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Differential Amplifiers

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Presentation on theme: "Differential Amplifiers"— Presentation transcript:

1 Differential Amplifiers
Why differential? • Well suited for integrated circuit (IC) fabrication • Less sensitive to noise • Possibility to couple amplifier stages without capacitors sedr42021_0701.jpg Figure 7.1 The basic MOS differential-pair configuration.

2 Operation with a Common-Mode Input Voltage
sedr42021_0702.jpg Figure 7.2 The MOS differential pair with a common-mode input voltage vCM.

3 Operation with a Differential Input Voltage
sedr42021_0704.jpg 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.

4 Large-Signal Operation
sedr42021_0705.jpg Figure 7.5 The MOSFET differential pair for the purpose of deriving the transfer characteristics, iD1 and iD2 versus vid = vG1 – vG2.

5 sedr42021_0706.jpg 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.

6 sedr42021_0707.jpg 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.

7 Small-Signal Operation of The MOS Differential Pair
Differential Gain sedr42021_0708a.jpg 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.

8 Common-Mode Gain and Common-Mode Rejection Ratio
sedr42021_0710a.jpg Figure (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.”

9 The BJT Differential Pair
sedr42021_0712.jpg Figure The basic BJT differential-pair configuration.

10 sedr42021_0714.jpg Figure Transfer characteristics of the BJT differential pair of Fig assuming a . 1.

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