TRANSISTORS 



Of the possible circuits to compensate for these temperature effects, the 

 most widespread is shown in Figure 45.22. A stabilizing resistor R^ is 

 connected in the emitter circuit, and the base bias is derived from a potential 

 divider R1-R2 across the battery. If R^ and Ro are chosen so that the 

 current up them is much larger than the base current, then the mean 

 potential of the base below earth is merely {/?2/(^i + -^2)}^- Since the 

 base-emitter voltage with transistors is small, about 100 mV, the expression 



Figure 45.22 



{R^liRi + ^2)}^ ^Iso gives with sufficient accuracy the voltage across Rg, 

 defining the emitter current at 



i?2 



R1 + R2 



V 



Of this, a fraction given very roughly by 1//5 flows out of the base, and the 

 remainder out of the collector. The total collector current is composed of 

 the true transistor collector current /94, plus the leakage current (/? + l)4(o)- 

 Any tendency for the leakage current to increase produces an increase in 

 the potential difference across R^ and a reduction in the emitter-base voltage. 

 Because the emitter-base junction is a diode biased in the forward direction, 

 the voltage-current relation is exponential and a small proportional reduc- 

 tion in voltage produces a much larger proportional reduction in base 

 current. Thus an increase in (/? + 1)7^(0) is partially offset by a reduction 

 in ^I,. 



At first sight it appears that the compensation breaks down when ^I^ is 

 reduced to zero, and that therefore the collector current can never be less 

 than (^ + l)/c(o). In fact this is not so, because of the peculiar shape of 

 the base-emitter characteristic for transistors in the earthed-emitter mode. 

 It is clear from Figure 45.10 that the direction of the base current reverses 

 before the emitter-base voltage has fallen to zero. Thus as {^ + l)^c(o) 

 increases, a point is reached at which base current begins to flow into, 

 instead of out of, the transistor. /54 changes sign and the value of collector 

 current is maintained. 



An exact analysis of temperature compensation circuits is complicated 

 and cannot be undertaken here. Oakes^ and Stuart-Monteith* may be 

 consulted. In brief, the greater R^ in comparison with the other circuit 

 resistance, the better the stabilization. Compensation for the output stage 

 of the amplifier in Figure 45.15 may be carried out along approximate 

 lines as follows: 



690 



