What Is an Audio Transistor?

G.W. Poulos

Transistors are electronic devices that perform switching or amplification functions on electrical signals. An audio transistor, in simplest terms, is any transistor intended to work with audio signals. Typically, an audio transistor is an amplification type transistor used to boost weak audio signals, such as in radios, to levels usable by other electronic devices, such as filters, equalizers, other amplifiers, and speakers.

A transistor helps amplify signals.
A transistor helps amplify signals.

Standard audio transistors are typically NPN-type bipolar junction transistors. They are made of three pieces of either germanium or silicon that have been treated with either a positively or negatively charged material. In an NPN-type audio transistor, two of the pieces are negative and one is positive.

Transistors are typically identified by a printed code, which often includes one or two letters followed by a series of numbers.
Transistors are typically identified by a printed code, which often includes one or two letters followed by a series of numbers.

The three pieces of the transistor are then fused together in a line, with the positively charged section sandwiched between two negatively charged sections, creating two junction points. Next, a wire lead attaches to each of the three sections of semiconductor material. Finally, when the entire transistor is coated with an insulting covering that will also help it dissipate heat, the transistor is complete.

One negative piece of semiconductor is the collector of the transistor and is the portion that receives the incoming audio signal. The positive section is the base of the transistor and is where the voltage will be applied that will boost the incoming signal. Finally, the remaining negatively charged section is the transistor’s emitter, and it is from here that the amplified audio signal will pass from the transistor to the rest of the circuitry.

In amplification applications, the transistor receives an audio signal through its collector and a separate signal is applied to its base. The output at the transistor’s emitter will be identical to the collector signal, with the exception that it will increase in power by an amount proportional to the signal applied to the base. Additionally, varying the signal applied to the base will vary the amplification of the audio signal leaving the transistor.

Transistors can vary in a number of ways. Depending on their design, they can have differing amounts of amplification potential and signal fidelity. Audio transistors tend to be discrete, that is isolated, and have excellent signal fidelity qualities.

There are a number of different types of audio transistors, each performing a given function to process the audio signal. Often, the difference between these types is merely a matter of degrees. For example, one type might amplify an audio signal only to a modest level, such as in a preamplifier, with only enough power to drive a set of headphones.

Another type of audio transistor, a power-audio transistor, boosts that preamp signal to a point capable of driving large loudspeakers. Sometimes, these types of transistors can create a signal that is hundreds of watts in strength, such as in the case of amplifiers used at concerts. Additionally, new types of transistors, such as field transistors with broader ranges of capabilities, are finding roles as audio transistors capable of taking a small signal to the point of driving a speaker.

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