An active circuit is any type of circuit component with the ability to electrically control electron flow (electricity controlling electricity). In order for a circuit to be properly called electronic, it must contain at least one active device. Components incapable of controlling current by means of another electrical signal are known as passive devices. Resistors, capacitors, inductors, transformers, and even diodes are all considered passive devices.

Transistors

A Transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits.

There are many types of transistors, but the most common are Bipolar Junction Transistors and Metal Oxide Semiconductor Field Effect Transistors, known as MOSFETs, or just "FETs" for short. The schematic symbols for Bipolar Junction Transistors are shown below:

The schematic symbols for MOSFETs are shown as:

Try this MOSFET Simulator to visulize the operation of a MOSFET, or this MOSFET Video,

You can think of the MOSFET as simple switch, when the Gate pin of the MOSFET is active, then current is allowed to flow from the Source pin to the Drain pin of the MOSFET. This is the basis of how all integrated circuits function.

These are called Active Circuits because they can amplify voltage or current, not just consume it. For example, here is a very simple amplifier circuit, that could be used to drive a small speaker. It uses a complementary transistor pair (NPN & PNP) to make a Class AB Amplifier with just a few resistors and capacitors. It makes a small headphone or speaker amplifier.

Operational Amplifiers

An operational amplifier (often op-amp or opamp) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output.

The schematic symbol for an OpAmp is:

Opamps can be used in a number of Analog amplifier and processing circuits, like active filters. The following circuit is an Non-Inverting Amplifier:

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Further (Links)

• Transistor Video