Electronic Troubleshooting
Chapter 6
Power Amplifiers
Power Amplifiers
• Characteristics
• When an amplifier deliverers more than a few milliwatts
• Often drives low impedance loads such as speakers
• Power Amps Topics Covered
•
•
•
•
•
•
Complementary Symmetry Output Stage
Crossover Distortion and Reducing it
Adding a Driver to the Complementary Symmetry Stage
Quasicomlementary Amps
Transformer-coupled Push-pull Circuit
High Power MOSFET Amps
Complementary Symmetry Output Stage
• Characteristics
• Built with a matched
pair of NPN and PNP
transistors
• Circuit Overview
• Simlified version
• R1-R2 voltage divider
holds both base leads at
½ Vcc
• Output stabilized at the
same voltage as the
common base voltage
Complementary Symmetry Output Stage
• Operation
• AC input - continued
• Positive going transition
• Q1 acts like a emitter follower
• Negative going transition
• Q1 turns off
• Q2 turns on
• Notice the current flow
directions
• Output voltage
• Peak value at a theoretical value
of ½ Vcc
• Theoretical Peak-Peak range
• Vcc
Complementary Symmetry Output Stage
• Operation
• Turn-on Sequence
• Before the coupling Cap charges to 10V, the 10V on the base of
Q1 turns it on hard
• Path to ground for the emitter current is through load resistor (8Ω)
• Significant current - only limited by the load resistor
• As the Cap charges the Output voltage rises towards ½ Vcc
• If the output goes above 10V Q1 turns off
• Q2 turns on and provides a discharge path for the Cap
• Once the Cap is charged to ½ Vcc both transistors are off
• AC input signal applied to the common base pins
through a input coupling Cap
• Positive going transition
• Q1 conducts
Complementary Symmetry Output Stage
• Operation
• Output current
• Peak value at a theoretical value of Vcc/2RL
• Theoretical Peak-Peak range = Vcc/RL
• Power (Maximum)
• Needs RMS value of the AC voltages
1
Vrms  0.707  VP  0.707  VCC
2
V
I rms  0.707  I P  0.707  CC
2 RL
Pout  I out( rms) Vout( rms)
• Each transistor is on ½ of the circuit operation
thus only supplies ½ of the power supplied to the load
Complementary Symmetry Output Stage
• Operation
• Output current
• Peak value at a theoretical value of Vcc/2RL
• Theoretical Peak-Peak range = Vcc/RL
• Power
• Needs RMS value of the AC voltages
Vrms
1
 0.707  VP  0.707  VCC
2
I rms  0.707  I P  0.707 
VCC
2 RL
Pout  I out( rms) Vout( rms)
Pout  0.707 I out( peak )  0.707Vout( peak )  0.5I out( peak ) Vout( peak )
Complementary Symmetry Output Stage
• Operation
• Power
• Each transistor is on ½ of the circuit operation
thus only supplies ½ of the power supplied to the load
PPeakTranRating
Pout 0.5I out( peak ) Vout( peak ) I out( peak ) Vout( peak )



2
2
4
• The average power supplied would be equal to the power
supplied during a ½ cycle spread over the full cycle (or second)
– THUS ¼ of the RMS power
PAveTranRating 
PPeakTranRating
2
Pout 0.5I out( peak ) Vout( peak ) Prms



4
4
4
• Sample Problem 6-1 on page 137
• In Class: 6-5, 6-6, 6-7, 6-8
Crossover Distortion
• Characteristics
• Neither transistor is conducting
for a period of time
• When vin is between +0.7 and 0.7V
• The Amp off time causes a
deformed output wave
• Called Crossover distortion
• Also generates odd harmonics
Reducing Crossover Distortion
• Cure to reduce distortion
• Insert a diode between the
bases of Q1 and Q2
• Operation
• Both transistors are forward
biased to about 0.35 V
• Will reduce crossover distortion
• Some distortion will remain
• D1 doesn’t rectify the input
• Acts like a 0.7V battery in the
circuit
Adding a Driver
to the Complementary Symmetry Stage (CSS)
• Key Aspects
• If the input signal to the CSS is too
small
• Add an amplifier – aka Driver to
the input stage
• Q3 replaces R2 in the
previous drawing
• Q3 acts as a directly
coupled amplifier
tied to the CSS
• However it has a non-apparent feedback circuit
• It’s voltage source is the output of the CSS
Adding a Driver
• Key Aspects
• Q3 acts as ~~ continued
• RA and RB provide VB for Q3
• Operation
• Start-up
• Q3 is off the moment power is applied
• R1 pulls the base of Q1 towards Vcc. The emitter of Q1 (point
X) follows
• As point X goes positive RA pulls the base of Q3 positive and
starts to turn Q3 on
• When point X reaches ½ Vcc VB of Q3 should be 0.7V
• If X goes to high Q3 turns on harder; then bases of Q1 & Q2
will go lower; then point X goes back to ½ Vcc
Adding a Driver
• Operation
• Temperature Stability
• Example: If Q3 heats up and IC3 increases
• Bases of Q1 and Q2 go lower
• Q1 conducts less, Q2 conducts more
• Voltage at point X goes lower
• VB3 goes lower and IC3 decreases
• Less net change due to the feedback
• Real Example
• Fig 6-6 on page 140
• Highlighted section
• Has a circuit similar too the previous one (also on page 139)
• Drawn differently with a few changes
Adding a Driver
• Real Example
• Highlighted section
• Notice the added 1Ω emitter resisters on Q4 and Q5
• Limits current during thermal run-a-way
