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  1. ELECTRONICSPart II EE 213 H. Chan; Mohawk College

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. Graphical Picture IBQ IC Ib IB5 Ic IB4 Q IB3 ICQ IB2 IB1 VCEQ VCE Vce H. Chan; Mohawk College

  8. 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

  9. 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

  10. h-parameter Ratios Common-Emitter Common-Base Common-Collector H. Chan; Mohawk College

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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 RE110 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

  20. 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

  21. 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

  22. 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

  23. Darlington Pair +VCC • Ie2bac2Ie1 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

  24. 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

  25. 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

  26. Comparing CE, CC, & CB Amplifiers H. Chan; Mohawk College

  27. 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

  28. 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

  29. 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

  30. 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

  31. 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

  32. 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

  33. 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

  34. 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

  35. 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

  36. 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

  37. 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

  38. 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

  39. 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

  40. 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

  41. Power & Efficiency: Class A Amplifier • Power gain, Ap = AiAvbDCRc/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

  42. 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

  43. 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

  44. 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

  45. 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

  46. 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

  47. 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

  48. 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

  49. 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

  50. 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