Lecture 13 MOSFET Differential Amplifiers Microelectronic circuits by

Lecture 13 MOSFET Differential Amplifiers Microelectronic circuits by Meiling CHEN 1

topics • Ideal characteristics of differential amplifier – Input differential resistance – Input common-mode resistance – Differential voltage gain – CMRR • Non-ideal characteristics of differential amplifier – Input offset voltage – Input biasing and offset current • Differential Amplifier with active load • Frequency response Microelectronic circuits by Meiling CHEN 2

MOS differential pair Figure 7. 1 The basic MOS differential-pair configuration. Microelectronic circuits by Meiling CHEN 3

Common mode operation BJT’s differential pair VCM no bound Q 1 and Q 2 in saturation mode Figure 7. 2 The MOS differential pair with a common-mode input voltage v. CM. Make sure current source is working Microelectronic circuits by Meiling CHEN 4

Exercise 7. 1 Saturation mode Microelectronic circuits by Meiling CHEN 5

Figure 7. 3 (Continued) Microelectronic circuits by Meiling CHEN 6

Large signal operation Figure 7. 5 The MOSFET differential pair for the purpose of deriving the transfer characteristics, i. D 1 and i. D 2 versus vid = v. G 1 – v. G 2. Microelectronic circuits by Meiling CHEN 7

Figure 7. 6 Normalized plots of the currents in a MOSFET differential pair. Note that VOV is the overdrive voltage at which Q 1 and Q 2 operate when conducting drain currents equal to I/2. Microelectronic circuits by Meiling CHEN 8

More k is bigger more linear range of vid 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. Microelectronic circuits by Meiling CHEN 9

7 -2. 1 Small signal operation (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. (c) An alternative way of looking at the small-signal operation of the circuit. Microelectronic circuits by Meiling CHEN 10

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ro effects Microelectronic circuits by Meiling CHEN 12

Differential-mode equivalent circuit Microelectronic circuits by Meiling CHEN 13

Common-mode gain et CMRR (1) Half circuit of differential pair (2) Full circuit Microelectronic circuits by Meiling CHEN 14

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Non zero common gain due to RD mismatch Microelectronic circuits by Meiling CHEN 16

Non zero common gain due to gm mismatch Figure 7. 11 Analysis of the MOS differential amplifier to determine the common-mode gain resulting from a mismatch in the gm values of Q 1 and Q 2. Microelectronic circuits by Meiling CHEN 17

Microelectronic circuits by Meiling CHEN 18

Input offset voltage Figure 7. 25 (a) The MOS differential pair with both inputs grounded. Owing to device and resistor mismatches, a finite dc output voltage VO results. (b) Application of a voltage equal to the input offset voltage VOS to the terminals with opposite polarity reduces VO to zero. Microelectronic circuits by Meiling CHEN 19

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Differential amplifier with active load Active load 1. Differential gain 2. Common-mode gain et CMRR Microelectronic circuits by Meiling CHEN 22

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Differential-mode equivalent circuit with active load Passive load active load Microelectronic circuits by Meiling CHEN 24

Common-mode equivalent circuit with active load Microelectronic circuits by Meiling CHEN 25

1. Find the transconductance Gm Figure 7. 29 Determining the short-circuit transconductance Gm ; io/vid of the active-loaded MOS differential pair. Microelectronic circuits by Meiling CHEN 26

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2. Find the output resistance Ro 3. Find the differential gain Microelectronic circuits by Meiling CHEN 28

Common-mode gain et CMRR Figure 7. 31 Analysis of the active-loaded MOS differential amplifier to determine its common-mode gain. Microelectronic circuits by Meiling CHEN 29

Microelectronic circuits by Meiling CHEN 30