Electronic Troubleshooting Chapter 6 Power Amplifiers

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

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 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 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 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

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

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

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

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

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 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

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

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

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

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