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 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
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
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
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. 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 TP’s 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