The electrical ratings of transistors are generally given in terms of a few basic characteristics. Since these characteristics are, for the most part, different from those used to describe vacuum tubes or other electronic components, it is important that the electronic technician, student, and serviceman become familiar with them.
1. TRANSISTOR TYPES
Transistors can be divided into two general types, according to the method of construction employed point contact and junction. In addition, both basic types can be further subdivided. Point contact transistors can be classed as having either a PType or an NType base. A further subdivision into triodes and tetrodes is also possible. Junction transistors may be classified as PNP or NPN. Still further classifications may be introduced as additional transistor types are produced. For example, a number of manufacturers are developing multi purpose transistors, high power transistors, and other special types. At present, there is considerable variation in terminal lead connections between transistors supplied by different manufacturers. However,there is a definite tendency toward standardization and, in the future, we may expect most transistors to have similar lead connections. However, even if the lead connections are known, special care should be exercised to check the transistor types used whenever assembling new circuits or repairing transistorized equipment. The application of improper power supply voltage polarities, even if only momentarily, may easily ruin a transistor.
2. PHYSICAL CHARACTERISTICS
All commercially available transistors are physically quite small, with overall dimensions, exclusive of leads, seldom exceeding a fraction of an inch. Their weight, too, is generally minute, usually but a tiny part of an ounce. The semiconductor crystal itself may be molded in plastic or glass, or both, for protection and mechanical stability. There may be an outer case or shell of metal. Some manufacturers also offer hermetically sealed units, in which the transistor crystal is sealed in a vacuum. Both ambient temperature and humidity conditions may affect transistor operation. At this writing, no manufacturer recommends operation of their units above 100°C, and many specify a maximum ambient temperature of 40 or 50°C. As far as moisture is concerned, this problem is only important if the transistors used are not hermetically sealed, and if the humidity, under operating conditions, is likely to be high. In such cases, special care must often be taken, when designing the transistorized equipment, to insure a reasonably well sealed case.
3. ELECTRICAL CHARACTERISTICS
Manufacturers of transistors generally tabulate the electrical specifications of their units in threeways, as follows:
- MAXIMUM RATINGS,
- TYPICAL OPERATING CHARACTERISTICS, also called AVERAGE CHARACTERISTICS,
- CHARACTERISTIC CURVES (or CHARTS)
The Maximum Ratings are those that must not be exceeded when operating the units. These values are especially important to the experimenter, to the electronics engineer, and to any worker designing or assembling new circuits in which transistors are employed. In most cases, the Maximum Ratings will include the Maximum Collector Voltage, Maximum Collector Current, Maximum Emitter Current,and Maximum Collector Dissipation. In the case of point contact transistors, the Maximum Peak Inverse Emitter Voltage may also be included. Voltages are generally given with respect to the base, with the values indicated in volts. Currents are given in milliamperes or micro amperes,and the power dissipation values are given in watts or milliwatts, at a specific ambient temperature. The Typical Operating Characteristics are given to serve as a guide to the engineer or designer building new equipment, or to the service manor technician called on to service transistorized apparatus. These values may vary widely,depending on the "operating conditions" chosen by a particular manufacturer as typical. From a practical viewpoint, the Typical Operating Conditions need not be followed exactly in designing transistor circuits, provided the Maximum Ratings are not exceeded. Under Typical Operating Characteristics will generally be listed such values as Collector Voltage, Emitter Current, Current Amplification Factor, Cut off Current, Power Gain, Input Impedance (or Resistance), Output Impedance, Output Capacitance, Noise Factor, Frequency Cut Off, Power Output, and Distortion. Several additional values may be given by some manufacturers, including Base Resistance, Output Admittance, and other characteristics, but the values listed above are the most common. The Collector Voltage and Emitter Current are given in volts and milliamperes (or micro amperes), respectively. These values establish the basic conditions under which the other characteristics have the indicated numerical values. In addition, it is generally customary to indicate the circuit arrangement used to obtain the values given in the manufacturer's technical data sheet. Many of the values listed, especially those for Power Gain, Input and Output Impedance, Power Output and Output Capacitance, may have quite different values with other circuit arrangements. If not otherwise stated, the Typical Operating Characteristics given by a manufacturer generally