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Differential Amplifier

• Input of every operational amplifier is a differential amplifier

• Performance of the differential pair depends critically on the matching between the both sides of the circuit

• Differential amplifiers utilize more components

• Differential circuit are much less sensitive to the noise and interference as only difference signal between two input is sensed

• Enables biasing and coupling of the amplifier without bypass & coupling capacitors

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Figure 7.1 The basic MOS differential-pair configuration.

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Differential Amplifier

• Q1& Q2 are two matched transistors

• Constant current source (I) provides dc biasing & is an ideal source

• RD1=RD2

• It is essential that the MOSFET remain in saturation mode all the time

Microelectronic Circuits - Fifth Edition Sedra/Smith

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Figure 7.2 The MOS differential pair with a common-mode input voltage vCM.

Microelectronic Circuits - Fifth Edition Sedra/Smith

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Figure 7.3 Circuits for Exercise 7.1. Effects of varying vCM on the operation of the differential pair.

Microelectronic Circuits - Fifth Edition Sedra/Smith

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Figure 7.3 (Continued)

Microelectronic Circuits - Fifth Edition Sedra/Smith

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Figure 7.4 The MOS differential pair with a differential input signal vid applied. With vid positive: vGS1 > vGS2, iD1 > iD2, and vD1 < vD2; thus (vD2 - vD1) will be positive. With vid negative: vGS1 < vGS2, iD1 < iD2, and vD1 > vD2; thus (vD2 - vD1) will be negative.

Microelectronic Circuits - Fifth Edition Sedra/Smith

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Figure 7.5 The MOSFET differential pair for the purpose of deriving the transfer characteristics, iD1 and iD2 versus vid = vG1 – vG2.

Microelectronic Circuits - Fifth Edition Sedra/Smith

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Figure 2.4 Representation of the signal sources v1 and v2 in terms of their differential and common-mode components.

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Figure 7.8

Small-signal analysis of the MOS differential amplifier:

The circuit with a common-mode voltage applied to set the dc bias voltage at the gates and with vid applied in a complementary (or balanced) manner.

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Figure 7.8 Small-signal analysis of the MOS differential amplifier

The circuit prepared for small-signal analysis.

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Figure 7.8 Small-signal analysis of the MOS differential amplifier:

An alternative way of looking at the small-signal operation of the circuit.

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Figure 7.9 (a) MOS differential amplifier with ro and RSS taken into account.

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Figure 7.9 Equivalent circuit for determining the differential gain.

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Figure 7.10 The MOS differential amplifier with a common-mode input signal vicm.

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Figure 7.10 Equivalent circuit for determining the common-mode gain (with ro ignored). Each half of the circuit is known as the “common-mode half-circuit.”

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Figure 7.11 Common-mode gain resulting from a mismatch in the gm values of Q1 and Q2.

Assignment # 2

Exercise 7.5 and 7.6Problem 7.15, 7.16, 7.17

Due date: 20th October 2011