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ELECTRONICS Part II. EE 213. Course Content. Part I Introduction to Oscilloscope Measurements Introduction to Semiconductors Diode Applications Special-Purpose Diodes Bipolar Junction Transistors (BJTs) Bipolar Transistor Biasing. Course Content (cont’d). Part II
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ELECTRONICSPart II EE 213 H. Chan; Mohawk College
Course Content • Part I • Introduction to Oscilloscope Measurements • Introduction to Semiconductors • Diode Applications • Special-Purpose Diodes • Bipolar Junction Transistors (BJTs) • Bipolar Transistor Biasing H. Chan; Mohawk College
Course Content (cont’d) • Part II • Small-Signal BJT Amplifiers • Power Amplifiers • Field-Effect Transistor Biasing • Small-Signal FET Amplifiers • Amplifier Frequency Response • Transistor Voltage Regulators • Introduction to Thyristors H. Chan; Mohawk College
Intro to Small-Signal Amplifiers • The Q-point of a transistor is set by dc biasing • Small-signal amplifiers are designed to handle small ac signals that cause relatively small variations about the Q-point. • Convention used for dc and ac values: • dc values, e.g. IE , RE • ac values, e.g. Ie (rms is assumed unless otherwise stated), Re , r’e (internal r of t’sistor) • instantaneous values, e.g. ie H. Chan; Mohawk College
Basic Small-Signal Amplifier +VCC Ic ICQ Vb RC R1 VBQ C2 Ib Rs C1 IBQ VCEQ Vs RL RE R2 Vce C1 and C2 block dc voltage but pass ac signal. H. Chan; Mohawk College
Small-Signal Amplifier Operation • Coupling capacitors C1 and C2 prevent Rs and RL from changing the dc bias voltages • Vs causes Vb and Ib to vary slightly which in turn produces large variations in Ic due to b • As Ic increases, Vce decreases and vice versa • Thus, Vc (output to RL) is 180o out of phase with Vb H. Chan; Mohawk College
Graphical Picture IBQ IC Ib IB5 Ic IB4 Q IB3 ICQ IB2 IB1 VCEQ VCE Vce H. Chan; Mohawk College
h Parameters • h (hybrid) parameters are typically specified on a manufacturer’s data sheet. • hi: input resistance; output shorted • hr : voltage feedback ratio, input open • hf : forward current gain; output shorted • ho : output conductance; input open • Each h parameter has a 2nd subscript letter to designate configuration, e.g. hfe, hfc, hfb H. Chan; Mohawk College
Amplifier Configurations • Common-emitter amplifier: emitter is connected to ground, input is applied to base, and output is on collector • Common-collector: collector is grounded, input is at base, and output is on emitter • Common-base: base is grounded, input is at emitter, and output is on collector H. Chan; Mohawk College
h-parameter Ratios Common-Emitter Common-Base Common-Collector H. Chan; Mohawk College
h-parameter Equivalent Circuit Iin hi Vin Vout hfIin ho hrVout • The above diagram is the general form of the h-parameter equivalent circuit for a BJT. • For the three different amplifier configurations, just add the appropriate second subscript letter. H. Chan; Mohawk College
r Parameters • The resistance, r, parameters are perhaps easier to work with • aac : ac alpha (Ic/Ie) • bac : ac beta (Ic/Ib) • re’ : ac emitter resistance • rb’ : ac base resistance • rc’ : ac collector resistance Relationships of h and r parameters: aac = hfb ; bac = hfe H. Chan; Mohawk College
