Amp output transistors are used for improving the incoming current gain to match it with the amperage rating of the load (speakers).
Output transistors are regular bipolar or FET transistors used to provide extra current to speakers. Usually, non-FET output transistors are used with driver transistors for increasing the total current gain. This is because most output transistors have a small HFE gain. So, a driver transistor is installed in front of output transistors in the amp to increase the current gain.
MOSFET transistors are usually used as standalone output transistors. They don’t need driver transistors because they boast a high HFE gain.
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What Output Transistors Do?
Output transistors essentially amplify the current of the incoming signal so that it can match the amperage rating of the connected speakers. More importantly, they do this current amplification at a comparatively low voltage supply. Without going into the complex electronics, let’s try to understand how output transistors do that.
An amp output transistor, like any regular transistor, has three components base, emitter, and collector. The amount of current flowing between the collector and emitter (output) depends on the voltage at the base. Output transistors can drive a large amount of current from the collector to the emitter (the load) even with a small signal at the base pin.
In other words, you can drive a large amount of current to the load (in this case it is speakers) by having a small signal at the “control” (base) pin.
To further amplify the current gain of the output transistor, another transistor can be installed in front of it.
Amp Output Configurations
There are essentially three types of amp output configurations you can find inside different car amplifiers. All these configurations/arrangements use more than one transistor.
Darlington Complementary
In this configuration, all the output transistors are arranged as emitter followers (emitter of one NPN transistor following the emitter of the other PNP transistor). This is the most basic amp output configuration.
Quasi Complementary
In this amp output transistors arrangement, all transistors are NPN. Also, here the emitters of one set of transistors follow the collectors of the other set of transistors. This configuration is used in the majority of contemporary amplifiers. It produces a higher gain than the Darlington arrangement.
Compound Complementary
Compound Complementary configuration entails the directly (without resistors) connected collectors of PNP transistors with the collectors of NPN transistors. Compound complementary configuration is better than the rest of the two because it can provide higher gain. Also, it remains stabilized during temperature variations.
