Lecture 12 Single Stage FET Amplifiers: Common Gate Amplifier Common Drain Amplifier The Building Blocks of Analog Circuits - II In this lecture you will learn: • Common Gate (CG) and Common Drain (CD) Amplifiers • Small signal models of amplifiers ECE 315 – Spring 2007 – Farhan Rana – Cornell University The Common Source Amplifier VDD R IOUT + iout VOUT vout D ID + id RS G + + I + i R v G g L VOUT vout s - - S + VBIAS - An attribute of the common source amplifier: The input resistance is very large: Rin Not suitable for current amplifiers or transimpedance amplifiers ECE 315 – Spring 2007 – Farhan Rana – Cornell University 1 The Common Gate Amplifier VDD R Assume an AC short I + i I + i IS + is D d OUT out R S + I + i + G g RL VOUT vout IBIAS + v s - - VBIAS - The gate terminal is “common” between the input and the output The common gate amplifiers are useful when small input resistances and large output resistances are desired in amplifiers (they also have good high frequency performance – but that will come later in the course) But the current gain is unity! Note: The bulk is not tied to the source ECE 315 – Spring 2007 – Farhan Rana – Cornell University The Common Gate Amplifier: Same Topology, Different Look VDD R IOUT + iout I + i D D d IG + ig G + Assume an S RL VOUT vout IS + is - + AC short V BIAS RS - IBIAS + v s - The gate terminal is “common” between the input and the output The common gate amplifiers are useful when small input resistances and large output resistances are desired in amplifiers But the current gain is unity! Note: The bulk is not tied to the source ECE 315 – Spring 2007 – Farhan Rana – Cornell University 2 Choice of Substrate Potentials D S B D S B N+ P+ NN P+ P P N+ N well P Substrate In order to keep the source and drain PN-junctions with the substrate (or bulk) reverse biased at all times, i) The P-substrate (for NFETs) is generally tied to the most negative voltage in the circuit ii) The N-substrate or N-well (for PFETs) is tied to the most positive voltage in the circuit ECE 315 – Spring 2007 – Farhan Rana – Cornell University The Common Gate Amplifier: Small Signal Model i ig Gate Drain id out + v gs r g v o + - m gs gmbvbs R vout RL + Source - - vbs RS + vin + v s - - Base Note: vbs v gs ECE 315 – Spring 2007 – Farhan Rana – Cornell University 3 The Common Gate Amplifier: Short Circuit Current Gain ig Gate Drain id + v gs r g v o - m gs gmbvbs R Source - iout vbs + i test Base Short circuit current gain: itest id iout iout Ai 1 Short circuit current gain is unity! itest ECE 315 – Spring 2007 – Farhan Rana – Cornell University The Common Gate Amplifier: Input Resistance i ig Gate Drain id out + v gs r g v o + - m gs gmbvbs R vout RL Source vs vgs - - vbs i test + + Base vtest - Input resistance: v v gm gmb ro 1 out gs itest id vgs id gm gmb vgs ro R || RL ro v vtest vgs ro R || RL gs R || RL Rin gm gmb vgs id itest itest gm gmb ro 1 ro ro v R can be small if: g g v gs in m mb gs r i o ggrmmbo1 d R || R 1 L ro rRRoL ||1 RR || L gm gmb ro 1 R 1 vgs in Small r R || R ggrmmbo1 gg mmb r o o L ECE 315 – Spring 2007 – Farhan Rana – Cornell University 4 The Common Gate Amplifier: Open Circuit Voltage Gain ig Gate Drain id + v gs r g v o - m gs gmbvbs R + Source - vout vbs - + itest + Base vtest - Open circuit voltage gain: g g r 1 1 v v v i R R m mb o v g g R || r v out gs out d gs m mb o gs id gm gmb vgs ro R ro ro v v 1 vgs R out out g g v i Av gm gmb R || ro m mb gs d vtest vgs ro ro ro gm gmb ro 1 id vgs If: ro R gm gmb ro 1 1 Av gm gmb R || ro ~ gm gmb R || ro Large if R is large ro ECE 315 – Spring 2007 – Farhan Rana – Cornell University The Common Gate Amplifier: Output Resistance ig Gate Drain id + v gs r g v o itest - m gs gmbvbs R Source - + vs vgs v bs vtest - + RS Base RD Output resistance: vtest v gs v gs id gm gmb v gs ro Rs 1 ro || || Rs gm gmb vgs vtest ro vtest roRs RD ro Rs 1 ro gm gmb id 1 ro || || Rs gm gmb ECE 315 – Spring 2007 – Farhan Rana – Cornell University 5 The Common Gate Amplifier: Output Resistance ig Gate Drain id + v gs r g v o itest - m gs gmbvbs R Source - + v bs vtest - + RS Base RD Output resistance: RD ro Rs roRs gm gmb Rout R || RD Rout depends on Rs R || ro Rs roRs gm gmb Rout can be large if R