Download - Differential Amplifiers
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Differential Amplifiers
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Outline
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Single-Ended Versus Differential Operation
The transitions disturbthe differential by equal amounts, leavingthe difference in tact.
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Immunity to Supply Noise
If VDD changes by ∆V, Vout changes bythe 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 lowVin,CM turns offDevices.
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Basic Differential Pair
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Schematic of Differential Amplifier
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Input/Output Characteristics
Independent of Vin,cm
Maximum SlopeThus maximum Gain
Minimum Slope
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Range of Vin,cm
Lower bound of Vin,cm:VP should be sufficiently high in order for M3 to actas a current source.Upper bound of Vin, cm
M1 and M2 need to remain in saturation.
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Sensitivity to Vin, cm
M3=Linear M3=Linear 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 Vin,
CM
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Maximum Allowable Output Swing
The higher the input CM level, the smallerthe allowable output swings.
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Transconductance
∆Vin1Represents the maximum differential signal a differentialpair can handle.
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Linearity
W/L increasesISS Constant
Constant W/L ISS increases
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Determinations of Small Signal Gain
1. CS with resistive source degeneration
2. Thevenin Resistance3. Cascode 4. Superposition Principle
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CS with resistive source degeneration
Interpretation: The resistance at the drainDivided 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 amplifiercan 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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SimulationVin,m=1 mVVout,m=8.735 mVAv=-8.735
Calculations:Gm=1mSro=30.53 KOhmRL=12 KohmAv=-Gm(ro||RL)=-8.615
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Common-Mode Response
• Sensitivity of Vout,CM due to Vin,CM
• In the presence of resistor mismatch• In the presence of transistor
mismatch• Common Mood Rejection Ratio
(CMRR)
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Sensitivity of Vout,CM due to Vin,CM
Vin,CM ↑, VP ↑, I(RSS) ↑,VX,V↓
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Output CM Sensitivity due to Vin, CM
Vout,m=0.285 mV
Vin,cm=1 mV
RL=12 KGm=1.043 mSGds3=58.29 uS
Av, CM(Analytical)=0.343Av, 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 significantat 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: 1.176 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