics II Dr. Suketu Naik
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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ECE 3120 Microelectronics II Dr. Suketu Naik
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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ECE 3120 Microelectronics II Dr. Suketu Naik
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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ECE 3120 Microelectronics II Dr. Suketu Naik
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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ECE 3120 Microelectronics II Dr. Suketu Naik
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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ECE 3120 Microelectronics II Dr. Suketu Naik
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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ECE 3120 Microelectronics II Dr. Suketu Naik
Cascoding: Improving gain and output resistance
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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ECE 3120 Microelectronics II Dr. Suketu Naik
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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ECE 3120 Microelectronics II Dr. Suketu Naik
Cascoding: Improving gain and output resistance
Double Cascoding
Q: What are the
limitations to
Cascoding?
With an ideal current source load,
total gain = ? total output resistance = ?
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ECE 3120 Microelectronics II Dr. Suketu Naik
Cascoding: Improving gain and output resistance
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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ECE 3120 Microelectronics II Dr. Suketu Naik
Advantages
Disadvantages
Cascoding: What are the advantages and disadvantages?
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ECE 3120 Microelectronics II Dr. Suketu Naik
Cascoding: Improving gain and output resistance
BJT Cascode
Current
Source or
Active Load
Amplifier
Stages
Limitation of BJT Cascode: max
possible Ro = β2 ro2
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