Lecture 19 OUTLINE Common gate stage Source follower Reading Chapter 7 3 7 4 EE105 Fall 2007 Lecture 19 Slide 1 Prof Liu UC Berkeley Diode Connected MOSFETs Diode connected NMOSFET RX 1 ro1 g m1 Small signal analysis circuit Diode connected PMOSFET 1 RY ro 2 gm2 Small signal analysis circuit Note that the small signal model of a PMOSFET is identical to that of an NMOSFET EE105 Fall 2007 Lecture 19 Slide 2 Prof Liu UC Berkeley Common Gate Amplifier Stage An increase in Vin decreases VGS and hence decreases ID The voltage drop across RD decreases Vout increases The small signal voltage gain Av is positive Av g m RD EE105 Fall 2007 Lecture 19 Slide 3 Prof Liu UC Berkeley Operation in Saturation Region For M1 to operate in saturation Vout cannot fall below Vb VTH Trade off between headroom and voltage gain EE105 Fall 2007 Lecture 19 Slide 4 Prof Liu UC Berkeley I O Impedances of CG Stage 0 Small signal analysis circuit for determining input resistance Rin 1 Rin gm EE105 Fall 2007 Small signal analysis circuit for determining output resistance Rout Rout RD Lecture 19 Slide 5 Prof Liu UC Berkeley CG Stage with Source Resistance Small signal equivalent circuit seen at input vX 1 gm 1 RS gm vin For 0 vout vout v X 1 g m RD vin v X vin g m RS 1 EE105 Fall 2007 Lecture 19 Slide 6 Av RD 1 RS gm Prof Liu UC Berkeley The output impedance of a CG stage with source resistance is identical to that of CS stage with degeneration Small signal analysis circuit for determining output resistance Rout Rout rO 1 g m RS RS 1 g m rO RS rO EE105 Fall 2007 Lecture 19 Slide 7 Prof Liu UC Berkeley CG Stage with Biasing R1 and R2 establish the gate bias voltage R3 provides a path for the bias current of M1 to flow vout R3 1 g m g m RD vin R3 1 g m RS EE105 Fall 2007 Lecture 19 Slide 8 Prof Liu UC Berkeley CG Stage with Gate Resistance For low signal frequencies the gate conducts no current Gate resistance does not affect the gain or I O impedances EE105 Fall 2007 Lecture 19 Slide 9 Prof Liu UC Berkeley CG Stage Example Small signal equivalent circuit seen at input vX 1 1 g m1 g m 2 1 1 RS g m1 g m 2 vout v X g m1 RD Av v X vin 1 g m1 g m 2 RS EE105 Fall 2007 Small signal equivalent circuit seen at output 1 vin vin 1 g m1 g m 2 RS 1 Rout1 g m1rO1 RS rO1 gm2 1 Rout g m1rO1 RS rO1 RD gm2 Lecture 19 Slide 10 Prof Liu UC Berkeley Source Follower Stage vout rO RL Av 1 vin 1 r R O L gm Small signal analysis circuit for determining voltage gain Av vin v1 vout EE105 Fall 2007 Equivalent circuit vout g m v1 ro RL g m vin vout ro RL Lecture 19 Slide 11 Prof Liu UC Berkeley Source Follower Example In this example M2 acts as a current source Av EE105 Fall 2007 Lecture 19 Slide 12 rO1 rO 2 1 rO1 rO 2 g m1 Prof Liu UC Berkeley Rout of Source Follower The output impedance of a source follower is relatively low whereas the input impedance is infinite at low frequencies thus it is useful as a voltage buffer Small signal analysis circuit for determining output resistance Rout Rout EE105 Fall 2007 1 1 rO RL RL gm gm Lecture 19 Slide 13 Prof Liu UC Berkeley Source Follower with Biasing RG sets the gate voltage to VDD RS sets the drain current Solve the quadratic equation to obtain the value of ID Assuming 0 1 W 2 I D nCox VDD I D RS VTH 2 L EE105 Fall 2007 Lecture 19 Slide 14 Prof Liu UC Berkeley Supply Independent Biasing If Rs is replaced by a current source the drain current ID becomes independent of the supply voltage VDD EE105 Fall 2007 Lecture 19 Slide 15 Prof Liu UC Berkeley
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