6.012 Microelectronic Devices and Circuits Spring 2001 Lecture 191 Lecture 19 Transistor Amplifiers (I) CommonSource Amplifier April 24, 2001 Contents: 1. Amplifier fundamentals 2. Commonsource amplifier 3. Commonsource amplifier with currentsource supply Reading assignment: Howe and Sodini, Ch. 8, 8.18.6
6.012 Microelectronic Devices and Circuits Spring 2001 Lecture 192 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 Spring 2001 Lecture 193 1. Amplifier fundamentals Goal of amplifiers: signal amplification. V v OUT v IN vout output signal v IN V input signal 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 Spring 2001 Lecture 194 Signal could be represented by current or voltage four broad types of amplifiers: R S voltage v s v amplifier out R S i out transconductance amplifier i s R S transresistance amplifier v out i out i s R S current amplifier
6.012 Microelectronic Devices and Circuits Spring 2001 Lecture 195 More realistic transfer characteristics: v OUT Q output signal v IN input signal 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 unusual components, must be small (in Si area)
6.012 Microelectronic Devices and Circuits Spring 2001 Lecture 196 2. CommonSource Amplifier Consider the following circuit: V =V DD R D i R signal source R S i D signal load v OUT V GG V =V SS Consider it first unloaded by. How does it work? V GG, R D and W/L of MOSFET selected to bias transistor in saturation and obtain desired output bias point (i.e. V OUT = 0). v GS i D i R v out A v = v out < 0; output out of phase from input, but if amplifier well designed, A v > 1. [watch notation: v OUT (t) =V OUT v out (t)]
6.012 Microelectronic Devices and Circuits Spring 2001 Lecture 197 Load line view of amplifier: I R =I D load line V DD V SS R D V GG V SS =V DD V SS V GG V SS 0 V SS V DD V GG V SS =V T V OUT Transfer characteristics of amplifier: V OUT V DD V SS 0 V T V DD V SS V GG V SS Want: Bias point calculation; smallsignal gain; limits to signal swing frequency response [in a few days]
6.012 Microelectronic Devices and Circuits Spring 2001 Lecture 198 Bias point: choice of V GG, W/L, and R D to keep transistor in saturation and to get proper quiescent V OUT. Assume MOSFET is in saturation: I D = W 2L µ nc ox (V GG V SS V T ) 2 If we select V OUT =0: I R = V DD V OUT R D Then: I D = I R = W 2L µ nc ox (V GG V SS V T ) 2 = V DD R D V GG = 2V DD R D W L µ nc ox V SS V T
6.012 Microelectronic Devices and Circuits Spring 2001 Lecture 199 Smallsignal voltage gain: draw smallsignal equivalent circuit model: R D G v in v gs gm v gs D r o v out S v in gm v in r o //R D v out v out = g m v in (r o //R D ) Then unloaded voltage gain: A vo = v out v in = g m (r o //R D )
6.012 Microelectronic Devices and Circuits Spring 2001 Lecture 1910 Signal swing: V DD signal source R S R D v OUT V GG V SS Upswing: limited by transistor going into cutoff: v out,max = V DD Downswing: limited by MOSFET entering linear regime: V DS,sat = V GS V T or v out,min V SS = V GG V SS V T Then: v out,min = V GG V T
6.012 Microelectronic Devices and Circuits Spring 2001 Lecture 1911 Effect of input/output loading: V DD signal source R S R D i R i D i L v OUT V GG V SS Bias point not affected because selected V OUT =0. Upswing limited by resistive divider: v out,max = V DD R D Downswing not affected by loading Voltage gain: input loading (R S ): no effect because gate does not draw current; output loading ( ): detracts from voltage gain because it draws current. A v = g m (r o //R D // ) <g m (r o //R D )
6.012 Microelectronic Devices and Circuits Spring 2001 Lecture 1912 Generic view of loading effect on smallsignal operation: Twoport network view of smallsignal equivalent circuit model of voltage amplifier: R in is input resistance R out is output resistance A vo is unloaded voltage gain R s R out v in R in Avo v in v out input loading unloaded circuit output loading Voltage divider at input: v in = R in R in R S Voltage divider at output: Loaded voltage gain: v out = A vo v in R out A v = v out = R in A vo R in R S R out
6.012 Microelectronic Devices and Circuits Spring 2001 Lecture 1913 Calculation of input resistance, R in : load amplifier with apply test voltage (or current) at input, measure test current (or voltage) For commonsource amplifier: i t v t v gs g m v gs r o //R D i t =0 R in = v t i t = No effect of loading at input.
6.012 Microelectronic Devices and Circuits Spring 2001 Lecture 1914 Calculation of output resistance, R out : load amplifier at input with R S apply test voltage (or current) at output, measure test current (or voltage) For commonsource amplifier: R S v gs g m v gs r o //R D i t vt v gs =0 g m v gs =0 v t = i t (r o //R D ) R out = v t i t = r o //R D
6.012 Microelectronic Devices and Circuits Spring 2001 Lecture 1915 Twoport network view of commonsource amplifier: R s R out v in R in Avo v in v out input loading unloaded circuit output loading A v = v out = R in A vo = g m (r o //R D ) R in R S R out r o //R D Or: A v = g m (r o //R D // )
6.012 Microelectronic Devices and Circuits Spring 2001 Lecture 1916 Design issues of commonsource amplifier (unloaded): Examine bias dependence: A vo = g m (r o //R D ) g m R D Rewrite A vo in the following way: A vo g m R D = W 2I D L µ V DD nc ox I D V DD ID Then, to get high A vo : V DD I D Both approaches imply R D = V DD I D Consequences of high R D : large R D consumes a lot of Si real state large R D eventually compromises frequency response Also, it would be nice not to use any resistors at all! Need better circuit.
6.012 Microelectronic Devices and Circuits Spring 2001 Lecture 1917 3. Commonsource amplifier with currentsource supply V DD i SUP signal source RS i D signal load v OUT V GG V SS Loadline view: i SUP =I D load line V GG V SS =V DD V SS I SUP V GG V SS 0 V SS V DD V GG V SS =V T V OUT
6.012 Microelectronic Devices and Circuits Spring 2001 Lecture 1918 Current source characterized by high output resistance: r oc. Then, unloaded voltage gain of commonsource stage: A vo = g m (r o //r oc ) significantly higher than amplifier with resistive supply. Can implement current source supply by means of p channel MOSFET: V DD V B i SUP signal source R S i D v OUT V GG V SS
6.012 Microelectronic Devices and Circuits Spring 2001 Lecture 1919 Relationship between circuit figures of merit and device parameters Remember: g m = 2 W L µ nc ox I D Then: r o 1 λ n I D L I D Circuit Parameters Device A vo R in R out Parameters g m (r o //r oc ) r o //r oc I SUP W µ n C ox L adjustments are made to V GG so none of the other parameters change CSamp with current supply source is good voltage amplifier (R in high and A v high), but R out high too voltage gain degraded if r o //r oc.
6.012 Microelectronic Devices and Circuits Spring 2001 Lecture 1920 Key conclusions Figures of merit of an amplifier: gain signal swing power consumption frequency response robustness to process and temperature variations Commonsource amplifier with resistive supply: tradeoff between gain and cost and frequency response. Tradeoff resolved by using commonsource amplifier with current source supply. Twoport network computation of voltage gain, input resistance and output resistance of amplifier.