r-Parameter Equivalent Circuits C C C aacIe bacIb aacIe bacIb rc’ rb’ B B B re’ Ib Ib re’ re’ 25 mV/IE Ie E E E Generalized r-parameter equivalent circuit for BJT Simplified r-parameter equivalent circuit and symbol of BJT H. Chan; Mohawk College
Difference between bac and bDC IC IC ICQ Q DIC { Q IB IB { 0 IBQ 0 DIB bDC = ICQ/IBQ bac = DIC/DIB bDC and bac values will generally not be identical and they also vary with the Q-point chosen. H. Chan; Mohawk College
Common-Emitter Amplifier +VCC C1 : for input coupling C3 : for output coupling RC R1 Vout C1 Vin C3 RL R2 RE C2 C2 : for emitter bypass H. Chan; Mohawk College
DC Analysis of CE Amplifier +VCC RC If bDCRE = RIN(base) >> R2, then R1 VC VB VE VE = VB - VBE ; R2 RE VC = VCC - ICRC H. Chan; Mohawk College
AC Analysis of CE Amplifier Input and output resistance: RC Rin(tot) = R1//R2//Rin(base) where Rin(base) = bacre’ ac ground C Vout Rout RC(not including RL) bacIb B Voltage gain of CE amplifier: Vin re’ R1 R2 E If RL is included: Rout = RC//RL, and C1, C2, and C3 are replaced by shorts assuming XC 0 H. Chan; Mohawk College
Overall Voltage Gain of CE Amplifier Voltage gain without emitter bypass capacitor, C2 : Rs Vout Vb Vs R1//R2 Rc = RC//RL Rule of thumb on emitter bypass capacitor: XC2 RE/10 H. Chan; Mohawk College
Stability of Voltage Gain +VCC Bypassing RE produces max. voltage gain but it is less stable since re’ is dependent on IE and temperature. RC C3 R1 C1 By choosing RE110 re’, the the effect of re’ is minimized without reducing the voltage gain too much: R2 RE1 Av -Rc/RE1 C2 RE2 Rin(base) = bac(re’+RE1) Swamped amplifier H. Chan; Mohawk College
Current Gain and Power Gain Ic Rs Ib Is Vs R1//R2 Rc Base to collector current gain is bac but overall current gain is Ai = Ic/Iswhere Overall power gain is: Ap = Av’Ai H. Chan; Mohawk College
Common-Collector Amplifier DC analysis: +VCC R1 VE = VB - VBE C1 IE = VE / RE Vin C2 VC = VCC Vout The CC amplifier is also known as an emitter- follower since Vout follows Vin in phase and voltage. R2 RE RL H. Chan; Mohawk College
AC Analysis of CC Amplifier aacIe Iin Vin If Re >> re’, then, Av 1, and Rin(base) bacRe re’ R1//R2 Vout Rin(tot) = R1//R2//Rin(base) Ie Rout (Rs/bac)//Re (very low) Re= RE//RL Ai = Ie/Iin bac(if R1//R2>> bacRe) Vin = Ie(re’ + Re) Vout = IeRe Ap = AvAi Ai H. Chan; Mohawk College
Darlington Pair +VCC • Ie2bac2Ie1 bac1bac2Ie1 • So,bac(overall) = bac1bac2 • Assuming re’ << RE, Rin = bac1bac2RE • Darlington pair has very high current gain, very high Rin, and very low Rout - ideal as a buffer Ib1 bac1 bac2 Ie1 Ie2 RE H. Chan; Mohawk College
Common-Base Amplifier +VCC CB amplifier provides high Av with a max. Ai of 1. RC R1 Vout C2 There is no phase inversion between Vout and Vin. C3 C1 RL DC formulas are identical to those for CE amplifier. Vin R2 RE H. Chan; Mohawk College
AC Analysis of CB Amplifier Vout Ic Rc Rin(emitter) = re’//RE B re’ If RE >> re’, then Vin E Rin(emitter) = re’ RE Rout Rc Ai 1; and Ap Av H. Chan; Mohawk College
Comparing CE, CC, & CB Amplifiers H. Chan; Mohawk College
Multistage Amplifiers Av1 Av2 Av3 Avn Vout Vin n amplifier stages in cascade • Overall voltage gain, AvT = Av1Av2Av3 . . . Avn = Vout/Vin • Overall gain in dB, AvT(dB) = Av1(dB)+Av2(dB) + . . . +Avn(dB)where, Av(dB) = 20 log Av • The purpose of a multistage arrangement is to increase the overall voltage gain. H. Chan; Mohawk College
