Electronic Troubleshooting. Chapter 5 Multistage Amplifiers

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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 C C Freq of AC signal under amplification High enough to yield insignificant impedance, X C for C C Determining impedance seen by AC signals DC Power supplies appear as a ground/common Equivalent impedance seen by the output of Q 1

5 Two Stage Capacitively Coupled Characteristics r R R R r L1 C1 3 4 in Q2 Gain of the first stage A V1 = r L1/r e1 Gain of the second stage A V2 = r L2 /r e2 Total Gain A V(tot) = A V1 x A V2 Sample Problem Given: v in = 2mV, A V1 = 40, A V2 = 60 Find voltages at points X and Y on the drawing v 2mV 40 80mV vy vx Av2 80mV mV 4.8V X

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 3 rd 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 X C 1 2 fc the increasing impedance of CC as the Freq of the input is decreasing As can be seen in the coupling circuit to the right X C at lower freq decrease the input signal for the second stage At DC C C 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 of its max» Power delivered is ½ or -3dB» The freq at which this happens is the lower -3dB point or f 1 Example Problem See middle of page 98 f ( R r ) C C1 in(2 ndstage) C

10 Frequency Response of Cascaded Stages Freq response of amplifiers is limited High Freq limiting Shunting Caps cause high frequency limiting Q1 shunted by C CE Q2 input shunted by C BE or C in The composite shunting Cap for all the coupling circuit wiring C S is the parallel combination Same for R eq f 2 is the freq at which X C = R t The half power point or -3dB point See example problem Mid-page on 99 f R e q C S

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 baseemitter diode The characteristic curve is only linear over a small range See the negative transition of I b Will yield» Distorted I c» Distorted v O

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 R E and R F

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 thee second stage I C and collector voltage V Q1 will have the same form

16 Distortion Reduction Negative Negative Feedback 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 A between Open and Closed loop gain R F v( ClosedLoop) 1 RE

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 V RE Can be changed by: Changing R E or V E I E V R E E I C I E V B 0.7v R E V V I R C CC C C

18 Direct Coupled Amplifiers Simple Amp without Feedback Characteristics A V1 =R C1 /r e1, A V2 =R C2 /R E2, A V2 is usually much smaller than A V1 Problems with circuit As Q 1 temperature increases» I C increases» V C(Q1) decreases» Changes are amplified by Q 2 Direct coupling increases temperature instability

19 Direct Coupled Amplifiers Simple Amp with Feedback Characteristics Forward biased on Q1 comes from V RE Divided by R1 and R2 Follow startup Q1 off V B(Q2) goes positive Q2 turns on and V E grows V B(Q1) goes positive Q1 turns on I RC1 increases, V B(Q2) decreases V B(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 I C1 increases V C1 and V B2 decreases V E decreases, thus V B1 decreases Q1 then conducts less Thus V C1 increases End result a temperature change causes less change in output C E 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 I C1 increases V C1 and V B2 decreases V E decreases, thus V B1 decreases Q1 then conducts less Thus V C1 increases End result a temperature change causes less change in output C E 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 X 101 is direct coupled to Base of X 102 Base of X 101 is biased off of R114 through R104 Temp Stability X What is the circuit that links the collector of X 102 to the emitter of X 101?

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 I E s ~ 1mA» Both collectors = 10V and V D =0V

24 Differential Amplifiers Characteristics Temperature stability Due to identical transistors if the temperature rises both have the same current increase and V D stays the same Walk through One input has a more positive value» That transistor conducts More, V E increases, V C decreases» The other transistor conducts less and V C 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 Characteristics Have unity gain Emitter Followers Output in phase with Input No collector resistor Output from emitter Provides current gain without loading the input circuit R E = R L for given circuit r in = 80 x 1kΩ

27 Emitter Followers Actual Circuits Load for the DC Amp V Q1 sees 5K Ω 30KΩ The output can drive a 3KΩ with less than 10% change in output

Module 4 Unit 4 Feedback in Amplifiers

Module 4 Unit 4 Feedback in Amplifiers Module 4 Unit 4 Feedback in mplifiers eview Questions:. What are the drawbacks in a electronic circuit not using proper feedback? 2. What is positive feedback? Positive feedback is avoided in amplifier