1. Electronic Troubleshooting
Chapter 6
Power Amplifiers

2. Power Amplifiers Characteristics Power Amps Topics Covered
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
Reducing Crossover Distortion
Adding a Driver to the Complementary Symmetry Stage
Quasicomlementary Amps
Transformer-coupled Push-pull Circuit
High Power MOSFET Amps

3. 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

4. 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

5. 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

6. 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
Each transistor is on ½ of the circuit operation
thus only supplies ½ of the power supplied to the load

7. 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

8. 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
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
Sample Problem 6-1 on page 137
In Class: 6-5, 6-6, 6-7, 6-8

9. Reducing 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
Call Crossover distortion
Also generates odd harmonics

10. Reducing Crossover Distortion
Cure to reduce distortion
Insert a diode between the base ins 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

11. 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

12. Adding a Driver Key Aspects Operation Q3 acts as ~~ continued Start-up
RA and RB provide VB
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

13. Adding a Driver Operation Real Example 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

14. Adding a Driver Real Example Highlighted section
Notice the added 1Ω emitter resisters on Q4 and Q5
Limit 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
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

15. 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 increased by the low resistance values of R15 & R14
Thus the gain for Q3 is greater, AV3 = rL3/re3

16. 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

17. 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

18. Quasicomlementary Amps
Characteristics
Actual circuit
See page 144
Power Amplifier circuit is shown in the shaded area

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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