is large ECE 315 – Spring 2007 – Farhan Rana – Cornell University The Common Gate Amplifier: Transimpedance Gain ig Gate Drain id + v gs r g v o - m gs gmbvbs R + Source - vout vbs - + itest + Base vtest - Transimpedance gain: itest id vout idR Rm R itest itest Rm can be large if R is large ECE 315 – Spring 2007 – Farhan Rana – Cornell University 6 The Common Gate Amplifier: A Current Buffer ig Gate Drain id + iout v gs r g v o + - m gs gmbvbs R vout RL Source - - vbs + is RS Base Compare with the standard current amplifier model: Rout can be large i i in out R is small + + in i R Ai iin R v R s S Rin out out L A is unity - - i It is a good current buffer!! ECE 315 – Spring 2007 – Farhan Rana – Cornell University The Common Gate Amplifier: A Transimpedance Amplifier ig Gate Drain id + iout v gs r g v o + - m gs gmbvbs R vout RL Source - - vbs + is RS Base Compare with the standard transimpedance amplifier model: Rout can be large R iin out Rin is small + + + i R R i v R can be large s S Rin - m in out RL m - - It is a decent transimpedance amplifier!! ECE 315 – Spring 2007 – Farhan Rana – Cornell University 7 The Common Drain Amplifier (or the Source Follower) VDD R I + i D D d IG + ig G RS S IOUT + iout + IS + is v s - + + Ro RL VOUT vout - VBIAS - The drain terminal is “common” between the input and the output The common drain amplifiers are useful when large input resistances and small output resistances are desired in voltage amplifiers The voltage gain is less than unity! Note: The bulk is not tied to the source ECE 315 – Spring 2007 – Farhan Rana – Cornell University The Common Drain Amplifier: Small Signal Model ig Gate Drain id + v gs ro R Rs - gmvgs gmbvbs i + s v s Source - + - v R bs vout R o + L Base - Note: vbs vout ECE 315 – Spring 2007 – Farhan Rana – Cornell University 8 The Common Drain Amplifier: Open Circuit Voltage Gain i Drain i g Gate d vd + v gs ro R - gmvgs gmbvbs + vtest is - Source - + v R bs v o + out Base - Open circuit voltage gain: vtest vgs vout vgs idRo vd vout id gmv gs gmbvout ro vout gmrovgs 1 gmbro vout id i R v Ro ro R g v g v d out m gs mb out r 1 1 g r g r g r o v mb o v m o v v m o out R r R gs r R test out r R gmrov gs 1 gmbro vout o o o o id ro R 1 1 gmbro gmro gmro vout vtest Ro ro R ro R ro R gmroRo v r R A out o 1 v g g r R Less than unity – but can be very vtest 1 m mb o o ro R close to unity ECE 315 – Spring 2007 – Farhan Rana – Cornell University The Common Drain Amplifier: Input Resistance ig Gate Drain id + itest v gs ro R - gmvgs gmbvbs i + s vtest Source - + v - R bs v R o + out L Base - Input resistance: vtest Rin Large itest ECE 315 – Spring 2007 – Farhan Rana – Cornell University 9 The Common Drain Amplifier: Output Resistance i Drain i g Gate d vd + v gs ro R - gmvgs gmbvbs R i s is test Source - v + R bs v o + test Base - Output resistance: vtest vd vtest itest id id gmv gs gmbvtest Ro ro v g r v 1 g r v id R vtest test m o gs mb o test gmv gs gmbvtest ro Ro ro R gmrov gs 1 gmbro vtest vtest gmro 1 gmbro id vtest ro R Ro ro R 1 itest 1 gm gmb ro 1 Rout can be small if: Rout vtest Ro ro R ro >> R (gm+gmb) ro >>1 and/or if Ro is small ECE 315 – Spring 2007 – Farhan Rana – Cornell University The Common Drain Amplifier: A Voltage Buffer ig Gate Drain id + v gs ro R Rs - gmvgs gmbvbs i + s v s Source - + v - R bs v R o + out L Base - Compare with the standard voltage amplifier model: Rout can be small Rin is large Rout RS A can be almost + + + v v R A v vout R unity + in in - v in L v s - - - It is a good voltage buffer!! ECE 315 – Spring 2007 – Farhan Rana – Cornell University 10 The CS, CG, and CD Topologies The common source (CS) topology: 1) Large input resistance 2) Large output resistance 3) Decent voltage gain 4) A decent voltage amplifier 5) A decent transconductance amplifier The common gate (CG) topology: 1) Can have small input resistance 2) Large output resistance 3) Decent voltage gain 4) Unity current gain 5) A good current buffer 6) A decent voltage amplifier 7) A decent transimpedance amplifier The common drain (CD) topology: 1) Large input resistance 2) Small output resistance 3) Less than unity voltage gain 4) A good voltage buffer ECE 315 – Spring 2007 – Farhan Rana – Cornell University 11.