Amplifier Configurations in Microelectronic Circuits | ECE 3040, Study notes of Electrical and Electronics Engineering

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ECE 3040 - Dr. Alan DoolittleGeorgia Tech Lecture 27 Amplifier Configurations Reading: CE/CS: Jaeger 13.6, 13.9, 13.10, 13.11 CC/CD: Jaeger 14.1, 14.3 CB/CG: Jaeger 14.1, 14.4 and Notes ECE 3040 - Dr. Alan DoolittleGeorgia Tech Amplifier Configurations Voltage Amplifier: Voltage input and Voltage output The controlled source is a Voltage-controlled-Voltage Source Av=Open Circuit Voltage Gain can be found by applying a voltage source with Rs=0, and measuring the open circuit output voltage(no load or RL=infinity) Source LoadAmplifier A m pl ifi er In pu t R es is ta nc e So ur ce In pu t R es is ta nc e L oa d R es is ta nc e A m pl ifi er O ut pu t R es is ta nc e •Any signal source has a finite “source resistance”, RS . •The amplifier is often asked to drive current into a load of finite impedance, RL (examples: 8 ohm speaker, 50 ohm transmission line, etc…) ECE 3040 - Dr. Alan DoolittleGeorgia Tech Amplifier Configurations Transconductance Amplifier: Voltage input and Current output The controlled source is a Voltage-controlled-Current Source Gm=Transconductance Gain can be found by applying a voltage source with Rs=0, and measuring the short circuit output current (No Load or RL=0) •Only the voltage vin is amplified to iout=Gmvin. •Since Rs and Rin form a voltage divider that determines vin, you want Rin as large as possible for maximum transconductance gain. •Since RL and Rout form a current divider that determines iout, you want Rout as large as possible for maximum transconductance gain. ECE 3040 - Dr. Alan DoolittleGeorgia Tech Amplifier Configurations Transresistance Amplifier: Current input and Voltage output The controlled source is a Current-controlled-Voltage Source Rm=Transresistance Gain can be found by applying a current source with Rs=infinity, and measuring the open circuit output voltage (RL=infinity) •Only the current iin is amplified to vout=Rmiin •Since Rs and Rin form a current divider that determines iin, you want Rin as small as possible for maximum transresistance gain. •Since RL and Rout form a voltage divider that determines vout, you want Rout as small as possible for maximum transresistance gain. ECE 3040 - Dr. Alan DoolittleGeorgia Tech Amplifier Configurations Input Resistance With the load resistance attached… Apply a test input voltage and measure the input current, Rin=vt/it Or Apply a test input current and measure the input voltage, Rin= vt/it Output Resistance With all input voltage sources shorted and all input current sources opened… Apply a test voltage to the output and measure the output current, Rout=vt/it Or Apply a test current to the output and measure the output voltage, Rout= vt/it ECE 3040 - Dr. Alan DoolittleGeorgia Tech Transistor Amplifier Configurations Common Emitter and Common Source rπ rπ is replaced with an open circuit for the MOSFET case gmvπ or gmvgsR in MOSFET for the 1||2BJT for the ||1||2 RRRinorrRRRin == π MOSFET BJT for the || orRrRout co= rο Rc R ou t Previously, we have analyzed voltage gain. Now let us look at the amplifier input and output resistance (these are small signal parameters): Portion due to bias circuitry Portion due to transistor Portion due to bias circuitry Portion due to transistor ECE 3040 - Dr. Alan DoolittleGeorgia Tech Transistor Amplifier Configurations Summary of Common Emitter and Common Source Characteristics •Very Large Voltage Gain •Inverting Voltage Gain (due to –gmro) •High Input Impedance •High Output Impedance Now let us consider the other two configurations of transistor amplifiers: •Common Gate/Common Base •Common Drain/Common Collector These properties make the CE/CS configuration very good for high gain stages of amplifiers. ECE 3040 - Dr. Alan DoolittleGeorgia Tech Transistor Amplifier Configurations Common Collector and Common Drain Collector (or Drain) is neither an input or output Input is Base (or Gate) Output is Emitter (or Source) B