Two-stage CE Amplifier +VCC R3 R7 R1 R5 C5 C3 C1 Vout Vin Q2 Q1 R2 R4 C2 R6 R8 C4 Capacitive coupling prevents change in dc bias H. Chan; Mohawk College
Analysis of 2-stage CE Amplifier DC Analysis: VB, VE, VC and IC are identical to those for 1-stage CE ampl. Vin R1//R2 R3 R5//R6 Rin(base2) AC Analysis: Rc1 = R3//R5//R6//Rin(base2) ; AvT = Av1Av2 H. Chan; Mohawk College
2-stage Direct-Coupled Amplifier +VCC R5 R3 R1 Vout Vin Q2 Q1 Note: No coupling or bypass capacitors R6 R2 R4 H. Chan; Mohawk College
Notes on Direct-Coupled Amplifier • DC collector voltage of first stage provides base-bias voltage for second stage • Amplification down to 0 Hz is possible due to absence of capacitive reactances • Disadvantage - small changes in dc bias voltages due to temperature or power-supply variations are amplified by succeeding stages H. Chan; Mohawk College
Transformer-Coupled Multistage Amplifier +VCC R1 R3 T3 T2 T1 R4 R2 Vout C5 Vin C3 C1 Q2 Q1 R5 C2 R6 C4 H. Chan; Mohawk College
Notes On T’former-Coupled Amplifiers • Transformer-coupling is often used in high-frequency amplifiers such as those in RF and IF sections of radio and TV receivers. • Transformer size is usually prohibitive at low frequencies such as audio. • Capacitors are usually connected across the primary windings of the transformers to obtain resonance and increase selectivity. H. Chan; Mohawk College
Typical Troubleshooting Process • Identify the symptom(s). • Perform a power check. • Perform a sensory check. • Apply a signal-tracing technique to isolate the fault to a single circuit. • Apply fault-analysis to isolate the fault further to a single component or group of components. • Use replacement or repair to fix the problem. H. Chan; Mohawk College
Power Amplifiers • Power amplifiers are large-signal amplifiers • They are normally used as the final stage of a communications receiver or transmitter to provide signal power to speakers or to a transmitting antenna. • Four classes of large-signal amplifiers will be covered: class A, class B, class AB, and class C. H. Chan; Mohawk College
Class A Amplifier Characteristics • Q-point is centred on ac load line. • Operates in linear region (i.e. no cutoff or saturation) for full 360o of input cycle. • Output voltage waveform has same shape as input waveform except amplified. • Can be either inverting or noninverting. • Maximum power efficiency is 25%. H. Chan; Mohawk College
Class A Operation: DC Load Line +VCC IC(sat) occurs when VCE 0, so RC R1 VCE(cutoff) occurs when IC 0, so VCE(cutoff) VCC Vout C1 Vin C3 IC RL R2 RE IC(sat) C2 Q ICQ VCE VCEQ VCC H. Chan; Mohawk College
Class A Operation: AC Load Line From Q-point to cutoff point: DIc = ICQ ; DVce’ = ICQRc Vce(cutof) = VCEQ + ICQRc Vin IC R1//R2 Rc Ic(sat) DIc’ Q Rc = RC//RL ICQ DVce’ From Q-point to saturation point: DVce = VCEQ ; DIc’ = VCEQ/Rc Ic(sat) = ICQ+DIc’ = ICQ + VCEQ/Rc VCE VCEQ Vce(cutoff) H. Chan; Mohawk College
Maximum Class A Operation IC AC load line Q-point is at centre of ac load line for max. voltage swing with no clipping. Ic(sat) Q ICQ 0 VCE To centre Q-point: VCEQ = ICQRc Ic(sat) = 2ICQ Vce(cutoff) = 2VCEQ VCEQ Vce(cutoff) 0 H. Chan; Mohawk College
Non-linearity of re’ • For large voltage swings, re’25mV/IE is not valid. • Instead, the average value of re’ = DVBE/DIC should be used for Av formula. • Non-linearity of re’ leads to distortion at output. • Reduce distortion by setting Q-point higher or use swamping resistor in the emitter. IC Q VBE H. Chan; Mohawk College
