 # Microelectronic circuits by Meiling CHEN 1 Lecture 13 MOSFET Differential Amplifiers.

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Microelectronic circuits by Meiling CHEN 1 Lecture 13 MOSFET Differential Amplifiers

Microelectronic circuits by Meiling CHEN 2 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 3 Figure 7.1 The basic MOS differential-pair configuration. MOS differential pair

Microelectronic circuits by Meiling CHEN 4 Figure 7.2 The MOS differential pair with a common-mode input voltage v CM. Common mode operation Q 1 and Q 2 in saturation mode BJT ’ s differential pair V CM no bound Make sure current source is working

Microelectronic circuits by Meiling CHEN 5 Exercise 7.1 Saturation mode

Microelectronic circuits by Meiling CHEN 6 Figure 7.3 (Continued)

Microelectronic circuits by Meiling CHEN 7 Figure 7.5 The MOSFET differential pair for the purpose of deriving the transfer characteristics, i D1 and i D2 versus v id  v G1 – v G2. Large signal operation

Microelectronic circuits by Meiling CHEN 8 Figure 7.6 Normalized plots of the currents in a MOSFET differential pair. Note that V OV 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 9 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 V OV. More k is bigger more linear range of v id

Microelectronic circuits by Meiling CHEN 10 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 v id 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. 7-2.1 Small signal operation (differential gain)

Microelectronic circuits by Meiling CHEN 11

Microelectronic circuits by Meiling CHEN 12 r o effects

Microelectronic circuits by Meiling CHEN 13 Differential-mode equivalent circuit

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

Microelectronic circuits by Meiling CHEN 15

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

Microelectronic circuits by Meiling CHEN 17 Figure 7.11 Analysis of the MOS differential amplifier to determine the common-mode gain resulting from a mismatch in the g m values of Q 1 and Q 2. Non zero common gain due to g m mismatch

Microelectronic circuits by Meiling CHEN 18

Microelectronic circuits by Meiling CHEN 19 Figure 7.25 (a) The MOS differential pair with both inputs grounded. Owing to device and resistor mismatches, a finite dc output voltage V O results. (b) Application of a voltage equal to the input offset voltage V OS to the terminals with opposite polarity reduces V O to zero. Input offset voltage

Microelectronic circuits by Meiling CHEN 20

Microelectronic circuits by Meiling CHEN 21

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

Microelectronic circuits by Meiling CHEN 23

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

Microelectronic circuits by Meiling CHEN 26 Figure 7.29 Determining the short-circuit transconductance G m ; i o / v id of the active-loaded MOS differential pair. 1. Find the transconductance G m

Microelectronic circuits by Meiling CHEN 27

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

Microelectronic circuits by Meiling CHEN 29 Figure 7.31 Analysis of the active-loaded MOS differential amplifier to determine its common-mode gain. Common-mode gain et CMRR

Microelectronic circuits by Meiling CHEN 30

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