In m xlcrn electronic circuits, discrete sransistors are used primarily for applications in -hich only one c,· a small number of devices arc required, and in applications where substantial power is dissipated. Although older designs, composed entirely of discrete devices, can still be found in large numbers, most new circuits containing a large number of transistors arc constructed in integrated-circuit fom •. I- many applications, both discrete and integrated components are used, In thes applications, the integrated circuit typically performs complex, low-level and a discrete transistor then drives a power-consuming 10 d such as an indicator lamp or an audio speaker. This use of the transistor is an example of it provides a link between a device having limited power capabilities and a load that by standard TO-numbers. Three leads arc brought out through each enclosure to permit external connection to the transistor’s emitter, base, and collector. In some power transistors, rated for high power dissipation, the collector is attached and electrically common to the metal case. (The majority of the power dissipated in a transistor occurs at the collector-base juncti6n, since the collector voltage is usually the largest voltage in the device.) A transistor manufacturer uses a consistent scheme that can be followed to’ identify the base. emitter, and collector terminals for a given case type. For example, in the TO-39 case, the three leads are attached in a scmicirrelur duster and a metal tab on the case is adjacent to the emitter. The base is the center lead in the cluster and the collector is the remaining lead. A discrete transistor of a specific type, having registered J AN (military) spccificanons. is identified by a number with the prefix 2N. While all transistors having the same number may not be identical, they are all designed to meet the same performance specifications related 10 vollage (/Ild current limits, power dissipation. operating temperature range. and parameter variations. More than one mauuf’acturcr may produce a transistor with a given 2N number. Many manufacturers also produce “comrnercial ‘t-grade devices that do not have 2N designations. Figure 4-47(a) and 4-47(b), see page 132. shows parts of a typical set of
Figure 4-46 docs not show some other specifications that are usually furnished by a manufacturer, including small-signal characteristics. Small-signal characteristics are associated with the ac operation of a transistor, which we will cover in Chapter 5. Other specifications often furnished by the manufacturer include graphs showing additional parameter variations with temperature. voltage. and current
TRANSISTOR CURVE TRACERS
We have mentioned that characteristic curves are seldom included in transistor specifications. These vary widely among transistors of a given type and are rarely used for circuit design purposes. However, in areas such as component testing, preliminary circuit development; and research. it is ofteri useful to he able to study the characteristic curves of a single device and to obtain important parameter values from the curves. Recall that parameters such as currents. saturation voltages, and the Early voltage can be discerned from appropriate sd’ of characteristic curves.
The most widely used method for obtaining a set of characteristic curves is by use’ of an instrument called a transistor curve tracer. A curve tracer is basically an oscilloscope equipped with circuitry that automatically steps the currents (or voltages) in a semiconductor device through a range of values and displays the family of characteristic curves that result. Selector switches allow the user to set the maximum value and the increment (step) value ‘of each current or voltage applied to the device. For example, to obtain a family of transistor collector characteristics, the user might set the base current increment to be 10 J-LA, the maximum collector voltage to 25 V, and the number of steps to be 10. The characteristics would then be displayed as a family of curves showing Ie versus Vn for III = 0, 10 J-LA, 20 J-LA, …. Figure 4-48 shows a typical curve tracer
The curve tracer from which this display was obtained permits the user to select a value of series collector resistance (Rd. which. for the display shown. was set to 2 kn. Notice that the base current curves become shorter with increasing current. An imaginary line connecting the right-hand tips of each curve represents the load line for the circuit. This load line is seen to intersect the VcE-axisat 13 V and the
One convenient feature of a curve tracer is that it permits a user to expand or contract the display in different regions of the characteristic curves by adjusting the sensitivity and range controls.
Figure 4-49(b) shows collector characteristics of the same transistor when the curve tracer settings are adjusted to generate larger values of VCE. In this example, the honzontal .sensiti\oi y’.i 2 V/divi ion, he vert; 31 ” nsitivity is 0.2 mA/division, and the base current increment is 2 p.A. With these settings, the breakdown characteristics arc clearly evident. For example, at In = 12 p.A and Vo: = 12 V, it can be seen that the transistor is in its breakdown region and that the collector current is approximately 0.84 mA. .
Most curve tracers can be used to obtain characteristic curves for devices other than transistors. Some even have special adapters that allow the testing of integrated circuits. Figure 4-50 shows photographs of diode characteristics that were obtained from a curve tracer display. The to and reverse characteristics are shown in Figure 4-50(a), wit, -t orizantel sensitivity of 10 V/division: It can be seen that the diode enters breakdown at a reverse voltage of about 25 V. With this scale the forward characteristic essentially coincides with the vertical axis. However, when the sensitivity is set to 0.2 V/division, the forward characteristic appears as sr-: :’\ in Figure 4-S0(b) and can be examined in det it. (The origin of the axes is at. .! center of the display.) We see that the knee of the characteristic occurs at about 0.62 V, and.there is sufficient detail to compute de and ac resistances in the forward region.