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 Req
• 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 IEs 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 1kO

27
Emitter Followers

• Actual Circuits
• Load for the DC Amp
• VQ1 sees 5K O 30KO
• The output can drive a 3KO with less than 10
  change in output

28
Complete Amp System

• Complete channel of old tape recorder
• Input Section
• Mic jack at top Tape heads below
• Input amplifier (aka preamp) X101 and X102
• Audio Frequency (AF) amplifier
• Another two stage amp after R119, the volume
  control pot (top left of part 2 page113)
• Output driver
• Emitter follower, X105, driving the headphone
  output top right of part 2 (page 113)
• C122 couples AC signal only to headphones
• AC output is also rectified and feed to the
  Play/Record Level meter.
• Record amplifier
• Part 2 (page 113) Mid-page on right

29
Left Channel of Tape Recorder (Part 1)
30
Complete Amp System

• Complete channel of old tape recorder
• Record amplifier
• Single stage amp for recording, X106.
• Voltage divider biasing (R138 and R139)
• Capacitive input coupling C125.
• High frequency noise roll-off (attenuation)
• For Example C102 goes from collector of X101 to
  base
• It appears to Amp input signal as much larger
  (value multiplied by gain of that stage)
• 33pF looks like 3300 pF to the input signal
• Prevents oscillations caused by high frequency
  noise
• Troubleshooting
• Inject small AC signal on the left (input) side
• Trace signal through amplifier chain
• Amplitudes should increase as you move to the
  right except for X105 no amplitude gain.

31
Left Channel of Tape Recorder (Part 2)
32
Complete Amp System

• Complete channel of old tape recorder
• Troubleshooting
• As with all circuits - If output has problems
• Check supply voltage, if OK
• Check convenient Mid-point of circuit, if OK
• Check a convenient midpoint of the remaining part
  of the circuit that has the malfunction in it
• Repeat until problem is found
• Frequency Response (aka Tone Control)
• S2 used to select from two different R-C circuits
  for tone control (S1 selects Record or Playback
  modes)
• Fig 5-23 shows coupling circuit between TPs 21
  and 28 in Normal
• Playback (below)

33
Complete Amp System

• Complete channel of old tape recorder
• Frequency Response (aka Tone Control)
• S2 used to select from two different R-C circuits
  for tone control (S1 selects Record or Playback
  modes)
• At 100Hz (figure 5-24b next slide)
• C113 reactance approx. 39k ohms
• C114 reactance approx. 390k ohms
• As the frequency increases the signal feeding the
  second two stage amplifier increases. The
  circuit acts as a High Pass filter. See below
• S2 in chrome position connects different RC
  coupling

34
Analyzing the Tone Control