4/7/2008 Lecture 19 OUTLINE • Common‐gate stage • Source follower • Reading: Chap. 7.3‐7.4 EE105 Spring 2008 Lecture 19, Slide 1Prof. Wu, UC Berkeley Common‐Gate Stage AvmD= gR • Common‐gate stage is similar to common‐base stage: a rise in input causes a rise in output. So the gain is positive. EE105 Spring 2008 Lecture 19, Slide 2Prof. Wu, UC Berkeley EE105 Fall 2007 1 4/7/2008 Signal Levels in CG Stage • In order to maintain M1 in saturation, the signal swing at Vout cannot fall below Vb‐VTH EE105 Spring 2008 Lecture 19, Slide 3Prof. Wu, UC Berkeley I/O Impedances of CG Stage 1 R = in λ =0 RRout= D gm • The input and output impedances of CG stage are similar to those of CB stage. EE105 Spring 2008 Lecture 19, Slide 4Prof. Wu, UC Berkeley EE105 Fall 2007 2 4/7/2008 CG Stage with Source Resistance 1 g vv= m Xin1 + RS gm 1 vv g AgR==out x m vmD1 vvxin + RS gm R gR ==D mD 1 1+ gRmS + RS gm • When a source resistance is present, the voltage gain is equal to that of a CS stage with degeneration, only positive. EE105 Spring 2008 Lecture 19, Slide 5Prof. Wu, UC Berkeley Generalized CG Behavior Rgout= (1++g mrR O) S r O • When a gate resistance is present it does not affect the gain and I/O impedances since there is no potential drop across it (at low frequencies). • The output impedance of a CG stage with source resistance is identical to that of CS stage with degeneration. EE105 Spring 2008 Lecture 19, Slide 6Prof. Wu, UC Berkeley EE105 Fall 2007 3 4/7/2008 Example of CG Stage voutg m1R D ⎡ ⎛⎞1 ⎤ = Rout≈ ⎢⎥gr m11 O⎜⎟|| R S+ r O 1 || R D vggRin1++() m12 m S ⎣⎦⎢⎥⎝⎠gm2 • Diode‐connected M2 acts as a resistor to provide the bias current. EE105 Spring 2008 Lecture 19, Slide 7Prof. Wu, UC Berkeley CG Stage with Biasing vout Rg3 ||( 1/ m) =⋅gRmD vRin3 ||() 1/ gR m+ G • R1 and R2 provide gate bias voltage, and R3 provides a path for DC bias current of M1 to flow to ground. EE105 Spring 2008 Lecture 19, Slide 8Prof. Wu, UC Berkeley EE105 Fall 2007 4 4/7/2008 Source Follower Stage Av <1 EE105 Spring 2008 Lecture 19, Slide 9Prof. Wu, UC Berkeley Source Follower Core vrR|| out= O L 1 vin + rROL|| gm • Similar to the emitter follower, the source follower can be analyzed as a resistor divider. EE105 Spring 2008 Lecture 19, Slide 10 Prof. Wu, UC Berkeley EE105 Fall 2007 5 4/7/2008 Source Follower Example rr|| A = OO12 v 1 + rrOO12|| gm1 • In this example, M2 acts as a current source. EE105 Spring 2008 Lecture 19, Slide 11 Prof. Wu, UC Berkeley Output Resistance of Source Follower 11 Rout=≈||rR O || L || R L ggmm • The output impedance of a source follower is relatively low, whereas the input impedance is infinite (at low frequencies); thus, a good candidate as a buffer. EE105 Spring 2008 Lecture 19, Slide 12 Prof. Wu, UC Berkeley EE105 Fall 2007 6 4/7/2008 Source Follower with Biasing 1 W 2 ICVIRV=−−μ ( ) DnoxDDDSTH2 L • RG sets the gate voltage to VDD, whereas RS sets the drain current • The quadratic equation above can be solved for ID EE105 Spring 2008 Lecture 19, Slide 13 Prof. Wu, UC Berkeley Supply‐Independent Biasing • If Rs is replaced by a current source, drain current ID becomes independent of supply voltage. EE105 Spring 2008 Lecture 19, Slide 14 Prof. Wu, UC Berkeley EE105 Fall 2007 7.