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65.5%. If the output transistors were perfect switches the efficiency would be 78%.
The efficiency of a class B amplifier changes with output power. Let s examine a simple
example. Let s say we have a power supply of +/- 50v. We also have a 10 ohm load (easy for
calculation). Let s assume the output moves 10 volts positive. Then 20 volts until it reaches
the rail of 50 volts. The output transistors are perfect for this example, NO LOSSES.
Output voltage Output current Voltage left Dissipation in the
amps Across the output transistor in watts
Output transistor
0 0 50 0
10 1 40 40x1=40
20 2 30 30x2=60
30 3 20 20x3=60
40 4 10 10x4=40
50 5 0 0x5=0
+50 VOLTS +50 VOLTS +50 VOLTS +50 VOLTS +50 VOLTS
OUTPUT OUTPUT OUTPUT OUTPUT OUTPUT
TRANSISTOR TRANSISTOR TRANSISTOR TRANSISTOR TRANSISTOR
+50 VOLTS +40 VOLTS +30 VOLTS +20 VOLTS +10 VOLTS
0 VOLTS 10 VOLTS 20 VOLTS
30 VOLTS 40 VOLTS
10 Ohm LOAD 10 Ohm LOAD 10 Ohm LOAD 10 Ohm LOAD 10 Ohm LOAD
0 Amps 1 Amps 2 Amps 3 Amps 4 Amps
So as you can see the dissipation in the output transistors increase to a peak and then
decrease. If we did this volt by volt maximum dissipation in the output transistors would be at
44% of absolute unclipped power.
The class A-B amplifiers we use and talk about are operating in class A mode only to
extremely low power levels. Let s see what s happening. The 50w/ch amplifier is set to idle at
say 50mA (0.05 amperes) and we have a 4 ohm load. Remember Ohm s Law. IxIxR= Power.
0.05x0.05x4=0.01 watts. Yes 0.01 watts or 10mW. A typical 50 watt amplifier runs in class A
up to TEN THOUSANDTHS OF A WATT, NO MORE NO LESS.
Lastly Class D
Class D (PWM) amplifiers.
Class D
Class D
This type of amplifier uses MOSFETS as switches. A high frequency carrier is mixed with the
audio signal and the output Mosfets are on or off depending on the average level of the audio
signal. Simply put when a positive pulse of audio exceeds the absolute value of the carrier,
then the positive Mosfet turns on. This action happens at the frequency of the carrier
(Typically > 100KHz). A low pass filter removes the carrier from the signal to be applied to the
speaker and what is left is amplifies audio. There are numerous ways of achieving this result
but at the end of the day the Low Pass filter must be used to remove the high frequency
carrier. Class D amplifiers for low frequencies are fine but in our opinion they kind of suck for
full range. Due to the fact that the output Mosfets are either on or off, there are much smaller
losses than their analog counterparts. Efficiencies as high as 95% are attainable but typically
80-90% is practical and this varies with output power and load. The higher the power, the
higher the efficiency, the lower the load, the lower the efficiency. The efficiency numbers
manufacturers quote are those at maximum output into the highest impedance (4 ohms?) but
this is misleading since who can play their amplifier at maximum power?
Copyright Information This document including all text, diagrams and pictures, is the
property of Zed Audio Corporation and is Copyright 2005.
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