A transistor amplifier is an electronic circuit that uses a semi-conducting transistor instead of a tube or integrated circuit chip to amplify electrical signals. Typically used in audio applications, a transistor amplifier provides excellent performance in a relatively small package. It has largely replaced the vacuum tube signal amplifier and remains a strong competitor to the more modern integrated circuit (IC) amplifier.
Before the invention of the transistor in 1947, amplifiers used vacuum tubes. Vacuum tubes were large, bulky, fragile and inefficient, and they required time to warm up. Transistors eliminated all of these problems while also offering the ability to amplify signals with much less distortion. In addition, they were able to output more powerful signals, allowing some transistor amplifiers to output hundreds of watts per channel. Their small size and low power consumption also made possible the invention of battery-powered portable audio components, such as transistor radios.
The structure of a transistor amplifier circuit is relatively simple. In it, a power supply is connected to the transistor’s collector terminal, and the signal to be amplified gets fed into the base terminal. The transistor uses the signal at the base to determine how much power from the collector flows through its gate to the emitter terminal, which transfers the amplified signal. If a transistor is compared to a faucet valve, the collector would be the supply pipe, the emitter would be where the water comes out, and the base would be the hand that turns the spigot on, off or somewhere in between.
Amplifiers using IC chips began to replace the transistor amplifier in the 1960s. The IC chip combined multiple electronic components onto one small piece of silicon, allowing it to do more in much less space. Bad sound quality and very limited power output capabilities plagued these types of amplifiers. Over the years, though, the technology has improved to the point that most portable and lower-cost home audio components use IC amplifiers.
Even with low-cost ICs, many home audio components still use transistor amplifiers, although they are frequently described as discrete amplifiers. This circuit type is more prevalent in power amplifiers and in the final output stage of amplifiers, both of which take the line-level signal from the preamplifier and amplify it for output to speakers. Some high-end source components and preamplifiers also use transistor amplifiers, though. In either case, these amplifier circuits use metal oxide semiconductor field-effect transistors (MOSFETs) as the source of amplification.