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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