Electronic Troubleshooting

1. Electronic Troubleshooting Chapter 5 Multistage Amplifiers

2. Overview • When more amplification is required than can be supplied by a single stage amp • A second stage is added • Or more stages are added • Aspects that are covered • Capacitively Coupled Stages • Testing and Troubleshooting • Frequency Response of Cascaded Stages • Using Negative Feedback • Direct Coupled Amplifiers

3. Overview • Aspects that are covered • Differential Amplifiers • Emitter Followers • Analysis of a Complete Amplifier System

4. Two Stage Capacitively Coupled • Characteristics • Two stages coupled by • Cap – CC • Freq of AC signal under amplification • High enough to yield insignificant impedance, XC for CC • Determining impedance seen by AC signals • DC Power supplies appear as a ground/common • Equivalent impedance seen by the output of Q1

5. Two Stage Capacitively Coupled • Characteristics • Gain of the first stage AV1 = rL1/re1 • Gain of the second stage AV2 = rL2/re2 • Total Gain AV(tot) = AV1 x AV2 • Sample Problem • Given: vin = 2mV, AV1 = 40, AV2 = 60 • Find voltages at points X and Y on the drawing

6. Testing a two-stage amplifier • Check the output of the last stage • Should have non-distorted signal of appropriate magnitude • If bad check at the output of each stage • Remove from consideration all properly functioning parts of the circuit

7. Troubleshooting Cascade Stages • Test the power supply voltages – If Good ↓ • Insert small AC signal • Signal Characteristics • Few millivolts • Into first stage • Follow the testing chart • Page 95 and 96 • Quickly sets focus on defective part of circuit • Divide and fix strategy • Walk through assuming R2 is an open – 3rd para on page 97

8. Frequency Response of Cascaded Stages • Frequency response of amplifiers is limited • At both high and low frequencies around the operating band • Low Freq limiting • Attenuation of the output is directly related to the increasing impedance of CC as the Freq of the input is decreasing • As can be seen in the coupling circuit to the right • XC at lower freq decrease the input signal for the second stage • At DC CC is an open

9. Frequency Response of Cascaded Stages • Frequency response of amplifiers is limited • Low Freq limiting • A Thevenin equivalent circuit simplifies the analysis • When XC = RC1 + r in(2nd stage) • Vin to the second stage is 0.707 of its max • Power delivered is ½ or -3dB • The freq at which this happens is the lower -3dB point or f1 • Example Problem • See middle of page 98

10. Frequency Response of Cascaded Stages • Freq response of amplifiers is limited • High Freq limiting • Shunting Caps cause high frequency limiting • Q1 shunted by CCE • Q2 input shunted by CBE or Cin • The composite shunting Cap for all the coupling circuit wiring • CS is the parallel combination • Same for Req • f2 is the freq at which XC = Rt • The half power point or -3dB point • See example problem • Mid-page on 99

11. Frequency Response of Cascaded Stages • Amplifier Frequency Response Curve

12. Distortion Reduction –Negative Feedback • Prime Cause – Large driving signal • Results of such distortion are illustrated below • Unequal positive and negative transitions on the output

13. Distortion Reduction –Negative Feedback • Prime Cause – Large driving signal • Distortion results from the characteristics of the base-emitter diode • The characteristic curve is only linear over a small range • See the negative transition of Ib • Will yield • Distorted Ic • Distorted vO

14. Distortion Reduction –Negative Feedback • Negative Feedback • Characteristics • Supplies fraction of the output back to the input • Connection to the emitter yields negative feed back • Feedback voltage scaling • Voltage divider of RE and RF

15. Distortion Reduction –Negative Feedback • Negative Feedback • Effects of negative feedback • Pre-distorts the output of the first stage to yield an undistorted output from the second stage • Will help counter act the distortion generated in the second stage • IC and collector voltage VQ1 will have the same form

