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Differential Amplifiers
Why differential?
• Well suited for
integrated circuit
(IC) fabrication
• Less sensitive
to noise
• Possibility to
couple amplifier
stages without
capacitors
Figure 7.1 The basic MOS differential-pair configuration.
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Operation with a Common-Mode Input Voltage
Figure 7.2 The MOS differential pair with a common-mode input voltage vCM.
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Operation with a Differential Input Voltage
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.
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Large-Signal Operation
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iD = k n (VGS - Vt )2
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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.
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Figure 7.6 Normalized plots of the currents in a MOSFET differential pair. Note that VOV is the overdrive voltage at which Q1 and Q2 operate when
conducting drain currents equal to I/2.
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Figure 7.7 The linear range of operation of the MOS differential pair can be extended by operating the transistor at a higher value of VOV.
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Small-Signal Operation of The MOS Differential Pair
Differential Gain
Figure 7.8 Small-signal analysis of the MOS differential amplifier: (a) 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. (b) The circuit prepared for small-signal analysis.
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Common-Mode Gain and Common-Mode Rejection Ratio
Figure 7.10 (a) The MOS differential amplifier with a common-mode input signal vicm. (b) 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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The BJT Differential Pair
Figure 7.12 The basic BJT differential-pair configuration.
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Figure 7.14 Transfer characteristics of the BJT differential pair of Fig. 7.12 assuming a . 1.
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