One of the key challenges in many audio output stages, and many other output stages is to be able to provide a stage that is able to have two devices, one for each half of the cycle.
One of the most popular solutions is known as a quasi-complementary symmetry output or a pseudo complementary output providing virtually the same configuration on either side, but one being the mirror of the other.
If a single transistor was used one side would need to be PNP and the other NPN. This was not always totally viable using a single transistor. In the past, NPN power transistors offered much better performance when compared to PNP power transistors. As a result the quasi complementary output was became an ideal option. It had the advantage that both output transistors were the same and often transistors such as the old 2N3055 could be used.
Quasi complementary symmetry transistor output stage
The quasi-complementary symmetry output or a pseudo complementary output is based around the use of a Darlington pair and a Sziklai pair - one used for each half of the waveform.
In this quasi complementary symmetry amplifier, the transistor pair formed from Q2 and Q3 is a Darlington, and the transistor pair formed from Q4 and Q5 is a Sziklai or complementary pair.
The diodes provide the correct bias between the two pairs of transistors, accommodating the required number of base emitter junctions. The variable resistor, VR1 enables the correct bias current to be set through the diodes to ensure that crossover distortion is minimised.
Although the circuit has been widely used in the past, quasi complementary symmetry amplifiers are used less these days because NPN and PNP pairs with very similar performance are available.
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