16. Distortion Reduction –Negative Feedback • Negative Feedback • Effects of negative feedback • The more feedback the less distortion • However the more feedback the less gain • Gain with Feedback • Called Closed Loop Gain • When open loop gain (without feedback) is large compared to closed loop gain • At least a factor of 10 or more between Open and Closed loop gain

17. Direct Coupled Amplifiers • Characteristics • Used when low frequency or DC signals are amplified • For example DC signals in a power regulator, or the outputs of thermocouples • Simple circuit (typical of Output stages) • Transistor current controlled by VRE Can be changed by: • Changing RE or VE

18. Direct Coupled Amplifiers • Simple Amp without Feedback • Characteristics • AV1 =RC1/re1 , AV2 =RC2/RE2 , AV2 is usually much smaller than AV1 • Problems with circuit • As Q1 temperature increases • IC increases • VC(Q1) decreases • Changes are amplified by Q2 • Direct coupling increases temperature instability

19. Direct Coupled Amplifiers • Simple Amp with Feedback • Characteristics • Forward biased on Q1 comes from VRE • Divided by R1 and R2 • Follow startup • Q1 off VB(Q2) goes positive • Q2 turns on and VE grows • VB(Q1) goes positive • Q1 turns on • IRC1 increases, VB(Q2) decreases • VB(Q1) reaches 0.7V quickly • At stability VRE depends on the ratio of R1 & R2

20. Direct Coupled Amplifiers • Simple Amp with Feedback • Characteristics • Temperature Stability • Q1 heats up and IC1 increases • VC1 and VB2 decreases • VE decreases, thus VB1 decreases • Q1 then conducts less • Thus VC1 increases • End result a temperature change causes less change in output • CE was added to make a good low frequency Amp • No effect on DC input signals

21. Direct Coupled Amplifiers • Simple Amp with Feedback • Characteristics • Temperature Stability • Q1 heats up and IC1 increases • VC1 and VB2 decreases • VE decreases, thus VB1 decreases • Q1 then conducts less • Thus VC1 increases • End result a temperature change causes less change in output • CE was added to make a good low frequency Amp • No effect on DC input signals

22. Direct Coupled Amplifiers • Real Sample Circuit • See Figure 5-14 on page 106 • Walk-through • Collector of transistor X101 is direct coupled to Base of X102 • Base of X101 is biased off of R114 through R104 –Temp Stability • What is the circuit that links the collector of X102 to the emitter of X101?

23. Differential Amplifiers • Characteristics • Used to amplify differences between two signals • Can use transistors, Tubes, or Linear ICs • This chapter deals with the transistor version • Requires two identical transistors and a common emitter resistor • Both are forward biased • -15 Supply • Both emitters at -0.7V • Both IE’s ~ 1mA • Both collectors = 10V and VD =0V

24. Differential Amplifiers • Characteristics • Temperature stability • Due to identical transistors if the temperature rises both have the same current increase and VD stays the same • Walk through • One input has a more positive value • That transistor conducts More, VE increases, VC decreases • The other transistor conducts less and VC Increases • VD is proportional to the inputs but larger • Example problem on top of page 108

25. Differential Amplifiers • Characteristics • Walk through • Impractical to use very high voltage supplies • Use a constant current source instead • RE can be adjusted for a more accurate current amount

26. Emitter Followers • Characteristics • Have unity gain • Output in phase with Input • No collector resistor • Output from emitter • Provides current gain without loading the input circuit • RE = RL for given circuit • rin = 80 x 1kΩ

27. Emitter Followers • Actual Circuits • Load for the DC Amp • VQ1 sees 5K Ω ||30KΩ • The output can drive a 3KΩ with less than 10% change in output

28. Left Channel of Tape Recorder (Part 1)

29. Left Channel of Tape Recorder (Part 2)

30. Coupling Circuit

31. Analyzing the Tone Control

32. Analyzing the Tone Control

33. Playback Signal (Part 1)

34. Playback Signal (Part 2)

35. Record Amp