Transistor Circuit Design Tutorial

- introduction or tutorial in the basics of designing transistor circuits: circuit types, design methodologies, equations and techniques.

Transistor circuit design lies at the heart of today's electronics revolution.

Since the transistor was first developed in the late 1940s, it has become a part of everyday life.

Transistor circuits are used in an ever increasing number of items of equipment ranging from radios to mobile phones, computers to washing machines, automobiles to scientific equipment and much more.

The transistors may be in the form of discrete components, or more commonly in the form of integrated circuits. However the same basic design principles are required whether the transistor may be found.

Bipolar transistor basics

Obviously the key component in any transistor circuit is the transistor itself. These components can be obtained in a discrete form, or they may be within an integrated circuit.

The transistors are manufactured in a variety of formats and can be obtained to fulfil a variety of roles from small signal to high power, and audio to RF and switching.

Note on the Bipolar Transistor:

The bipolar transistor transformed the world of electronics. It is a key component for the electronics industry. The components consist a thin base region of either n or p-type semiconductor sandwich between layers of the opposite type. They can be manufactured in many way, often being included within integratde circuits.

Click on the link for further information about the Bipolar Transistor

Circuit design requirements

Before starting on the design of the transistor circuit, it is necessary to define the requirements.

Without knowing what is required of the circuit, it is not possible to design the circuit. There are no aims for it.

There can be a number of parameters required in the requirements for the transistor circuit design:

  • Voltage gain:   The voltage gain is often a key requirement. It is the output signal voltage divided by the input signal voltage.
  • Current gain:   This is the gain of the transistor circuit in terms of current. For example a circuit driving a loudspeaker will need to have a large current gain to be able to provide sufficient current to drive the loudspeakers.
  • Input impedance:   This is the impedance that the previous stage will see when it is providing a signal to this transistor circuit in question.
  • Output impedance:   The output impedance is also important. If the transistor circuit is driving a low impedance circuit, then its output must have a low impedance, otherwise a large voltage drop will occur in the transistor output stage.
  • Frequency response:   Frequency response is another important factor that will affect the transistor circuit design. Low frequency or audio transistor circuit designs may be different to those used for RF applications. Also the choice of the transistor and capacitor values in the circuit design will be greatly affected by the required frequency response.

Many of these requirements are linked, For example low impedance outputs are likely to need a high current gain. As a result when undertaking the design of the transistor circuit, it is necessary to have a grasp of the overall circuit and what it needs to do.

By Ian Poole

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