1. Cascode Amplifier
Module Title: Analog and mixed signal circuit design
Date: 2nd July 2012
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2. Contents
• Introduction
• Cascode amplifier
• Small signal model
• Gain calculation
• Cascode Current mirror
• Cascode differential amplifier
• Conclusion
• References
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3. Multi-Stage Amplifiers (Cascading)
• We can cascade different types of amplifiers to get desired overall
characteristics. Often want:
– High input impedance
– High gain
– Low output impedance
– Higher gain
• Mix and match cascades of different types of amplifiers to get desired
result
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4. Introduction
• Constructed using two transistors, one operating as CS and the other as
a CG
• Best of both the circuit configurations namely,
– Moderate input impedance and high voltage gain of CS configuration
– Very large output impedance, large output voltage swing and much larger band
width of CG configuration.
• Disadvantage: Requires two transistors and a relatively high supply
voltage.
• Applications: Used in current mirrors to increase output impedance, as
a modulator for amplitude modulation,
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5. Comparison of CSA with a Cascode amp
RL=300k
D
G
M1
VGS1=0.6V
S
5u/0.35u
vout
vin
Figure 1. Common source amplifier [Sharma 2010]
Figure 2. Cascode amplifier [Sharma 2010]
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6. Cascode amplifier
Figure 3. Cascode amplifier [Sharma 2010]
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7. Waveforms for Cascode amplifier for varying load
Figure 4. Waveforms for change of output with respect to load
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8. Small signal model
Figure 5. Small signal model of cascode amplifier [Sharma 2010]
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9. Gain calculation
Figure 6. Gain calculation [Sharma 2010]
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10. Cascode Current Mirror
• This circuit does not need an external voltage bias.
• The reference side of the mirror generates the bias voltages for both the
transistors of the cascode output side.
• However, this reduces the voltage range over which the output may swing.
Figure 7. (a) Current mirror (b) Cascode current mirror (c) Simulation waveforms
[Sharma 2010]
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11. Common-Source - Source-Follower Cascade
RD
M2
Rs
M1
vO
Cout
vS
IS1 CS IS2 RLD
Figure 8. Common source and source follower cascode
[Sharma 2010]
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12. Cascode Differential amplifier
• Use of cascoding to increase load resistance
• Cascode both the active loads and the differential pair
– Higher effective load resistance
– Higher ro for the differential pair
– Reduces Miller effect M7 M8
M5 M6
vo
Vbias
M3 M4
M1 M2
vid
I
Figure 9. Cascode differential amplifier [Baker 2010]
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13. Summary
• Best of both the circuit configurations (common source and common
gate)
• The cascode arrangement offers high gain, high stability and high input
impedance
• Wide applications as in current mirrors, modulators and differential
OpAmp
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14. References
• Dinesh Sharma (2010) CMOS Mixed Signal Design: OpAmp Design
[online] available from < www.ee.iitb.ac.in/~smdp/DKStutorials/
opamp-tutorial.pdf > [01st July 2012]
• Jacob Baker, R. (2010) CMOS: Circuit Design, Layout and Simulation.
3rd edn. Wiley-IEEE
• Phillip E. Allen., and Douglas R, Holberg. (2002) CMOS Analog Circuit
Design. 2nd edn. Oxford University Press
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