E C G S D DC Circuit ECE 3040 - Dr. Alan DoolittleGeorgia Tech Transistor Amplifier Configurations Common Collector AC Voltage Gain ( ) ( ) ( )( )( ) ( ) ( ) ( )( )( ) ( ) ( )( ) ( ) ( ) ( ) ( ) ( ) ( ) [ ]VVRg RgA RRg Rg RsRR RRA rgRRg Rg RsRR RRA RrR RrR R RsRR RRA RrRrRi RrRi RsRR RR v v v vA RsRR RRvv RrRrRiv RrRiv Lm Lm v oL o th m Lm v o m L o th m Lm v o o Loth Lo v oothb obo s th th o v sth oothbth oboo 1 1 1Rg and RsR1||R2for But 1 1||2 1||2 11 1||2 1||2 1 gby r denominato andnumerator gmultiplyin 7||||4R where 1 1 1||2 1||2 7||||41 7||||41 1||2 1||2 1||2 1||2 7||||41 7||||41 Lm m L ≅ + ≅ >>>>                     +      + +       + =                     + +      + +       + = + =      +++ +       + =       +++ +       + == + = +++= += α β ββ β β β β β β β π π π π Gain is positive and ~1 ECE 3040 - Dr. Alan DoolittleGeorgia Tech Transistor Amplifier Configurations Common Drain Conversion from DC to AC Equivalent Circuit AC Circuit ro DG S R7gmvGS DC Circuit ECE 3040 - Dr. Alan DoolittleGeorgia Tech Transistor Amplifier Configurations Common Emitter and Common Source DC Circuit converted to AC Equivalent (reduced) AC Circuit ro DG S gmvGS AC Circuit (reduced) R7 DG ECE 3040 - Dr. Alan DoolittleGeorgia Tech Transistor Amplifier Configurations Common Collector Output Resistance ( ) 4||1 1 1 1 , RrRwhere RRr i vR R v Rr v Rr v r vi Rr v r viii oL L o th x x BJTout L x th x o th x o x o th x o x ox = + + + == +      + −− + =+− + =+−−= β βββ π πππ Two resistors in parallel: RL, and Resistance in the base circuit is “multiplied” by transistor to decrease the output resistance ix Output Resistance: With all input voltage sources shorted and all input current sources opened, apply a test voltage to the output and measure the output current, Rout=vx/ix RL ECE 3040 - Dr. Alan DoolittleGeorgia Tech ix Transistor Amplifier Configurations Common Drain Output Resistance 4||1 1 , RrRwhere R gi vR R vvg r vvgi oL L m x x MOSFETout L x xm o x GSmx = + == +=+−= Two resistors in parallel: RL and inverse transconductance Output Resistance: With all input voltage sources shorted and all input current sources opened, apply a test voltage to the output and measure the output current, Rout=vx/i gmvGS RL ECE 3040 - Dr. Alan DoolittleGeorgia Tech Transistor Amplifier Configurations Summary of Common Collector and Common Drain Characteristics •Unity Voltage Gain •Non-Inverting Voltage Gain •Very High Input Impedance •Low Output Impedance These properties make the CC/CD configuration very good for impedance transformation, I.E. “buffering” high impedances to low impedances. CC/CD configurations are good for output stages of amplifiers due to their very low output impedance, I.E., very little voltage drop in the output resistance of the amp. ECE 3040 - Dr. Alan DoolittleGeorgia Tech Transistor Amplifier Configurations Common Base DC Circuit converted to AC Equivalent (reduced) DC Circuit AC Circuit vo gmvπ rorπ CB E Note: Jaeger let’s ro go to infinity which makes the math dramatically easier ECE 3040 - Dr. Alan DoolittleGeorgia Tech Transistor Amplifier Configurations Common Base AC Equivalent (reduced) AC Circuit vo gmvπ rorπ CB E ro rπ gmvπ vπ vπ rorπ gmvπ vπ vo KRRR V RR RvV sth s s sth 73.14|| 8667.0 4 4 == = + = AC Circuit (reduced) ECE 3040 - Dr. Alan DoolittleGeorgia Tech Transistor Amplifier Configurations Common Base Voltage Gain ro rπ gmvπ vπ vo KRRR V RR RvV sth s s sth 73.14|| 8667.0 4 4 == = + = iC iB iE i π π π ππ π π π π π π π π ππ π π π ππ π π vR r v r R r R r g vv vgv R r R r g vvsoR r vv vgRiv also r r v R r v r vv vgv riRiv r v r vv vgi iivgiii th o L o L o m mth L o L o m oL o o mLCo th o o mth BthEth o o mE BmBCE +                               +                                     + + − +=−             + + −=              + +−=−=       +              +      + +=− −−=       +      + += ++=+= 1 1 1 1 , , ECE 3040 - Dr. Alan DoolittleGeorgia Tech Transistor Amplifier Configurations Common Base Input Resistance ro rπ gmvπ vπ=−vx vo iC iB -ix=iE i vx Input Resistance: With the load resistance attached apply a test