# What Are the Functions of a Transistor in a Circuit?

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Transistors are devices made from different types of semiconductors. When voltage and current are applied to one of the leads, it can control the current going through the other two leads. Until the 1950s, this function was performed by vacuum tubes, but transistors are smaller, more shock-resistant and cheaper to manufacture, and they require less power and can be reduced to fit on chips. Like vacuum tubes, transistors can perform a variety of functions.

## Electronic Switches

• Transistors can function as electronic switches. In many hobby projects, there will be only one transistor and it is functioning as a switch. A small change in electric potential on one of the leads of the transistor can turn on and off a large current flowing between the other two leads of the transistor. Power transistors can even allow a small current going to a push button to control a circuit carrying thousands of volts, which keeps dangerous levels of current away from human contact. In the proper circuits, the transistors can "latch," which means that once they are turned on they stay on even when the activating signal goes away.

## Amplifiers

• When the surrounding circuits are a little different, the same transistor that functions as a switch can work as an amplifier. A small signal on one of the leads is faithfully reproduced on a bigger scale in a larger current flowing through the other two leads. Amplifiers are essential to communications electronics because the signal that comes from the antenna is too weak to drive the speaker. It must be amplified -- usually through several stages -- to be useful for radios, cell phones and other audio devices.

## Transistor-Transistor Logic

• Transistor-transistor logic, or TTL, is a set of transistor circuits that implements logical operations, such as AND, OR and NOT. These logical circuits can be combined to make more complex logical circuits. Almost everything in the computer -- including mathematical operations like addition, subtraction, multiplication and division -- are implemented in TTL circuits. Even operations like moving memory around and checking error correcting codes will have a TTL component. Application specific integrated circuits, or ASIC -- chips that do a specific job -- are often networks of TTL circuits.

## Integrated Circuit Chips

• One of the great improvements of transistors over vacuum tubes is the way transistors can be reduced in size. You can hold one or two vacuum tubes in your hand, but you can hold a dozen transistors in your hand. In an integrated chip, you can hold millions of "transistors" in your hand. Transistors are made up of levels of different semiconductor materials. Integrated circuits are slabs of different semiconductor materials that are microscopically etched to make transistors, wires and other components. The good thing about information processing is that the components can be very small -- a switch is a switch, no matter how small it is. TTL can do the same job no matter how small the components are.

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