6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 19-1 Lecture 19 - Transistor Amplifiers (I) Common-Source Amplifier November 15, 2005 Contents: 1. Amplifier fundamentals 2. Common-source amplifier 3. Common-source amplifier with current-source supply Reading assignment: Howe and Sodini, Ch. 8, §§8.1-8.6 Announcements: Quiz 2: 11/16, 7:30-9:30 PM, open book, must bring calculator; lectures #10-18. Quiz 2 TA Review Session: 11/15, 7:30-9:30 PM,6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 19-2 Key questions • What are the key figures of merit of an amplifier? • How can one make a voltage amplifier with a single MOSFET and a resistor? • How can this amplifier be improved?6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 19-3 1. Amplifier fundamentals Goal of amplifiers: signal amplification. +V -V vIN vOUT + + --vIN vOUT input signal output signal RL Features of amplifier: • Output signal is faithful replica of input signal but amplified in magnitude. • Active device is at the heart of amplifier. • Need linear transfer characteristics for distortion not to be introduced.6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 19-4 Signal could be represented by current or voltage ⇒ four broad types of amplifiers: vs + − RS RL vout RL RS vs + − iout + − voltage amplifier transconductance amplifier + − voutRLRS is transresistance amplifier RLRS is iout current amplifier6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 19-5 More realistic transfer characteristics: vIN vOUT input signal output signal Q • Transfer characteristics linear over limited range of voltages: amplifier saturation. • Amplifier saturation limits signal swing. • Signal swing also depends on choice of bias point, Q (also called quiescent point or operating point). Other features desired in amplifiers: • Low power consumption. • Wide frequency response [will discuss in a few days]. • Robust to process and temperature variations. • Inexpensive: must minimize use of unusual compo-nents, must be small (in Si area)6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 19-6 2. Common-Source Amplifier Consider the following circuit: V+ =VDD vs VGG vOUT iR iD RD RS signal source + -signal load RL -V =VSS Consider it first unloaded by RL. How does it work? • VGG, RD and W/L of MOSFET selected to bias tran-sistor in saturation and obtain desired output bias point (i.e. VOU T = 0). • vGS ↑⇒iD ↑⇒iR ↑⇒vout ↓ • Av = vout < 0; output out of phase from input, but if vs amplifier well designed, |Av| > 1. [watch notation: vOU T (t)= VOU T + vout(t)]6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 19-7 Load line view of amplifier: IR=ID load line VDD-VSS VGG-VSS=VDD-VSS RD VGG-VSS VGG-VSS=VT 0 VSS VDD VOUT Transfer characteristics of amplifier: VOUT VGG-VSS0 VDD-VSSVT VSS VDD Want: • Bias point calculation; • small-signal gain; • limits to signal swing • frequency response [in a few days]� � � � � = � 6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 19-8 2 Bias point: choice of VGG, W/L, and RD to keep transistor in saturation and to get proper quiescent VOU T . Assume MOSFET is in saturation: W ID = µnCox(VGG − VSS − VT )2 2LVDD − VOU T IR = RD If we select VOU T =0: W ID = IR = µnCox(VGG − VSS − VT )2 = VDD 2L RD Then: 2VDDVGG � W + VSS + VTRD L µnCoxaded6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 19-9 2 Small-signal voltage gain: draw small-signal equiva-lent circuit model: G S D + -vin + -vgs + -voutgmvgs ro RD + -vin + -voutgmvin ro//RD vout = −gmvin(ro//RD) Then unlo voltage gain:unloaded voltage gain: voutAvo = = −gm(ro//RD) vin6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 19-10 2 Signal swing: VDD vs VGG vOUT RD RS signal source + -VSS • Upswing: limited by transistor going into cut-off: vout,max = VDD • Downswing: limited by MOSFET entering linear regime: VDS,sat = VGS − VT or vout,min − VSS = VGG − VSS − VT Then: vout,min = VGG − VT6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 19-11 2 Effect of input/output loading: VDD vs VGG vOUT iR iL iD RD RS RL signal source + -VSS • Bias point not affected because selected VOU T =0. • Signal swing: – Upswing limited by resistive divider: RL vout,max = VDD RL + RD – Downswing not affected by loading • Voltage gain: – input loading (RS ): no effect because gate does not draw current; – output loading (RL): RL detracts from voltage gain because it draws current. |Av| = gm(ro//RD//RL) <gm(ro//RD)6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 19-12 2 Generic view of loading effect on small-signal operation: Two-port network view of small-signal equivalent circuit model of voltage amplifier: Rin is input resistance Rout is output resistance Avo is unloaded voltage gain + -vin + -voutAvovin Rout + -Rin RS vs + -RL input unloaded circuit output loading loading vsVoltage divider at input: vin = RinRin+RS AvovinVoltage divider at output: vout = RLRout+RL Loaded voltage gain: vout Rin RLAv = = Avo vs Rin + RS RL + Rout6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 19-13 • Calculation of input resistance, Rin: - load amplifier with RL - apply test voltage (or current) at input, measure test current (or voltage) For common-source amplifier: it + vt + -vgs gmvgs ro//RD RL -vtit =0 ⇒ Rin = = ∞ it No effect of loading at input.6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 19-14 • Calculation of output resistance, Rout: - load amplifier at input with RS - apply test voltage (or current) at output, measure test current (or voltage) For common-source amplifier: + + --vgs vt it gmvgs ro//RDRS vgs =0 ⇒ gmvgs =0 ⇒ vt = it(ro//RD) vtRout = = ro//RDit6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 19-15 Two-port network view of common-source amplifier: RS Rout + -vin + -voutAvovin + -Rinvs + -RL input unloaded circuit output loading loading voutAv = vs Rin RL RL = Avo = −gm(ro//RD)Rin + RS RL + Rout RL + ro//RD Or: Av = −gm(ro//RD//RL)� � � �6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 19-16 2 Design issues of common-source amplifier (unloaded): Examine bias dependence: |Avo| =
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