Electronic Troubleshooting

Chapter 6Power 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

• Each transistor is on ½ of the circuit operationthus only supplies ½ of the power supplied to the load

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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

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Complementary Symmetry Output Stage• Operation• Power

• Each transistor is on ½ of the circuit operationthus 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

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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 verysimilar 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