differential amplifiers. outline single-ended versus differential operation the transitions disturb...
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Single-Ended Versus Differential Operation
The transitions disturbthe differential by equal amounts, leavingthe difference in tact.
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
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.
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
Maximum Allowable Output Swing
The higher the input CM level, the smallerthe allowable output swings.
Determinations of Small Signal Gain
1. CS with resistive source degeneration
2. Thevenin Resistance3. Cascode 4. Superposition Principle
CS with resistive source degeneration
Interpretation: The resistance at the drainDivided by the resistance in the source path
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)
SimulationVin,m=1 mVVout,m=8.735 mVAv=-8.735
Calculations:Gm=1mSro=30.53 KOhmRL=12 KohmAv=-Gm(ro||RL)=-8.615
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)
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)
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.
Diode Connected Load
Problem: Difficult to decrease (W/L)P without dropping the common mode voltage of Vout.
Addition of Current Source to Increase Voltage Gain
Reduce gm by reducing current rather than the aspect ratio.Reduce I(M3) and I(M4).