Maximum Limits

Author: Leonard Krugman

To use the transistor in practical circuits, it is necessary to be aware of its limitations. First, the transistor has limited power-handling capabilities. (The maximum power dissipation rating of a transistor is always specified in the manufacturer's rating sheet.) Because the dissipation rating is relatively low, the operating temperature of the transistor is usually kept in the general temperature range of 50°C to 60°C. Relatively low ambient temperatures are also desirable because germanium is temperature sensitive, and behaves erratically at higher temperatures. In addition, the operating range is limited by the maximum allowable collector voltage (a function of the Zener voltage, previously discussed), and the maximum collector current. (The values of these latter factors are also specified in the manufacturer's rating sheets.)

Fig. 4-16. Transistor collector I_C-E_C characteristic. Illustrating maximum limitation*.

Figure 4-16 illustrates the three maximum limitations of a typical transistor having the following specified ratings: maximum collector dissipation -100 milliwatts; maximum collector voltage -30 volts; and maximum collector current -15 milliamperes. The useful region of the collector current-voltage characteristics is necessarily limited to the area contained within these boundaries. In circuit application, none of the limiting factors can be ignored; exceeding any of the limits may damage the transistor. For example, assume that the transistor illustrated in Fig. 4-16 is operated as follows: collector current I_c - 4 milliamperes, collector voltage E_c = 20 volts, load resistance R_L = 10,000 ohms. Assume also that the a-c input signal causes a collector current variation of ±2 milliamperes. Thus, the output signal varies along the load line between the collector current limits of 2 to 6 milliamperes. The collector current never exceeds the maximum limit of 15 milliamperes, and at peak signal the collector dissipation is 40 volts times 2 milliamperes (80 milliwatts), which is well within the maximum power limits. However, the collector voltage is now 10 volts greater than the allowable limit of 30 volts. The transistor, therefore, would not be suitable for the assumed operation.