apply where the Grounded Base (or Common Base)circuit is employed. The Greek letter alpha (a) is used to indicate the Current Amplification Factor of a transistor. The alpha of junction transistors cannot be above1.0, but values appreciably above 1 may be obtained with point contact transistors. Alpha is related to the gain that can be obtained from a transistor and is defined as "the ratio of the change in collector current for a specific change in emitter current at a constant collector potential. "The Cut Off Current is the collector current drawn under conditions of zero base current, with a d.c. voltage applied between emitter and collector. Its value is generally listed only for junction transistors. The Cut Off Current usually has a value of from 5 to 25 micro amperes, although it may be higher or lower with individual units. Both the Power Gain and the Noise Factor are given in decibels (db). Internal noise, in transistors, is caused primarily by the molecular agitation resulting from the movement of holes and electrons through the material of the semiconductor. Although the Noise Factor for junction transistors is generally lower than that for point contact transistors, it is higher than that for vacuum tubes giving comparable gain. In the case of junction transistors, the Noise Factor may vary appreciably with the collector voltage used (and hence the operating point), with a low collector voltage generally giving minimum noise. The Input and Output Impedances are particularly important when coupling two or more stages, when using the transistor to drive other equipment, or when connecting a signal source to the input of a transistor. Both values will vary widely with the circuit arrangement employed, but in most cases the input impedance will have allow to medium value while the output impedance will be high.
These values are specified in ohms. The Output Capacitance is given in micromicrofarads (mmf). Typical values are generally appreciably less than 100 mmf. This characteristic is of special importance when using the transistor in wideband or high frequency circuits. The Frequency Cut Off (or Cut Off Frequency,as it is often called) is generally taken as the frequency where the value of alpha is 3 db down from its low frequency value (usually 1000 cps or less). As operating frequency is increased, the gain of transistor circuits decreases because of two actions in the transistor itself. The first is the time required for electrons to move through the crystal material and for holes to shift from place to place. Both of these movements are much slower than the movements of electrons through a vacuum tube, with the holes moving even slower than the electrons. The other factor is the internal capacities between elements of the transistor. It is possible to reduce the transit time required by electrons and holes in both point contact and junction transistors. In point contact transistors this is accomplished by reducing the spacing between contacts. Injunction transistors this is accomplished by using smaller dimensions between end contacts and by using a thinner base layer. Unfortunately, both of these construction changes, although they reduce transit time, also increase the internal capacities of the transistor,increasing the other limiting factor on frequency response. For practical purposes, junction transistors are generally limited to the audio range of frequencies, although especially selected units may operate satisfactorily at R.F at least into the AM Broadcast Band. Point contact transistors may generally be operated at much higher frequencies, and some units have been used as oscillators at hundreds of megacycles. Regardless of the type of transistor, however,it is usually possible to operate units satisfactorily at much higher frequencies as oscillators than as amplifiers. The Power Output obtained from a transistor will depend on the circuit employed, the d.c. operating currents and voltages, the amount of signal drive (in the case of an amplifier), and on the characteristics of the transistor itself. This value is generally given in milliwatts. The maximum Power Output that can be obtained from a specific transistor is dependent on the Maximum Power Dissipation of the unit and on the efficiency of the circuit employed. Distortion is expressed as a percentage and is important where the transistor is used in audio circuits. The Distortion obtained in a particular circuit increases as the operating voltages and currents approach the maximum rated values. In addition to the tabulated numerical values for the transistor characteristics, the majority of transistor manufacturers also supply CHARACTERISTIC CURVES (or CHARTS) as part of the technical data sheets on their units. These charts are roughly analogous to the Characteristic Curves furnished by vacuum tube manufacturers and serve approximately the same purpose to assist the electronics engineer or circuit designer in determining the proper operating conditions for a particular circuit. As in the case of vacuum tube curves, the data supplied must be considered as approximate only, and as typical only of the average characteristics of a particular type of transistor. Individual units may have somewhat different characteristics due to production tolerances.