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Common Gate Amplifier Is Often Used As a Current Buffer I.E

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Common Gate Amplifier Is Often Used As a Current Buffer I.E Lecture 20 Transistor Amplifiers (III) Other Amplifier Stages Outline • Common-drain amplifier • Common-gate amplifier Reading Assignment: Howe and Sodini; Chapter 8, Sections 8.7-8.9 6.012 Electronic Devices and Circuits—Fall 2000 Lecture 20 1 Summary of Key Concepts • Common-drain amplifier: good voltage buffer – Voltage gain » 1 – High input resistance – Low output resistance • Common-gate amplifier: good current buffer – Current gain » 1 – Low input resistance – High output resistance 6.012 Electronic Devices and Circuits—Fall 2000 Lecture 20 2 1. Common-drain amplifier • A voltage buffer takes the input voltage which may have a relatively large Thevenin resistance and replicates the voltage at the output port, which has a low output resistance • Input signal is applied to the gate • Output is taken from the source • To first order, voltage gain » 1 – vs » vg. • Input resistance is high • Output resistance is low – Effective voltage buffer stage How does it work? • vG •Þ iD cannot change Þ vS • – Source follower 6.012 Electronic Devices and Circuits—Fall 2000 Lecture 20 3 Biasing the Common-drain amplifier • VGG, ISUP, and W/L selected to bias MOSFET in saturation • Obtain desired output bias voltage – Typically set VOUT to”halfway” between VSS and VDD. • Output voltage maximum VDD-VDSsat • Output voltage minimum set by voltage requirement across ISUP. VBIAS = VGG = VGS + VOUT I = + SUP VGS VTn(VSB) W mnCox 2L 6.012 Electronic Devices and Circuits—Fall 2000 Lecture 20 4 Small-signal Analysis Unloaded small-signal equivalent circuit model: vin = vgs + vout vout = gmvgs(ro // roc ) Then: g = m » Avo 1 1 gm + ro // roc 6.012 Electronic Devices and Circuits—Fall 2000 Lecture 20 5 Input and Output Impedance Input Impedance : Rin = ¥ Output Impedance: 1 Rout = 1 gm + ro // roc 6.012 Electronic Devices and Circuits—Fall 2000 Lecture 20 6 Effect of Back Bias If MOSFET was not fabricated on isolated p-well, then body is tied to wafer substrate (connected to VSS) Two consequences: • Bias is affected – VT depends on VBS – VBS = VSS – VOUT ¹ 0 • Small signal figures of merit affected – Signal shows up between B and S – vbs = -vout 6.012 Electronic Devices and Circuits—Fall 2000 Lecture 20 7 Small-signal Analysis (with back-bias) gm gm Avo = » <1 1 g + g + gm + gmb + m mb ro // roc Also: 1 1 R = » out 1 g + g gm + gmb + m mb ro // roc 6.012 Electronic Devices and Circuits—Fall 2000 Lecture 20 8 Common-Drain Two-Port Model • Input resistance >> CS Amplifier – We want a large input resistance because the controlled generator is voltage controlled • Output resistance << CS Amplifier – We want a low output resistance to deliver most of the output voltage to the load 6.012 Electronic Devices and Circuits—Fall 2000 Lecture 20 9 Relationship between circuit figures of merit and device parameters: W gm = 2ID mnCox L g gmb = gm 2 -2fp - VBS Circuit Parameters Device* |A | R R Parameters vo in out gm µ 1 gm + gmb gm + gmb I - - SUP • ¯ W • - - ¯ C - - mn ox • ¯ L • - - • * the bias VGG is adjusted so that none of the other parameters change Common Drain amplifier is often used as a voltage buffer to drive small output loads (in multistage amplifiers, other stages provide the voltage gain). 6.012 Electronic Devices and Circuits—Fall 2000 Lecture 20 10 2. Common-Gate Amplifier: • A current buffer takes the input current which may have a relatively small Norton resistance and replicates the current at the output port, which has a high output resistance • Input signal is applied to the source • Output is taken from the drain • To first order, current gain » 1 – is » -iout.(Current Buffer) • Input resistance is low • Output resistance is high – Effective current buffer stage 6.012 Electronic Devices and Circuits—Fall 2000 Lecture 20 11 Biasing the Common-Gate Amplifier: Select ISUP, IBIAS, and W/L to obtain the proper quiescent IOUT and keep MOSFET in saturation. ISUP + IOUT + IBIAS = 0 Select bias such that IOUT=0 Þ VOUT = 0. Assume MOSFET in saturation (no channel modulation): W 2 I = m C (V - V ) = I = -I D 2L n ox GS T SUP BIAS But VT depends on VBS: VT = VTo + gn( -2fp - VBS - -2fp ) Must solve these two equations iteratively. 6.012 Electronic Devices and Circuits—Fall 2000 Lecture 20 12 Small-signal equivalent circuit (unloaded) iout iin = -iout ÞAio = =1 iin Aio is the short circuit current gain. Not surprising, since in a MOSFET: ig = 0 6.012 Electronic Devices and Circuits—Fall 2000 Lecture 20 13 Input Impedance Do KCL on input node: vt - it(roc // RL ) it - gmvt - gmbvt - = 0 ro Then: r // R 1 + oc L vt ro 1 Rin = = » i 1 g + g t gm + gmb + m mb ro 6.012 Electronic Devices and Circuits—Fall 2000 Lecture 20 14 Output Impedance Do KCL on input node: v t¢ + vgs i t¢ - gmvgs - gmbvgs - = 0 ro Notice also: vgs = -it¢ Rs Then: æ é ùö ç æ 1 ö ÷ Rout = roc // ro ê1+ Rs çgm + gmb + ÷ú » roc //[ro (1+ gmRs )] ç ç r ÷ ÷ è ë è o øûø 6.012 Electronic Devices and Circuits—Fall 2000 Lecture 20 15 Common-Gate Two-Port Model • The output resistance depends on the source resistance – The CG current buffer is not unilateral • Input resistance << CS Amplifier – We want a small input resistance because the controlled generator is current controlled • Output resistance >> CS Amplifier – We want a large output resistance to deliver most of the output current to the load 6.012 Electronic Devices and Circuits—Fall 2000 Lecture 20 16 Relationship between circuit figures of merit and device parameters: W g = 2I m C m D L n ox g gmb = gm 2 - 2fp - VBS 1 r » o l I n D Circuit Parameters Device* |A | R R Parameters io in out -1 1 roc //[ro (1+ gm R s )] gm + gmb I - ? SUP • ¯ W • - ¯ ? C - ? mn ox • ¯ L • - • ? * the bias VGG is adjusted so that none of the other parameters change Common Gate amplifier is often used as a current buffer i.e. transform a current source with medium source resistance to an equal current with high source resistance (in multistage amplifiers, other stages provide the current gain). 6.012 Electronic Devices and Circuits—Fall 2000 Lecture 20 17 What did we learn today? Summary of Key Concepts • Common-source amplifier: good voltage amplifier – Large voltage gain – High input resistance – Medium / high output resistance • Common-drain amplifier: good voltage buffer – Voltage gain » 1 – High input resistance – Low output resistance • Common-gate amplifier: good current buffer – Current gain » 1 – Low input resistance – High output resistance 6.012 Electronic Devices and Circuits—Fall 2000 Lecture 20 18.
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