Lecture 23 Frequency Response of Amplifiers (III) OTHER AMPLIFIER STAGES Outline 1. Frequency Response of the Common-Drain Amplifier Common-Drain Amplifier 2. Frequency Response of the Common-Gate Amplifier 6.012 Spring 2009 Lecture 23 11. Frequency Response of the Common-Drain Amplifier VDD signal source vs VBIAS vOUT iSUP RS RL + signal load 6.012 Spring 2009 Lecture 23 2 • Voltage gain ≈ 1 • High input resistance • Low output resistance • ⇒ Good voltage buffer Characteristics of CD Amplifier: VBIAS VSS -High-frequency small-signal model G S-B D Could use OTC to solve for bandwidth. To estimate bandwidth it is easier to use the 2-port models. 6.012 Spring 2009 Lecture 23 3RRLow Frequency Analysis Using 2-Port Model Capacitors -open circuit Avo = Rin RS + Rin (1) RL RL + Rout ≈ gm RL 1 + gm RL ≤ 1 In the calculation of the intrinsic voltage gain we assume that ro||roc was large. That is why we do not have RL|| ro||roc 6.012 Spring 2009 Lecture 23 4outLmLHigh Frequency Analysis Using 2-Port Model - Add capacitors AvC gs = RL R + R = gm RL 1+ g R Use Miller Approximation gs Rout + RL 1+ gm RL CM = Cgs 1− AvC gs( )= Cgs 1− gmRL 1+ gmRL = Cgs 1+ gmRL CT = Cgs 1− gmRL 1+ gmRL + Cgd RT = RS || Rin = RS Total capacitance at input Add time constant due to Cdb capacitance at output RCdb = Rout RL = Rout RL Rout + RL = RL 1+ gm RL 6.012 Spring 2009 Lecture 23 5Frequency Response of Common-Drain Amplifier ω3dB ≈ 1 RS C gs 1+ gm RL + Cgd + Cdb RL 1+ gm RL 6.012 Spring 2009 Lecture 23 6 If RSis not too high, bandwidth can be rather high and approach ωT.2. Frequency Response of the Common-Gate Amplifier VDD iSUP iOUT signal load VSS RL signal source • Current gain ≈ 1 • Low input resistance • High output resistance • ⇒ Good current buffer Characteristics of CG Amplifier: is VSS IBIASRS 6.012 Spring 2009 Lecture 23 7High Frequency Small Signal Model Could use OTC to solve for bandwidth. To estimate bandwidth it is easier to use the 2-port models. 6.012 Spring 2009 Lecture 23 8iR+R R+R ==High-frequency small-signal 2-port model Assume backgate is shorted to source Low frequency transfer function: Aio = iout = RS (1) Rout Aio is Rin + RS (1) RL + Rout Use OTC to find ωωωω3dB: Thevenin resistance across Cgs + Csb RTCgs = RS || Rin = RS || 1 / gm + gmb( )( ) Thevenin resistance across Cgd + Cdb ||)||)((|| LocSomoLoutTC RrRrgrRRR gd +== 6.012 Spring 2009 Lecture 23 9τ=((r+grR)||r))||RC+CHigh-frequency small-signal 2-port model con’t Open circuit time constants: τCgs +Csb =(RS || (1 / gm + gmb ))(Cgs + Csb ) ( )() τCgd +Cdb = ((r o + gm r o RS ) || r oc )) || RL( ) Cgd + Cdb ( ) Summing the open circuit time constants: ω3dB = 1 RS || (1 / gm + gmb )( )(C gs + Csb ) + ((r o + gm r o RS ) || r oc )) || RL( ) Cgd + Cdb ( ) 6.012 Spring 2009 Lecture 23 10 If RLis not too high, bandwidth can be rather high and approach ωT.What did we learn today? Summary of Key Concepts • Common-drain amplifier: – Voltage gain ≈ 1, Miller Effect nearly completely eliminates the effect of Cgs – If RS is not too high, CD amplifier has high bandwidth • Common-gate amplifier – No Miller Effect because there is no feedback capacitor – If RL is not too high, CG amplifier has high bandwidth • RS, RL can affect bandwidth of amplifiers. 6.012 Spring 2009 Lecture 23 11MIT OpenCourseWarehttp://ocw.mit.edu 6.012 Microelectronic Devices and Circuits Spring 2009 For information about citing these materials or our Terms of Use, visit:
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