THE EARTHED EMITTER TRANSISTOR 



Suppose we increase the battery voltage to 7-5, and lose the extra 1-5 V 

 across R^. We wish to keep the same working point, V^ — —2-5 volts, 

 /p = 3 mA. Then for an emitter current (approximately the same as the 

 collector current) of 3 mA, we need JR^ = 1-5 V/3 mA = 500 Q. The 

 emitter potential will then be 1-5 V negative to earth, and the base about 

 the same. Therefore 7-5 . {^2/(^1 + ^2)} r^iust equal 1-5. Suitable values 

 would be R2— 10 k, i?i = 40 k, giving the circuit of Figure 45.23a. 



AOk 



71/2V 



7V2V 



(a) 



(b) 



Figure 45.23 



There remains one thing to do. So far as signals are concerned, Kg is 

 part of the load in a manner analogous to the cathode resistor in a concertina 

 phase-splitter with valves, and introduces negative feedback. Where such 

 feedback is undesirable it may be removed by shunting i?, with a low 

 impedance to alternating currents; an electrolytic capacitor of the order of 

 100 [xP is suitable, to give Figure 45.23b as the complete circuit for the 

 stage. The base potential-divider slightly reduces the input resistance of 

 the stage, since the 10 and the 40 k resistors are both effectively in parallel 

 with it; a rather bigger coupling capacitor is therefore called for. Some 

 input signal power is lost in R^ and 7?2 5 their values should therefore not 

 be chosen too low, but equally they must not be too high or the stabilization 

 can be shown to be adversely affected. The values arbitrarily allotted are 

 typical. 



Other transistor parameters and equivalent circuits 



We now enumerate some of the other ways of considering transistors 

 which are extant, showing how to equate the alternative approaches to that 

 used in this chapter. 



The current generator in Figure 45.12 is sometimes replaced by a voltage 

 generator of output (5/iQ^^ (Figure 45.24). R^^ is a 'mutual resistance'; 



— • vWW 



-nAAV 



Hn^ 



--^S^KT 



R, 



Figure 45.24 



that is, the transistor is regarded as introducing so many volts in the collector 

 circuit per amp in the base circuit {cf. mutual conductance, so many amps in 

 the anode circuit per volt at the grid, with valves). The necessary conversion 

 factor is just R^ — ^R^. 



691 



