Electronic Troubleshooting
Chapter 5Multistage 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
• 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
2 40 80Xv mV mV
2 80 60 4800 4.8Y X vv v A mV mV V
243111 |||||| QinCLQL rRRRrr
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
2CX 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
11 (2 )
1
2 ( )C in ndStage C
fR r C
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
2e
1
2 R q S
fC
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 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
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
• At least a factor of 10 or more between Open and Closed loop gain
( ) 1Fv ClosedLoop
E
RA
R
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 byVRE Can be changed by:
• Changing RE or VE
EE
E
VI
R
0.7BC E
E
V vI I
R
C CC C CV V I R
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