Understanding Transistor Circuit Design: tutorial

It is possible to easily design many transistor circuits by understanding their operation and following some straightforward guidelines and applying some simple equations.

Transistor Circuit Design Tutorial Includes:
Transistor circuit design     Circuit configurations     Common emitter     Common emitter circuit design     Emitter follower     Common base    

See also: Transistor circuit types    

Transistor circuits lie at the very centre of today’s electronics technology. Although integrated circuits are used for most elements of circuits these days, basic transistor circuit design is often required in a variety of areas.

Understanding transistor circuit design is still important as it not only enables basic transistor circuits to be designed, but it also provides a greater understanding into the operation of integrated circuits that are based on bipolar transistor technology.

BC547 plastic leaded transistor: transistor gain Beta, is upwards of 110
BC547 Plastic leaded transistor

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.

Transistor basic structure and circuit symbols
Transistor basic structure and circuit symbols

Note on the Bipolar Transistor Device:

The bipolar transistor is a three terminal device which provides current gain where the collector current is Β times that of the base current. The bipolar transistor is widely available and its performance optimise over many years.

Read more about Bipolar Transistor Device & How it Works

The bipolar transistor was first invented in 1949 by a team of scientists working at Bell Labs in the USA. Its discovery makes interesting reading.

Note on Transistor History:

The bipolar transistor was invented by three researchers working at Bell Labroratories: John Bardeen, Walter Brattain, and William Schockley. They had been working on an idea that used a field effect to control the current in a semiconductor, but they were unable to make the idea work. They turned their focus onto another possibility and made a three terminal device using two closely spaced point contacts on a wafer of germanium. This idea worked and they were able to demonstrate it provided gain in late 1949.

Read more about Bipolar Transistor History

Transistor circuit design parameters

Before starting on the basics of transistor circuit design, it is necessary to define the requirements for the circuits: some of the main parameters associated with transistor circuits.

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.
  • Supply voltage and current:   In many circuits the supply voltage is determined by what is available. Also the current may be limited, especially if the finished transistor circuit design is to be battery powered.

These represent some of the major circuit parameters required for a transistor circuit design. Knowing these parameters can govern the choice of the circuit configuration, and it will certainly govern the determination of the component values and many other factors.

Accordingly it is necessary to know the parameters governing the operation of the transistor circuit before the design can be started.

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