The transistor enables many circuits to be designed with a variety of different functions. From amplifiers to buffers and filters to oscillators, current sources, long tailed pairs and many, many, more.
Whatever the circuit, the basic biasing conditions follow the same basic rules, but the topology of the circuit enables transistor to be used in a variety of ways to provide many different circuit functions.
There are many different types of transistor circuit. Each type has its own topology and fulfils a different function.
A selection of the different types of transistor circuit is given below:
- Common emitter transistor amplifier circuit: The common emitter amplifier is one of the most widely used types of transistor circuit. It is used in many applications where medium levels of input an output are acceptable and a medium voltage gain is needed. . Read more about the Common emitter transistor amplifier
- Emitter follower transistor circuit: This type of transistor circuit is often used where a high input resistance and low output resistance is needed. It acts as a buffer circuit. It is also known as a common collector circuit. Read more about the Common collector / emitter follower transistor amplifier.
- Common base transistor circuit: The common base form of transistor circuit is not as widely used as the common emitter and emitter follower types. It does find applications in some microphone amplifiers and also for VHF / UHF RF amplifiers. Read more about the Common base transistor amplifier.
- Darlington pair: The Darlington pair is a form of transistor circuit configuration that is used to provide very high levels of gain. With the emitter of the first transistor connected to the base of the second, it effectively gives beta multiplication. This type f transistor circuit can be used in many ways and can give excellent performance. Read more about the Darlington pair.
- Sziklai pair: The Sziklai pair has many similarities to the Darlington, but using a slightly different configuration it can be used with a Darlington in transistor amplifier output stages. Read more about the Sziklai pair.
- Current mirror circuit: This type of transistor circuit is used widely within integrated circuits. It has two arms and the current flowing in one arm is mirrored by the other. Read more about the Current mirror circuit.
- Long tailed pair: The transistor long tailed pair circuit is a basic form of differential amplifier which forms the basis of many op-amp circuits. Read more about the Long tailed pair amplifier.
- Constant current source: . Read more about the Active constant current source.
- Capacitance multiplier: The transistor capacitance multiplier circuit has the effect of multiplying the effective capacitance of a capacitor by the value of β of a transistor. Read more about the Capacitance multiplier.
- Two transistor amplifier: A useful little circuit using a PNP and an NPN transistor providing a defined level of gain, with possibility for more gain that that provided by a single transistor. Read more about the Two transistor amplifier
- High pass filter: Even though operational amplifiers are capable of providing very good high pass filters, there is sometimes the need for a simple transistor circuit to provide the same function. Read more about the High pass filter.
This gives an outline of some of the different transistor circuit types. There is a vast number of different circuit types, each with its own function and parameters. Using these it is possible to build much larger circuits providing more complex overall function.