Guide to Selecting Transistors [Understanding Electronics]

Transistors are specified by code letters and/or numbers, which is only a manufacturer's coding. Published circuit designs normally specify a particular type of transistor, all the associated component values-such as resistors-then being determined with respect to the characteristics of the particular transistor specified. No problem there. Simply use the specified transistor-unless, as can happen, you find that it is unobtainable.

In that case, there are basically three options. The first is to use an equivalent transistor, of different manufacture or type number, which has the same characteristics. For this you need a history of transistor equivalents from which to select an alternative. There are books available which give such equivalent listings, or, your local hobby shop, or parts distributor should be able to help.

Equivalents given in such listings are seldom exact equivalents. They are more likely to be near -equivalent with sufficiently close characteristics to be used in "basic" circuits where component values are not too critical.

Simple radio circuits are an example. In many cases with elementary circuits, almost any type of transistor of the same basic type (germanium or silicon), or better still, the same functional group, will work.

Information on functional grouping is harder to come by. Manufacturers group their products in this way, but suppliers usually only list their stocks by type number, which is not very helpful without manufacturers' catalogs to check on the functional group to which a particular transistor conforms. When you can find transistor types listed under functional groups, keep this material on file. It can be an invaluable guide in selecting transistors for a particular job.

PRO-ELECTRON CODING

From the start of production of transistors, manufacturers adopted their own individual form of coding, such as a letter identifying the manufacturer or general class of the group (functional group), with a number designating the particular design or development.

Several attempts have been made to find a "universal" code, the most acceptable of which is probably Pro-Electron coding. This consists of a two letter code followed by a serial number. The first letter indicates the semiconductor material:

A = germanium B = variable -capacitance diodes C = AF transistor (excluding power types) D = AF power transistor F = rf transistor (excluding power types) L = rf power transistor S = switching transistor (excluding power types) V = power type switching transistor Y = rectifier diode Z = zener diode The serial number following the letter code consists of three figures if the device is intended for consumer applications, such as radio receivers, audio amplifiers, television receivers, etc. A serial number consisting of a single letter followed by two figures indicates the device is intended for industrial or specialized applications.

If you find transistors described by this code, then you should have no trouble in deciding to which functional group they belong.

The third option, if you cannot obtain a transistor specified for a particular circuit (or where the transistor type is not specified at all), is first to identify to which functional group it belongs. If it is used in an audio-frequency amplifier, for example, it could be germanium group, or silicon.

Decide which group logically applies. Then choose an available transistor which belongs in this group and for which the characteristics are available (from the manufacturers data sheet). Then, recalculate the component values required around the characteristics of the chosen transistor.

Do not assume it will be a "near-equivalent" to the one used in the original circuit. It may be anything but (especially if the original one specified was a germanium transistor, for example, and you decide to use a silicon transistor as being in the same functional group).

IDENTIFYING TRANSISTORS BY SHAPE

While transistors are made in thousands of different types, the number of shapes in which they are produced is more limited and more or less standardized in a simple code--TO (Transistor Outline) followed by a number.

TO1 is the original transistor shape--a cylindrical "can" with the three leads emerging in triangular pattern from the bottom. Looking at the bottom, the upper lead in the "triangle" is the base, the one to the right (marked by a color spot) the collector and the one to the left the emitter. The collector lead may also be more widely spaced from the base lead than is the emitter lead.

In other TO shapes, the three leads may emerge in similar triangular pattern (but not necessarily with the same positions for base, collector, and emitter), or in-line. Just to confuse the issue, there are also sub -types of the same TO number shapes with different lead designations. The TO92, for example, has three leads emerging in line parallel to a flat side, or an otherwise circular "can," reading 1,2,3 from top to bottom, with the flat side to the right (looking at the bottom).

To complicate things further, some transistors may have only two emerging leads (the third connected to the case internally), and some transistor outline shapes are found with more than three leads emerging from the base. These, in fact, are integrated circuits (ICs), packaged in the same outline shape as a transistor. More complex ICs are packaged in quite different form.

Power transistors are easily identified by shape. They are metal cased with an elongated bottom and two mounting holes. In this case, there will only be two leads--the emitter and base-and these will normally be marked. The collector is connected internally to the can, and so connection to the collector is via one of the mounting bolts or bottom of the can.

Examples of transistor outline shapes together with typical dimensions and lead identification are given in Fig. 9-10.