PROPERTIES AND APPLICATIONS OF fl-p-U TRANSISTORS 537 



up to only 200 microamperes. It can be seen from the upper plot that the 

 collector circuit characteristics are still quite usefully straight and evenly 

 spaced in this micro-power range. In fact, for small signal operation it is 

 sufficient to use a collector voltage only a httle in excess of 0.1 volts and a 

 collector current a little in excess of 20 microamperes. This means that the 

 power required to bias the collector into the operating range amounts to 

 only a few microwatts. Contours are shown for 10, 50, and 100 microwatts 

 of power supply. 



This ability of the transistor to work with extremely small power con- 

 sumption is one of its most striking and perhaps most important features 



Fig. 6 — The low-frequency equivalent circuit of a transistor. 



When one considers that the total power consumption of a single transistor 

 stage can be smaller by many thousands of times than the power required 

 to heat the cathode in a vacuum tube, it is obvious that the advent of this 

 device will make possible many new kinds of application. 



Variation of Transistor Properties with Operating Point 



Ryder and Kircher^ have shown that it is convenient to analyze the 

 small signal properties of a transistor at low frequencies in terms of the 

 equivalent circuit of Fig. 6 where re is called the emitter resistance, rt is 

 called the base resistance, and r<. is called the collector resistance. The in- 

 ternal generator, tmie , is the active part of the circuit and in this respect 

 corresponds to the familiar fxCg of vacuum tube circuit theory. It is the 

 purpose of this section to show what values these quantities have for a 

 particular n-p-n transistor and to show how they vary with the applied 

 biases. This will form a basis for the next section in which these quantities 

 will be used to compute such things as the input and output impedances 

 and the gains of various transistor connections. 



Ryder and Kircher have shown that these four r's can be obtained di- 

 rectly from static characteristics such as those shown in Fig. 4 and Fig, 5. 

 In the case of n-p-n transistors, however, the magnitudes of these quan- 

 tities are such that it is difficult to obtain satisfactory accuracy in this way 

 and it has been more convenient to measure the 4-pole r's by a-c. methods. 



