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ATLCE - A4 01/03/2016
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Politecnico di Torino - ICT School
Analog and Telecommunication Electronics
A4 – MOS amplifier stages
» Operating point » Static In/Out characteristic» Voltage gain» Output dynamic range» Cascode circuits
AY 2015-16
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Lesson A4: MOS amplifier stages
• Setting the operating point (bias circuits)
• Static In/Out characteristic (load line)
• Small signal voltage gain
• Output dynamic range
• Circuit configuration– Common Source/Drain/Gate– Cascode circuits– Differential
• Ref.: F.Fiori: Introduction to CMOS Analog Circuits, CLUT, 2009– Chapter 1: MOS Devices; Chapter 2: Basic Gain Stages
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Operating point
• Rs defines operating point and gain
• Separation of bias and gain(as for BJT)
– Bias: R– Gain: R + C (+L)
or R-R-C Source feedback
• Nonlinear Id(Vgs)– Graphical/numeric solution– Square-law approx
» Two “solutions”, one good
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Basic CS circuit
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Common Source Vo(Vi)
• Set operating pointNegative slope: inverting amplifier
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CS gain
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Operating point and output swing
off
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Current source load
ID0
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Comparison with BJT active load
• Active load used also for BJT stages– Similar to complementary stages
• Benefit:– High dynamic impedance
• Problem: – Gain is sensitive to load
• Next step:– Use variable current source complementary stage
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Active complementary load
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CS (CE) stage frequency response
• LF limits– Series capacitors, high-pass cells– Source feedback network
• HF limits– Capacitors to GND, low-pass cells– Parasitic – Capacitors between points with “negative gain”
(G and D, Miller effect)
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HF equivalent circuit
Multiplied by Miller effect
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Frequency resp.: Miller approximation
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Source feedback (degeneration)
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Effect of Source feedback
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Common Drain (CD) basic stage
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Source follower Vo(Vi)
Positive slope: Av = 1 noninverting amplifier
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CD (CC) voltage gain
• High input impedance• Low output impedance• No voltage gain• Current gain
• Voltage follower behavior
Source follower
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Common Gate (CB) basic stage
VS VOUT
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CG (CB) voltage gain
• Low input impedance• High output impedance• Some voltage gain• No current gain
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Cascode circuit
• M1: CS stage– VD1 held at VG2, no voltage gain– CGD1 parasitic towards ground– No Miller effect (C multiplier)– Provides current gain
• M2: CG stage – Provides voltage gain– CGD2 parasitic towards ground– No Miller effect (C multiplier)
• Overall result– higher gain at high frequency
CGD1
CGD2
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Basic differential amplifier stage
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Differential stage transfer function
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Benefits of differential signals/circuits
• Exploit difference, not values
• Rejection of everything which can be considered “Common mode”
– Noise– Unbalance– …
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Several differential configurations
• Active load• Cascode• Folded cascode• ….
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Next: take into account nonlinearity
• Evaluate the effects of nonlinearity (BJT, then MOS)
• How to reduce the nonlinearity effects– Feedback,– Tuned circuits
• How to exploit nonlinearity– Using gain change compression, AGC, …– Exploit harmonics frequency multipliers
• Lab 2: – Large signal behaviour (nonlinear)
• Text reference (Del Corso):– Narrowband and tuned amplifiers: 1.2.3
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Lesson A4 – final test
• Set the operating point of MOS device.
• Describe the “load line” technique.
• Which are the benefits of active loads?
• Discuss output dynamic range with active load.
• Characteristics and applications of CS, CD, CG.
• Draw a CG circuit with bias network.
• Compare HF performance of CS and CG.
• Draw the schematic of CS with parasitic elements.
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