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Published byKelsie Nesmith Modified about 1 year ago

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Differential Amplifiers

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Outline

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Single-Ended Versus Differential Operation The transitions disturb the differential by equal amounts, leaving the difference in tact.

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Immunity to Supply Noise If VDD changes by ∆V, Vout changes by the same amount. Noise in VDD affects VX and VY, but not Vx-Vy

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Reduction of Coupled Noise Noise coupled from L3 to L1 and L2 to L1 cancel each other.

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Sensitivity to the Common mode level Excessive low V in,CM turns off Devices.

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Basic Differential Pair

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Schematic of Differential Amplifier

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Input/Output Characteristics Independent of V in,cm Maximum Slope Thus maximum Gain Minimum Slope

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Range of V in,cm Lower bound of V in,cm : VP should be sufficiently high in order for M3 to act as a current source. Upper bound of V in, cm M1 and M2 need to remain in saturation.

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Sensitivity to V in, cm M3=Linear M1=M2 =Off M1=M2 =Off M1=M2 =Off M1=M2 =On M1=M2 =On M1=M2 =On M3 in the linear region is modeled as a resistor

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Small signal Gain as a function of V in, CM

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Maximum Allowable Output Swing The higher the input CM level, the smaller the allowable output swings.

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Transconductance ∆V in1 Represents the maximum differential signal a differential pair can handle.

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Linearity W/L increases ISS Constant Constant W/L ISS increases

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Determinations of Small Signal Gain 1.CS with resistive source degeneration 2.Thevenin Resistance 3.Cascode 4.Superposition Principle

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CS with resistive source degeneration Interpretation: The resistance at the drain Divided by the resistance in the source path

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Treat M1 as a CS stage with resistive source degeneration to find VX/Vin

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Replace M1 by its Thevenin Equivalent Circuit If RS is sufficiently large, then the small signal gain of the amplifier can be obtained using thevenin’s equivalent circuit (see hand out)

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Gain of CG

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Replace M1 by its Thevenin Equivalent Circuit

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Small Signal Gain

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Half-Circuit Concept

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Conversion of Arbitrary inputs to Differential and Common-Mode Components

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Superposition Principle

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Schematic of Differential Amplifier

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Simulation Vin,m=1 mV Vout,m=8.735 mV Av= Calculations: Gm=1mS ro=30.53 KOhm RL=12 Kohm Av=-Gm(ro||RL)=-8.615

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Common-Mode Response Sensitivity of V out,CM due to V in,CM In the presence of resistor mismatch In the presence of transistor mismatch Common Mood Rejection Ratio (CMRR)

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Sensitivity of V out,CM due to V in,CM V in,CM ↑, V P ↑, I(RSS) ↑,V X,V↓

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Output CM Sensitivity due to V in, CM Vout,m =0.285 mV Vin,cm =1 mV RL=12 K Gm=1.043 mS Gds3=58.29 uS Av, CM(Analytical)=0.343 Av, CM(Simulation)=0.285 (Excluding gmb, ro)

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Common-Mode to Differential Conversion at High Frequencies Even if the output resistance of the current source is high, the common-mode to differential conversion becomes significant at high frequencies.

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Resistor Mismatch (from CS with resistive source degeneration)

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Common Mode to Differential Mode Conversion

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Voutp-Voutn Differential Mode signal at the output: uV

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Effect of CM Noise in the Presence of Resistor Mismatch Common Mode to Differential Conversion

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Transistor Mismatch

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Supply Noise Sensitivity

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CMRR

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Diode Connected Load Problem: Difficult to decrease (W/L)P without dropping the common mode voltage of Vout.

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Addition of Current Source to Increase Voltage Gain Reduce gm by reducing current rather than the aspect ratio. Reduce I(M3) and I(M4).

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Variable Gain Amplifier

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