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 Small signal operation (differential gain)

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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

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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

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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

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

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

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

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Microelectronic circuits by Meiling CHEN 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

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