Report

Differential Amplifiers Outline Single-Ended Versus Differential Operation The transitions disturb the differential by equal amounts, leaving the difference in tact. 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 Reduction of Coupled Noise Noise coupled from L3 to L1 and L2 to L1 cancel each other. Sensitivity to the Common mode level Excessive low Vin,CM turns off Devices. Basic Differential Pair Schematic of Differential Amplifier Input/Output Characteristics Minimum Slope Independent of Vin,cm Maximum Slope Thus maximum Gain Range of Vin,cm Lower bound of Vin,cm: VP should be sufficiently high in order for M3 to act as a current source. Upper bound of Vin, cm M1 and M2 need to remain in saturation. Sensitivity to Vin, cm M3 in the linear region is modeled as a resistor M1=M2 =On M1=M2 =Off M3=Linear M1=M2 =Off M1=M2 =On M3=Linear M1=M2 =Off M1=M2 =On M3=Linear Small signal Gain as a function of Vin, CM Maximum Allowable Output Swing The higher the input CM level, the smaller the allowable output swings. Transconductance ∆Vin1Represents the maximum differential signal a differential pair can handle. Linearity W/L increases ISS Constant Constant W/L ISS increases Determinations of Small Signal Gain 1. 2. 3. 4. CS with resistive source degeneration Thevenin Resistance Cascode Superposition Principle CS with resistive source degeneration Interpretation: The resistance at the drain Divided by the resistance in the source path Treat M1 as a CS stage with resistive source degeneration to find VX/Vin 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) Gain of CG Replace M1 by its Thevenin Equivalent Circuit Small Signal Gain Half-Circuit Concept Conversion of Arbitrary inputs to Differential and Common-Mode Components Superposition Principle Schematic of Differential Amplifier Simulation Vin,m=1 mV Vout,m=8.735 mV Av=-8.735 Calculations: Gm=1mS ro=30.53 KOhm RL=12 Kohm Av=-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) Sensitivity of Vout,CM due to Vin,CM Vin,CM ↑, VP ↑, I(RSS) ↑,VX,V↓ Output CM Sensitivity due to Vin, 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) 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. Resistor Mismatch (from CS with resistive source degeneration) Common Mode to Differential Mode Conversion Voutp-Voutn Differential Mode signal at the output: 1.176 uV Effect of CM Noise in the Presence of Resistor Mismatch Common Mode to Differential Conversion Transistor Mismatch Supply Noise Sensitivity CMRR 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). Variable Gain Amplifier