Bipolar Junction Transistors (BJTs)

What are BJTs?

Bi-polar transistors are amongst the most widely used devices for amplification of all types of electrical signals in discrete circuits, (circuits made from individual components rather than integrated circuits). They are also used in circuits together with I/Cs since it is often more practical in circuits where large power or high voltage is needed to use discrete output transistors, while using low voltage, low power I/Cs for most of the signal processing. It is also often more convenient to use a discrete transistor for an individual circuit within a larger system for which integrated circuits are not readily available. For example an integrated circuit may carry out all of the processing of the signals in a system but then pass the processed signal to a single discrete transistor for power amplification to drive a loudspeaker. Transistors come in many shapes and types. A selection of typical transistors is shown in Fig 2.1.1

Fig. 2.1.1 Typical Bipolar Transistors

• 1. AC127

  Germanium audio output transistor - found in vintage radios, and in some more modern circuits where sensitivity to heat is required.

• 2. BF318

  Silicon Video amplifier transistor uses a collector/emitter voltage (VCEO) of about 150V and will amplify frequencies up to 80MHz

• 3. BU208A

  Silicon output transistor use in TVs and large screen monitors. Can deliver high power and withstand pulse VCEO voltages of about 1000V. The metal case (normally bolted to a heat sink) is the collector connection.

• 4. BD124

  Silicon TV output transistor wth a lower power rating.

• 5. BC108

  General-purpose Silicon voltage amplifier transistor; the silver case with a small tab to identify the emitter connection is a standard TO39 package.

• 6. BD 131

  Silicon audio NPN output transistor in a TO26 package for mounting on a suitable heat sink; will dissipate 15W and is often used as part of a push-pull pair with a matched BD132 PNP transistor.

Transistor Principle

Isolatie

This animation shows the transistor effect as the transistor is made to alter its state from a starting condition of conductivity (switched 'on', full current flow) to a final condition of insulation (switched 'off', no current flow). The animation begins with current flowing through the transistor from the emitter (point E) to the collector (point C). When a negative voltage is applied to the base (point B), electrons in the base region are pushed ('like' charges repel, in this case both negative) back creating insulation boundaries. The current flow from point E to point C stops. The transistor's state has been changed from a conductor to an insulator.

Geleiding

This animation shows the transistor effect as the transistor is made to alter its state from a starting condition of insulation (switched 'off', no current flow) to a final condition of conductivity (switched 'on', full current flow). The animation begins with the transistor acting as an insulator. In order to have it conduct, positive voltage must be applied to the base (point B). As opposite charges attract (in this case, positive and negative), electrons are 'pulled' out of the insulation boundaries and flow out of the base region at point B. The barriers that once restricted flow of electrons from the emitter to the collector are diminished. Electrons begin to flow in at the emitter (point E), through the base to the collector (point C). The transistor's state has been changed from an insulator to a conductor.

Versterking

This animation demonstrates how a transistor functions in a circuit. As it begins, the transistor is acting as an insulator-- that is when there are no sound waves activating the transistor, it simply blocks the flow of current through the circuit. But as the microphone converts incoming sound waves into waves of positive electrical current, the current travels along the left side of the circuit to the transistor and pumps electrons out of the base region. A large surge of electrons rushes through the transistor and transforms the weak incoming current into a stronger copy of itself. This stronger current then travels along the right side of the circuit to the speaker where it exits as amplified sound. Met toestemming van Lucent. Copyright © 2001 Lucent Technologies. All rights reserved. Informatie: D.J.F. Scheper

How does a transistor work?

The design of a transistor allows it to function as an amplifier or a switch. This is accomplished by using a small amount of electricity to control a gate on a much larger supply of electricity, much like turning a valve to control a supply of water.

Transistors are composed of three parts – a base, a collector, and an emitter. The base is the gate controller device for the larger electrical supply. The collector is the larger electrical supply, and the emitter is the outlet for that supply. By sending varying levels of current from the base, the amount of current flowing through the gate from the collector may be regulated. In this way, a very small amount of current may be used to control a large amount of current, as in an amplifier. The same process is used to create the binary code for the digital processors but in this case a voltage threshold of five volts is needed to open the collector gate. In this way, the transistor is being used as a switch with a binary function: five volts – ON, less than five volts – OFF.

Semi-conductive materials are what make the transistor possible. Most people are familiar with electrically conductive and non-conductive materials. Metals are typically thought of as being conductive. Materials such as wood, plastics, glass and ceramics are non-conductive, or insulators. In the late 1940’s a team of scientists working at Bell Labs in New Jersey, discovered how to take certain types of crystals and use them as electronic control devices by exploiting their semi-conductive properties.Most non-metallic crystalline structures would typically be considered insulators. But by forcing crystals of germanium or silicon to grow with impurities such as boron or phosphorus, the crystals gain entirely different electrical conductive properties. By sandwiching this material between two conductive plates (the emitter and the collector), a transistor is made. By applying current to the semi-conductive material (base), electrons gather until an effectual conduit is formed allowing electricity to pass The scientists that were responsible for the invention of the transistor were John Bardeen, Walter Brattain, and William Shockley. Their Patent was called: “Three Electrode Circuit Element Utilizing Semiconductive Materials.”
There are two main types of transistors-junction transistors and field effect transistors. Each works in a different way. But the usefulness of any transistor comes from its ability to control a strong current with a weak voltage. For example, transistors in a public address system amplify (strengthen) the weak voltage produced when a person speaks into a microphone. The electricity coming from the transistors is strong enough to operate a loudspeaker, which produces sounds much louder than the person's voice.

JUNCTION TRANSISTORS

A junction transistor consists of a thin piece of one type of semiconductor material between two thicker layers of the opposite type. For example, if the middle layer is p-type, the outside layers must be n-type. Such a transistor is an NPN transistor. One of the outside layers is called the emitter, and the other is known as the collector. The middle layer is the base. The places where the emitter joins the base and the base joins the collector are called junctions.

The layers of an NPN transistor must have the proper voltage connected across them. The voltage of the base must be more positive than that of the emitter. The voltage of the collector, in turn, must be more positive than that of the base. The voltages are supplied by a battery or some other source of direct current. The emitter supplies electrons. The base pulls these electrons from the emitter because it has a more positive voltage than does the emitter. This movement of electrons creates a flow of electricity through the transistor.

The current passes from the emitter to the collector through the base. Changes in the voltage connected to the base modify the flow of the current by changing the number of electrons in the base. In this way, small changes in the base voltage can cause large changes in the current flowing out of the collector.

Manufacturers also make PNP junction transistors. In these devices, the emitter and collector are both a p-type semiconductor material and the base is n-type. A PNP junction transistor works on the same principle as an NPN transistor. But it differs in one respect. The main flow of current in a PNP transistor is controlled by altering the number of holes rather than the number of electrons in the base. Also, this type of transistor works properly only if the negative and positive connections to it are the reverse of those of the NPN transistor.

FIELD EFFECT TRANSISTORS

A field effect transistor has only two layers of semiconductor material, one on top of the other. Electricity flows through one of the layers, called the channel. A voltage connected to the other layer, called the gate, interferes with the current flowing in the channel. Thus, the voltage connected to the gate controls the strength of the current in the channel. There are two basic varieties of field effect transistors-the junction field effect transistor(JFET) and the metal oxide semiconductor field effect transistor (MOSFET). Most of the transistors contained in today's integrated circuits are MOSFETS's.