• Help equalize the peak currents of the two transistors even if
their β are different
• Notice the Cap (C10) from the output to R14-R15
• It is a large cap for the AC signals that are amplified
» It and the equivalent resistance have an RC time constant
much larger than the period of the signal
» Thus it doesn't discharge under normal operation
• C10 acts as a small battery and maintains the voltage drop
across R15 constant
» Thus no AC current flows through it and it appears as an
open to the AC signal
Adding a Driver
• Real Example
• Highlighted section
• Notice the Cap (C10) from the output to R14-R15
• Since R15 appears as an open the AC load seen by the driver
(Q3) isn’t decreased by the low parallel resistance values of
R15 & R14
» Thus the load seen Q3 and gain for Q3 is greater, AV3 =
rL3/re3
Quasicomlementary Amps
• Characteristics
• Similar to Complementary
• Used for high fidelity, high
power amplifier
• Analysis
• Without Q4
and Q5 it is very
similar to the previous circuit on page 139
• Two diodes used to further reduce crossover distortion
• Q2 & Q3 biased near cutoff
• Not enough current in R6 or R7 to turn either Q4 or Q5 on
• Q4 and Q5 are both NPN transistors
Quasicomlementary Amps
• Characteristics
• Operation
• Without a signal Q2 & Q3 are barely on
• Minimal current in R6 & R7
• Not enough to turn Q4 or Q5 on
• With signal
• On positive half cycle Q2 and Q4 drive the output
» With Q2 on – the voltage on R6 turns
Q4 on – thus raising the output voltage
• On negative half cycle Q3 and Q5 drive
the output
» Q3 turns on and the base of Q5 goes
positive and it turns on – output goes neg
Quasicomlementary Amps
• Characteristics
• Actual circuit
• See page 144
• Power Amplifier circuit is shown in the shaded area
Transformer-coupled Push-pull Circuit
• Characteristics
• Input to the power transistors is through a transformer
• Center tapped
• Bases of Q2
& Q3 on
opposite sides
of the secondary
• Q2 conducts
on positive transition
• Q3 conducts
on negative transition
• Transformers selected for impedance matching
• T2 – 8Ω speaker and a 10:1 turn ratio Q2 & Q3 see a 800 Ω load
Transformer-coupled Push-pull Circuit
• Characteristics
• Transformers
• Usually have a heavier metal cores
• Exact transformer replacements are critical for this type of circuit
• Expensive components that are avoided in designs if
Complementary Symmetry or Quasicomplementary circuits can
be used for coupling
• Operation
• See Figure 6-12 on page 145
Transformer-coupled Push-pull Circuit
• Characteristics
• Real circuit
• Fig 6-13 on page 146
• Uses a transformer on the input for coupling
• Output stage, quasicomplementary Amp matches the load
impedance
• High input impedance at T1
• Less drift in output without direct coupling
High Power MOSFET Amps
• Characteristics
• Usually Complementary
• Uses both N and P type
MOSFETs
• High output power over a
wide frequency
• i.e., 250 W, from 5 -1MHz
• Usually a simpler design
than comparable bipolar
Amps
High Power MOSFET Amps
• Characteristics
• Sample circuit – Previous slide or page 147
• Only Output stage shown (missing biasing and driver circuits)
• The P-type MOSFETs Q3 & Q4 have their sources tied to +75V
• The N-type MOSFETs Q5 & Q6 have their sources tied to -75V
• All the output transistor drains are connected to the Speaker
circuit
• Zener Diodes are used to prevent overdriving the output
transistors with more than 8.2 V
• The impedance of L1 and R7 are to balance the reactance of the
load at high frequencies
High Power MOSFET Amps
• Characteristics
• Operation
• Positive going input signal
• Base of Q1 goes positive and its emitter voltage follows, but 0.7
volts lower
» VGS for Q3 and Q4 goes smaller – they remain turned off
• Base of Q2 goes positive, Q2conducts less, emitter goes positive
» VGS for Q5 and Q6 turn on
» Voltage at point X goes negative
• Negative going input signal
• Same type of scenario – but Point X goes positive
High Power MOSFET Amps
• Troubleshooting tips
• Voltage at point X should be at 0VDC w/out input
• Should have 3.5 volts across both transistors
• If not, probably the base biasing of either Q1 or Q2 is off
• If 8.2V – check for open Q1/Q2 or biasing problem
Troubleshooting
• First steps – look for the obvious
• Smoke
• Signs of overheating
• Power cord - unplugged, Fuse blown, etc
• Flow Chart on page 150
• Notes on Transistor testing
• Check all junction in both directions – High one way –
other Low resistance
• Double check all removed transistors – parallel
components can cause bad in-circuit readings
Troubleshooting
• Notes on replacing components
• Try for exact replacements
• Research any substitution parts
• Shorted power transistor
• Replace part and restart the system gradually using a Variable
transformer as shown on page 152
• With out the full AC supply you may be able to ID a part that
caused the failure of the power transistor before blowing the
replacement one you installed
• See test setup on page 153