Basic Amplifier Stages II • CS - Example • Common Gate Amplifier
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Basic Amplifier Stages II • CS - Example • Common Gate Amplifier • Cascode Amplifier • Transistor Frequency Response Reading instructions: 185-187, 194-196, 205, 207-210, 214, 217-229,490-495 Modern Electronics: F7 Basic Amplifiers II 1 Basic Amplifier Stages II Gate biasing R 100kW V(t) VDD R1 3MW C1 0.01mF R2 1MW Parameters: k’=0.0015 A/V2 W/L=100 VT=0.5V VDD=5V VDD RD 47W l=0.1 C2 1mF RL 50W vOUT 1) Calculate bias points 2) What fraction of the current is ”used”? 3) Calculate medium frequency voltage gain Modern Electronics: F7 Basic Amplifiers II 2 Common Gate Amplifier VDD An common gate stage has • Low input resistance (transconductance) • High Output resistance (saturation) • Application: ”Current Buffer” io=ii RD vIN vOUT πΊπ = ππ π π = Source vsg 1/gm Drain 1 ππ π 0 = π π· gmvsg RD vo Gate Modern Electronics: F7 Basic Amplifiers II 3 Common Gate Amplifier VDD An common gate stage has • Low input resistance (transconductance) • High Output resistance (saturation) • Application: ”Current Buffer” io=ii • Addition of ro makes the amplifier ”bilateral” RD RS vIN RL vOUT πΊπ = ππ RS vi Source 1/gm ro π0 + π π· π πΏ π π = 1 + ππ ππ Drain gmvin RD RL vo π 0 = π π· Gate Modern Electronics: F7 Basic Amplifiers II π0 + π π 1 + ππ π0 π 0 ≈ π π· ππ π0 π π 4 Common Drain Amplifier – Source Follower πΊπ ≈ ππ VDD π π = ∞ π 0 = vIN RL vOUT π΄π = ππ 1 1 1 ππ + π + π 0 πΏ 1 1 1 ππ + π + π 0 πΏ <1 • Vout≈Vin-(VT+Vov) vgs • Level Shifter vi gmvgs ro RL vo • Voltage Buffer • Output stage – low output resistance Modern Electronics: F7 Basic Amplifiers II 5 2 minute exercise I VDD gm2, r02 Ix vx A diode connected FET can operate as a small signal resistor 1) Draw the small signal model of the circuit. 2) Calculate the equivalent resistance. R=Vx/Ix Modern Electronics: F7 Basic Amplifiers II 6 2 minute exercise II VDD gm2 • How do you use this to build a CS-amplifier using only transistors? • What is the corresponding voltage gain? Modern Electronics: F7 Basic Amplifiers II 7 Cascode VDD gm2vgs2 =-gm2vds1 RD M2 vo RD M1 vOUT vIN vi gm1vi ro1 πΊπ ≈ ππ1 • Two stage amplifier π π = ∞ • Increases output resistance • Good high frequency properties ro2 π 0 = π01 + π02 + ππ2 π01 π02 ≈ ππ2 π01 π02 Modern Electronics: F7 Basic Amplifiers II 8 Differential Amplifier VDD VDD RD π΄ππ π£ππ = ≈ −ππ π π· π£ππ RD vod -Vid/2 Vid/2 • Two input signals – opposite phase • Output signal – differential ITAIL • Rejects commonmode signals (eg. supply noise) • Amplifies differential signals Modern Electronics: F7 Basic Amplifiers II 9 2 minute problem – differential / single ended VDD RD VDD RD RD Vout vod Vid/2 -Vid/2 Vd If VDD varies with DV – (l=0, transistors in saturation) • How much does vod change? • How much does Vout change? Modern Electronics: F7 Basic Amplifiers II 10 High Frequency Transistors • Rd and Rs are usually small • Cgd<<Cgs RG + vin - Cgd + v1 gmv1 - Cgs + r0 vout - • The Cgd feedback capacitance complicates the analysis • We will use the Miller approximation Modern Electronics: F7 Basic Amplifiers II 11 Maximum current gain frequency: fT Cgd RG + vin - iin + v1 gmv1 - Cgs ππ − πππΆππ πππ’π‘ = β21 = ≈ πππ ππ(πΆππ + πΆππ ) ππ ≈ ππ πΆππ + πΆππ Unit current gain |h21|=1 when w=2p*fT ππ ≈ ππ 2π πΆππ + πΆππ r0 Maximum current gain (iout/iin) when output is shorted to ground 160 140 Current Gain (dB) iin 120 100 80 60 fT 40 20 0 10 5 10 10 Frequency (Hz) Modern Electronics: F7 Basic Amplifiers II 12 Current Best High Frequency Transistors • fT ~ 500-800 GHz • MSG/U: These are power gain curves. fmax ~ 1 THz • More about this in FFF115 High Speed Devices Modern Electronics: F7 Basic Amplifiers II 13 Miller Approximation RG + vin - Cgd + v1 gmv1 - Cgs RG r0 + + vout vin - - + gmv1 v1 - Cgs CM + vout r0 - • A feedback capacitance can be approximated as a larger capacitance at the input. • This is useful for simplifying calculations of the imput impedance / approximate transfer function • Not accurate for output impedance! Modern Electronics: F7 Basic Amplifiers II 14 Frequency Dependent Voltage Gain Cgd RG + vin - + + v1 gmv1 - Cgs RL - πΎ = −ππ π πΏ 25 1 20 ππ 1−π 1 −1 π1 = π πΊ (πΆππ + πΆππ (1 + ππ π πΏ )) • High gm • Small Cgs, Cgd! • Depends on RL Voltage Gain (dB) π΄π£ π ≈ πΎ vout 15 w-3dB=|p1| 10 5 0 5 10 10 10 Frequency (Hz) Modern Electronics: F7 Basic Amplifiers II 15 670 GHz Common Source Amplifier CS Amplifiers Input Output 5x Common Source Stages Modern Electronics: F7 Basic Amplifiers II 16
