Introducton to Amplfers Dad W. Graham West Vrgna Unersty Lane Department of Computer Scence and Electrcal Engneerng Dad W. Graham, 07
Small Wggles To Bg Wggles Amplfcaton s extremely mportant n electroncs Output from sensors s often small Could be n nv to µv range Too small to be processed n typcal electroncs (sgnals usually n the mv to V range) Often need to amplfy a sgnal to dre an put n the physcal world ex. Speaker or power amplfer (antenna) Hang good amplfers opens up a whole new realm of possbltes n crcut desgn
Ideal Amplfer Characterstcs Mnmal loadng effects Impedance from preous and subsequent stages do not mpact performance Predctable response Lnear amplfcaton performance Howeer, amplfers are made from nonlnear components (e.g. transstors) Gan equals desred alue 3
Loadng Effects 4
Input / Output elatonshps Dece Z n Z eason Independent Voltage Source Independent Current Source 0Ω Ω 0V put no oltage drop; no resstance 0V put replace wth a short crcut 0A put no current flows; nfnte resstance 0A put replace wth an open crcut Voltmeter Ω Ammeter 0Ω Mnmzes loadng effects No resstance n parallel Mnmzes loadng effects No resstance n seres VoltageControlled Voltage Source (VCVS) VoltageControlled Current Source (VCCS) CurrentControlled Voltage Source (CCVS) CurrentControlled Current Source (CCCS) Ω Ω 0Ω 0Ω 0Ω Ω 0Ω Ω 5
Input / Output Impedances Lessons learned Inputs Voltage sensng Want hgh nput mpedance Current sensng Want low nput mpedance Outputs Voltage put Want low put mpedance Current put Want hgh put mpedance 6
Input Impedances Inputs Voltage sensng Desre hgh nput mpedance Current sensng Desre low nput mpedance Voltage Sensng Current Sensng Norton Equalent Theenn Equalent I N N V sense sense V Th Th I sense sense V I sense N N sense I V / sense Th ( ) Th sense V sense I N N as sense I sense V Th / as 0 Th sense Norton and Theenn equalents represent the crcut we are sensng 7
Output Impedances Outputs Voltage puts Desre low put mpedance Current puts Desre hgh put mpedance Voltage Output Current Output Theenn Equalent Norton Equalent V Th Th I N N n V I n V V V V Th Th n as n Th Th 0 I I I I N N N as N n N Norton and Theenn equalents represent the crcut prodng the sgnal. n represents the nput mpedance of the subsequent stage. 8
TwoPort Models Most mportant parameter n an amplfer s gan Must determne how loadng affects gan Twoport models smplfes ths process Analogous to Norton/Theenn representaton TwoPort Equalent One parameter at each port s ndependent The other port s dependent on both the frst port and the twoport network Admttance Parameter Equatons Voltage s ndependent y y Current s dependent (Typcal of most oltagemode crcuts) y y 9
TwoPort Model Admttance Parameters TwoPort Equalent y y y y y 0 Input admttance Output short crcuted y y 0 0 eerse transconductance Input short crcuted Forward transconductance Output short crcuted y y y y y 0 Output admttance Input short crcuted 0
Unlateral TwoPort Model Typcally, there s no feedback y 0 y s referred to as transconductance (G m ) Conert admttances nto mpedances Z n nput mpedance Z put mpedance Unlateral TwoPort Model (Norton Output) Unlateral TwoPort Model (Theenn Output) Z Z n G m Z Z n a a Gan 0 G m Z
Connectng a TwoPort Network to a Crcut Theenn Source TwoPort Model Theenn Load s n n G m L a n n n n n n n s n s ( G ( )) m Gm L L Assumng s and L are fxed a as n a as
TwoPort Model Summary Z n G m Z A Z n 0 n n n 0 G m Z Z n Z a Z G m 0 0 n n 0 0 For oltagen oltage crcuts, we desre Z n Ω Z 0Ω 3
Lnear Amplfcaton 4
Lnear Amplfers Outsde World Sensor / Transducer Interface (Analog) µprocessor (Dgtal) Interface (Analog) Outsde World Amplfers, Flters, Data Conerters Acte crcuts (e.g. wth transstors) are lnear only oer a specfc range We wll constantly talk ab Largesgnal operaton Nonlnear equatons DC operatng pont, bas condtons Smallsgnal analyss Lnear equatons Amplfcaton regon Eery crcut must be analyzed wth both the large and smallsgnal analyses 5
LargeSgnal s. SmallSgnal Typcal Amplfer Transfer Functon V Gan Slope V n Lnear, hghgan regon (amplfer) Nonlnear portons Must frst do a largesgnal analyss to get the amplfer nto range Bas n the amplfcaton regon DC operatng pont (DC oltages and currents) from the largesgnal operaton Once n the amplfcaton regon, assume a smallsgnal nput Eerythng wll stay wthn the lnear regon / lnear range Lnear, tmenarant (LTI) analyss apples 6
LargeSgnal s. SmallSgnal Typcal Amplfer Transfer Functon V DC Operatng Pont V n Large sgnal (Bases / DC condtons) Moes amplfer nto range Amplfer s now ready to perform amplfcaton Small sgnal (AC nputs / puts) Small snusodal nputs makes bgger snusodal puts 7
Acheng Desred Gan 8
Acheng Desred Gan Need accurate, relable performance Amplfers are made from naccurate components Fabrcaton mperfectons Very hard to achee the exact gan you desre, wth some extra help Soluton Negate Feedback 9
Feedback Combnng a porton of the put wth the nput.e. feed back the put sgnal Can be poste or negate feedback Poste Feedback Add the put to the nput Tends to be a destablzng confguraton Typcally results n oscllatons Typcally undesred Negate Feedback Subtract the put from the nput Tends to be a stablzng confguraton Typcally results n the oerall crcut behang as desred Very desrable 0