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
3
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.
Microelectronic Circuits - Fifth
Edition Sedra/Smith
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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.
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Assignment # 2
Exercise 7.5 and 7.6
Problem 7.15, 7.16, 7.17
Due date: 20th October 2011