input voltage and measure the input current, Rin=vx/i L o L o m xo x o ox xmx xEx L o L o m o o o mE R r R r g vvand r v r vv vgi vvandii R r R r g vvand r v r vv vgi beforeFrom             + + =      +      − += −=−=             + + −=      +      + += 1 1 , 1 1 , , π π π π ππ π ECE 3040 - Dr. Alan DoolittleGeorgia Tech                               +                                     + + − + ==                               +                                     + + − +=       +                                     + + − += π π π rr R r R r g g i v R rr R r R r g gvi r v r R r R r g vv vgi o L o L o m m x x BJTin o L o L o m mxx x o L o L o m xx xmx 1 1 1 1 1 1 1 1 1 1 1 , Transistor Amplifier Configurations Common Base Input Resistance mm o o o o m BJTin m x x BJTin o gg r rg r R r g i v R rLetting 1 1 1 1 1 , , ≈= + = + =       + == ∞→ α β β β π π π π Input Resistance is very small! ECE 3040 - Dr. Alan DoolittleGeorgia Tech Transistor Amplifier Configurations Common Base Output Resistance βoib Rth Replace RL by a voltage source, vx rorπ vπ iCiB iE ix vx ve vr rorπ gmvπ vπ Result follows exactly after discussion in Jaeger, pages 668-670, and 683-684. ECE 3040 - Dr. Alan DoolittleGeorgia Tech Transistor Amplifier Configurations Common Gate Solution The Common Gate solution can be found by recognizing that the following translations can be made in our small signal model: ∞→ →⇒∞→ π αβ r oo 1 1 1 ,, + =→       +      + = thm Lm MOSFETv th thm Lm BJTv Rg Rg A r R Rg Rg A π m MOSFETin m BJTin g R r g R 1 1 1 ,, =→       + = π ( ) ththmoMOSFETout th th o th th mo th th o th th ooBJTout RRgrRRr Rr r Rr R rgr Rr Rr r Rr R rR ++=→ + +      + += + +      + += 1,, π π π π π π π β 11 ,, ==→≈= oBJTioBJTi AA αα ECE 3040 - Dr. Alan DoolittleGeorgia Tech Transistor Amplifier Configurations Summary of Common Base and Common Gate Characteristics •High Voltage Gain •Non-Inverting Voltage Gain •Very Low Input Impedance •Very High Output Impedance The input and output impedances are the opposite of what is typically needed for a voltage amplifier. Thus, Common Emitter/Source amplifiers are normally used instead of Common Base/Gate. ECE 3040 - Dr. Alan DoolittleGeorgia Tech Multistage Amplifier Configurations You can combine or Cascade configurations to produce “High Performance” amplifiers with High input impedance, low output impedance and huge voltage gains. C S pr ov id es H ig h In pu t I m pe da nc e, M od er at el y hi gh ne ga tiv e ga in C E p ro vi de s H ig h In pu t I m pe da nc e, hi gh g ai n, a nd co rr ec ts th e ne ga tiv e ga in fr om p re vi ou s st ag e C C p ro vi de s L ow ou tp ut Im pe da nc e, no g ai n, b ut m ai nt ai ns p os iti ve ga in fr om pr ev io us st ag e 21 2 , 1, v v v v v v v v v v A o inputths inputth s o v == v1 v2 vo ECE 3040 - Dr. Alan DoolittleGeorgia Tech Multistage Amplifier Configurations Continued….(For AC-Coupled amplifiers (capacitors between stages), the AC solution reduces to three circuits, each of which has a load dependent on the input resistance of the next stage!) ECE 3040 - Dr. Alan DoolittleGeorgia Tech Multistage Amplifier Configurations Continued... ( )( ) ( ) ( ) ( )( )( )                     ++       + ++−−===       ++       + =       +−=       += += ++−= −= −== + = )||(11 )||(1 )1||||()||||()1( )||(11 )||(1 )||(1 )||(1 )1||||( )||||( )||||(1 333 3 333 3 33122221111 32 3 1 21 333 3 333 3 3 333 3 3 333 3 4 43 331222 2 3 41222 2 3 21111 1 21 Lom Lom Lomom o ths th s o v Lom Lom o Lomoo Lomo o Lom inQom om thGS G s th Rrg r Rrg r RrRrgrRrg v v v v v v v v v v v vA Rrg r Rrg r v v Rrg r vvv Rrg r vv vvv RrRrg v v RRrg v v rRrg v v v v RR R v v π π ππ π π π π π π β β ECE 3040 - Dr. Alan DoolittleGeorgia Tech Multistage Amplifier Configurations AC-Coupled amplifiers (capacitors between stages), have one major limitation. They do not amplify low frequencies or DC voltages. To accomplish this, we must DC-Couple the stages as shown. Since the bias here is usually ~(2/3)Vcc (Vcc =15V in this example) , it is easier to use a PNP for the second stage so that VEB+IERE2 ~ (2/3)Vcc