Understanding Transistor Circuit Design: tutorial

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 electronic circuit design technology. Although integrated circuits are used for many circuits these days, basic transistor circuit design is often required in a variety of areas.

Although using discrete electronic components with transistors uses more components, it is possible to tailor the circuit to provide exactly the functionality that is required. Accordingly circuits using discrete transistors and a few additional electronic components is till at the heart of electronic circuit design .

This means that 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.

Bipolar transistor basics

Obviously the key electronic component in any transistor circuit is the transistor itself. These electronic 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.

They also come as PNP transistors and NPN transistors - of these NPN transistors are more widely used as tend to fit in with the negative ground system widely used, and also their performance is better in terms of speed.

Although NPN transistors are more widely used, this does not mean that PNP transistors are not used. They often find applications complementing NPN transistors and a few other circuits.

The bipolar transistor has been available for over seventy years - its technology is very well established, and although field effect transistor technology is probably more widely used in integrated circuits, bipolar transistors are still used in huge quantities in various analogue and digital circuits, both within integrated circuits and as discrete electronic components.

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.

Transistor circuit design parameters

Before starting on the electronic circuit design for a transistor circuit, 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 for electronic circuit design. The voltage gain of the circuit is the increase in voltage from the input to the output of the circuit. In terms of the mathematics, the voltage gain, A_v is the output voltage divided by the input voltage.

  Voltage gain is one of the key aims of many circuits because it enables the "size"

• Current gain:   The current gain of the circuit is often important, in electronic circuit design, especially where the circuit is driving a low impedance load. Often a circuit with no voltage gain, and only current gain is needed to enable a circuit with a relatively high impedance output to drive another circuit that has a lower impedance.

  There are many examples of this: an RF oscillator often needs a buffer stage to ensure that the oscillator circuit itself is not loaded unduly, but an output is needed to drive other circuits. Current gain is also used in power supply circuits where the series pass element of the voltage regulator needs to provide significant levels of current, but using a low current voltage reference. There are many other examples of where current gain is needed.

  Like the voltage gan, the current gain of a circuit compares the input and output levels, but in terms of current. The Current gain is equal to the output current divided by the input current.

• Input impedance:   The input impedance of a transistor circuit is always important. It determines the loading on the previous stage, and it is also important in RF circuits where impedance matching is an important parameter.

  In many electronic circuit designs, a high input impedance is desirable because it means that the previous stage is not unduly loaded. If the input impedance of the transistor circuit is too low then it will load the previous one, reducing the signal level and possibly causing distortion in some instances. Configuring a transistor stage to provide the right input impedance is a key element of the electronic circuit design process.

• 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 and in some instances distortion of the signal could occur.

  If the load impedance is low, then a circuit with a high current gain is typically needed and a suitable circuit format can be chosen during the electronic circuit design process. If a higher output impedance is allowable, then a circuit with a higher voltage gain is often more suitable.

• Frequency response:   Frequency response is another important factor that will affect the transistor circuit design. Low frequency or audio transistor circuit designs are very different to those used for RF applications. Also the choice of the electronic components within the circuit govern the response: the transistors, as well as the capacitor and resistor values in the electronic circuit design all affect the frequency response.

  In the early part of the circuit design, it is necessary to have a defined requirement for the frequency response needed, and then the circuit can be designed around the requirement.

• Supply voltage and current:   One of the key parameters for any circuit is the power requirements in terms of voltage and current required. In this way, it can be ensured that the right voltage is provided with the required current capability at the electronics circuit design stage.

• Power dissipation:   Another parameter very much allied to the voltage and current supplied to the circuit is the power that is dissipated. If the power dissipation is high, then arrangements may need to be made for cooling and generally removing heat from the circuit, and in particular any electronic components that may dissipate large amounts of heat. Typically this will be the transistor, but other components too may dissipate heat.

Transistor circuit function

There are many different functions that transistor circuits can perform. There are normally standard blocks for the common functions like amplifier, oscillator, filter, current source, differential amplifier and a host of others.

These standard circuit formats are widely used and can be adopted and the electronic component values determined during the electronic circuit design process.

The circuits often follow proven circuits that have been used for many years. These circuits have often been used with the old vacuum tube or thermionic valve technology and work equally well with bipolar transistors as well as field effect transistors, FETs, and sometimes even operational amplifiers.

The basic format is adopted and the values for the electronic components is determined to provide the required performance.Often this requires a little experimentation, but these days circuit simulation software is able to accurately replicate the operation for the circuit so that the electronic component values can be optimised for the required performance and functionality.

Transistor circuit configuration or topology

Whatever the overall function of the circuit, it is also necessary to consider the topology at the beginning of the electronic circuit design process.

Transistors circuits can be designed using different topologies, each one offering different characteristics, especially in terms of the input and output impedance.

These topologies of configurations are chosen according to the electronic circuit design requirements and include common emitter, common collector or emitter follower, and common base.

Transistor circuit design process

There are several stages to the transistor design process. These are typically taken in a logical order, but often there is some revisiting of the different stages to optimise the values of the various electronic components to provide the required overall performance.

• Determine requirements:   Determining the real requirements is an important stage, and getting this right will mean that the concept of the circuit does not change at a later date.

• Define circuit function & topology:   Once the overall requirements have been settled for the complete electronics device, it is necessary to decide upon the actual transistor circuit. For example there are many oscillator circuits, filters, amplifiers, etc. for transistors and the optimum type can be chosen for the particular requirement. This often also defines the actual circuit topology, i.e. the use of common emitter, common collector, common base, but if not this can form part of the overall decision making at this time, because loading on oscillators, gain, output impedance and the like can be considered at this time.

• Set up bias conditions:   In any circuit, one of the key features of the electronic circuit design is to ensure that the bias levels for the active devices: in this case the bipolar transistors are set correctly. If the bias is incorrect, the transistor circuit will not function. Determining the values of the electronic components (mainly the resistors) that set the bias is one of the key stages in the design.

• Determine functional electronic component values:   Along with setting the bias conditions, the values for the other electronic components to provide the circuit functionality need to be determined. This part of the electronic circuit design process proceeds along with setting the bias conditions, as the values for one will affect the other and vice versa.

• Revisit electronic component values for bias and function:   With the circuit values set, there is always a little iteration needed to balance the requirements for bias and circuit overall functionality. There is likely to be some iteration around this process.

• Test circuit:   Testing the circuit is a key element of any design. Often many laboratories will have circuit simulation software and therefore the circuit can be simulated before it is built to remove most of the issues. However the final test is to build and run the circuit under conditions as close as possible to the operational conditions.

• Rework and modify:   Often it will be necessary to modify the electronic circuit. If this is required, then it is reworked and tested with the new electronic component values, layout, etc.

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.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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