Power & Efficiency: Class A Amplifier • Power gain, Ap = AiAvbDCRc/re’ • Quiescent power, PDQ = ICQVCEQ • Output power, Pout = VceIc = Vout(rms)Iout(rms) • when Q-point is centred, Pout(max) = 0.5VCEQICQ • Efficiency, h = Pout/PDC = Pout/(VCCICQ) • When Q-point is centred, hmax = 0.25 • Max. load power, PL(max) = 0.5(VCEQ)2/RL H. Chan; Mohawk College
Class B Amplifier Characteristics • Biased at cutoff - it operates in the linear region for 180o and cutoff for 180o. • Class AB amplifier is biased to conduct slightly more than 180o. • Advantage of class B or class AB amplifier over class A amplifier - more efficient. • Disadvantage - more difficult to implement circuit for linear reproduction of input wave H. Chan; Mohawk College
Push-pull Class B Operation +VCC An npn transistor and a matched pnp transistor form two emitter- followers that turn on alternately. Since there is no dc base bias, Q1 and Q2 will turn on only when |Vin| is greater than VBE. This leads to crossover distortion. Q1 Vout Vin Q2 RL Q1 on -VCC Vout t Complementary amplifier Q2 on H. Chan; Mohawk College
Class AB Operation +VCC • The push-pull circuit is biased slightly above cutoff to eliminate crossover distortion. • D1 and D2 have closely matched transconductance characteristics ofthe transistors to maintain a stable bias. • C3 eliminates need for dual-polarity supplies. R1 C1 Q1 D1 C3 Vin D2 RL Q2 C2 R2 H. Chan; Mohawk College
Class AB Amplifier: DC Analysis +VCC • Pick R1 = R2, therefore VA=VCC/2 • Assuming transconductance characteristics of diodes and transistors are identical, VCEQ1=VCEQ2=VCC/2 • ICQ 0 (cutoff) R1 Q1 VCEQ1 D1 A D2 Q2 VCEQ2 R2 H. Chan; Mohawk College
Class AB Amplifier: AC Analysis IC • For max. output, Q1 and Q2 are alternately driven from near cutoff to near saturation. • Vce of both Q1 and Q2 swings from VCEQ = VCC/2 to 0. • Ic swings from 0 to Ic(sat) = VCEQ/RL Iout(pk) • Input resistance, Rin = bac(re’+RL) Ic(sat) ac load line Ic VCEQ VCE Vce H. Chan; Mohawk College
Power & Efficiency: Class B Amplifier • Average output power, Pout = Vout(rms)Iout(rms) • For max. output power, Vout(rms)= 0.707VCEQ and Iout(rms) = 0.707Ic(sat); therefore, Pout(max)=0.5VCEQIc(sat) = 0.25 VCCIc(sat) • Since each transistor draws current for a half-cycle, dc input power, PDC = VCCIc(sat)/p • Efficiency, hmax = Pout/PDC = 0.25p 0.79 • The efficiency for class AB is slightly less. H. Chan; Mohawk College
Darlington Class AB Amplifier +VCC In applications where the load resistance is low, Darlingtons are used to increase input resistance presented to driving amplifier and avoid reduction in Av. 4 diodes are required to match the 4 base- emitter junctions of the darlington pairs. R1 C1 Q2 D1 Q1 C3 Vin D2 D3 D4 Q4 RL Q3 C2 R2 H. Chan; Mohawk College
Class C Amplifier Characteristics • Biased below cutoff, i.e. it conducts less than 180o. • More efficient than class A, or push-pull class B and class AB. • Due to severe distortion of output waveform, class C amplifiers are limited to applications as tuned amplifiers at radio frequencies (RF). H. Chan; Mohawk College
Basic Class C Operation +VCC Vin VBB+VBE RC 0 Vo Vin Ic RB 0 tON T VBB The transistor turns on when Vin > VBB+VBE Duty cycle = tON / T H. Chan; Mohawk College