Electronic Troubleshooting
Chapter 5
Multistage Amplifiers
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
Overview
• Aspects that are covered
• Differential Amplifiers
• Emitter Followers
• Analysis of a Complete Amplifier System
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
Two Stage Capacitively Coupled
• Characteristics
rL Q1  rL1  RC1 || R3 || R4 || rinQ 2
• 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
vX  2mV  40  80mV
• Given: vin = 2mV, AV1 = 40, AV2 = 60
• Find voltages at points X and Y on the drawing
vY  vX  Av 2  80mV  60  4800mV  4.8V
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
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
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
1
XC 
2 fC
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
1
f1 
2 ( RC1  rin (2 ndStage) )CC
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
1
f2 
2 R e q CS
Frequency Response of Cascaded Stages
• Amplifier Frequency Response Curve
Distortion Reduction –Negative Feedback
• Prime Cause – Large driving signal
• Results of such distortion are illustrated below
• Unequal positive and negative transitions on the output
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 Ib
• Will yield
» Distorted Ic
» Distorted vO
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
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
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
Av (ClosedLoop )
• At least a factor of 10 or more
between Open and Closed loop gain
RF

1
RE
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
VE
IE 
RE
VB  0.7v
IC  I E 
RE
VC  VCC  IC RC
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
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
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
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
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?
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
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
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
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
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
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
Left Channel of Tape Recorder (Part 1)
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.
Left Channel of Tape Recorder (Part 2)
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)
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
Analyzing the Tone Control