1EE105 - Fall 2006Microelectronic Devices and CircuitsProf. Jan M. Rabaey (jan@eecs)Lecture 12: Amplifiers – General Concepts2Administrativia Hw 5 posted. Midterm 1 graded23Midterm 1 Total: 30 Average: 15.65 Median: 15.25 StdDev: 6.72 Max: 29 Min: 34Overview Last lecture– Voltage and Current Sources This lecture– Moving to Amplifiers (General)35A Current SourceiOUTvOUT+-ro2-nIREFn = (W/L)2/(W/L)1Region of validity?6Current Sinks and SourcesSink: output current goesto groundSource: output current comesfrom voltage supply47Current MirrorsIdea: we only need one reference current to set up all the currentsources and sinks needed for a multistage amplifier.8Application of Current Mirrors:Digital-to-Analog ConverterRREFMREFVDDRLvOUTD0D1D2D4Digital input: word D0D1D2D3 Æ voltages are either VDDor 0 V.Transistors M0, M1, M2, M3 have binary-weighted (W/L) ratiosM0M1M2M3iDoiD1iD2iD359ExampleInput word D0D1D2D3 = 0101IREF= 100 μA (W/L)REF= (W/L)0(W/L)1= 2(W/L)0(W/L)2= 2(W/L)1(W/L)3= 2(W/L)2Output voltage is vOUT= VDD– RL(iD0+ iD2)= VDD– RL(IREF+ 4 IREF)Sources of error: fab imprecision, channel-lengthmodulation, …10Moving to Amplifiers Sources: Signal, its source resistance, and bias voltage or current Load: Use resistor/current source or in Chap. 8, but could be a general impedance Port: A pair of terminals across which a voltage andcurrent are defined– Source, Load: “one port”– Amplifier: “two port”Amplifier Terminology611Two-Port Models (Amplifiers)iniCircuitBlockinv+−outv+−outi12One-Port Models (EECS 40) A terminal pair across which a voltage and associated current are definedCircuitBlockabv+−abithevvthevRabv+−abithevithevRabv+−abiOnly 1 independent variable713(Small-Signal) Two-Port Models General case: four variables, vin, iin, vout, iout:– Two independent, two dependent Four choices (Math 54):– iin= f(vin, vout), iout= f(vin, vout)– vin= f(iin, vout), iout= f(iin, vout)– iin= f(vin, iout), vout= f(vin, iout)– vin= f(iin, iout), vout= f(iin, iout)inv+−outv+−outiini14Small-Signal Two-Port Models We assume that input port is linear and that the amplifier is unilateral: – Output depends on input but input is independent of output. Output port : depends linearly on the current and voltage at the input and output ports Unilateral assumption is good as long as “overlap”capacitance is small (MOS)inv+−outv+−outiini815A Mathematical PerspectiveCan write linear system of equations for either ioutor voutin terms of two of iin, vin, iout, or vout: possibilities are12out in outivvαα=+34out in outiivαα=+56out in outvviαα=+78out in outviiαα=+What is physical meaning of α1? of α6?16EE Perspective Four amplifier types: determined by the output signal and the input signal … both of which we select (usually obvious)– Voltage Amp (VÆV)– Current Amp (IÆI)– Transconductance Amp (VÆI)– Transresistance Amp (IÆV) Need methods to find the 6 α parameters for the four models and equivalent circuits for unilateral two ports917Generalized AmplifiersvSBIASsvVv=+ovBIASVDDVSSVBIASIsiSRDDILDD DiIi=−()Dinifv=LRinvActive Device18Two-Port Small-Signal AmplifiersinisisRinRoutRLRiinAiCurrent AmplifiersvsRinRoutRLRvinAvinv+−Voltage Amplifier1019Two-Port Small-Signal AmplifierssisRinRoutRLRminRiiniTransresistance AmplifiersvsRinRoutRLRminGvinv+−Transconductance Amplifier20Input Resistance RinLooks like a Thevenin resistance measurement, but note that theoutput port has the load resistance attachedattachedRremovedRttinLSivR,=1121Output Resistance RoutLooks like a Thevenin resistance measurement, but note that theinput port has the source resistance attachedattachedRremovedRttoutSLivR,=22Finding the Voltage Gain AvKey idea: the output port is open-circuited and the source resistance is shorted1223Finding the Current Gain AiKey idea: the output port is shorted and the sourceresistance is removed ,0SLoutiinRRiAi→∞ ==24Finding the Transresistance Rm1325Finding the Transconductance Gm26Common-Source Amplifier (again)How to isolate DC level?1427DC BiasNeglect all AC signals5 V2.5 VChoose IBIAS, W/L28Load-Line Analysis to find QQDDDoutRDVVIR−=110kslope =0V10kDI =5V10kDI =1529Small-Signal AnalysisinR =∞30svsRinRoutRLRminGvinv+−Two-Port Parameters:Find Rin, Rout, GminR =∞mmGg=||out o DRrR=Generic Transconductance Amp1631Two-Port CS ModelReattach source and load
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