THE EARTHED EMITTER TRANSISTOR 



output resistances are concerned, might not be satisfactory in practice; 

 the performance would be highly dependent upon the ambient temperature. 

 If a transistor be connected as shown in Figure 45.19, a small base-to- 

 collector leakage current flows which is attributable to the presence of a 

 few unbound electrons within the lattice of the semi-conductor. This is 

 simply the same as saying that the backward resistance of a semi-conductor 

 diode is not infinite. The leakage current is called Ic^o), and is of the order of 



/c 



T. I J^ 



Working 

 point 



V, 



c 



Figure 45.19 Figure 45.20 Figure 45.2] 



10 fiA in a small transistor with a typical base-collector voltage at room 

 temperature. 



If now the transistor be connected in the grounded-emitter mode 

 (Figure 45.20) with the base open-circuited, the leakage current is found 

 to be about 30 times greater. The leakage current for the base- 

 collector junction has to come via the emitter-base junction. So far as the 

 transistor is concerned, it is as if the emitter-base current were due to a 

 base current being drawn off by an external input circuit; transistor action 

 occurs and an emitter-collector current /S times the emitter-base current 

 flows. The total collector current is (1 + ^)Icw> ^rid is evident in Figure 45.6 

 as the collector current which flows when /;, = 0. Unfortunately 7^,0, is 

 highly dependent upon the temperature of the base-to-collector junction. 

 In the small transistor considered 7^,0, might rise to 50 jiA at a junction 

 temperature of 50°C. 



Consider the effect of this on the output stage of the amplifier in Figure 

 45.15. At a junction temperature of 50°C an emitter-collector leakage 

 current of 50 //A (1 + 47) = 2-4 mA flows, producing an additional voltage 

 drop across the 1 kD. load of 2-4 V, and moving the working point to X in 

 Figure 45.16. On superposing input signal current on the standing base 

 bias current, the working point can move along the load line downwards 

 and to the right, but not upwards and to the left. The output waveforms 

 are 'cHpped' and the transistor is said to be 'bottomed'. 



In transformer-coupled stages the effect is much more serious. Here 

 the load resistance, so far as the steady operating conditions are concerned, 

 is very low, being merely the primary resistance of the transformer. The 

 effect of increase of temperature is to move the working point towards the 

 hyperbola of maximum collector power dissipation. Because the collector 

 voltage is approximately maintained, the rising leakage current across the 

 base-collector junction generates heat which further raises the junction 

 temperature. A cumulative action — 'thermal runaway' — occurs which 

 eventuaUy destroys the transistor (Figure 45.21). 



689 



