TRANSISTORS 



Taking Rq as 0, the output resistance is 2-5 D. + 150/50 

 and Carroll give the forward stabilization ratio 



5-5 Q. Evans 



h^L 



const 



for the circuit as 



RL + r,+ 



(^, 



If the stabilized output voltage is 6 and the current output is 60 mA, Rj^ = 

 100 Q. and F= 1/3,300. They point out that this simple circuit gives a 

 satisfactory forward stabilization, but that, in that /?out = 5-5 Q., the back- 

 ward stabihzation is to be regarded as poor. 



It is not necessary to use a battery as a voltage reference source. Beside 

 the 'Accumulateur Etanche', mentioned earlier in this Part, there is an 

 interesting device which corresponds in semi-conductor circuitry to the 



Forward 

 current 



Backward 



volts 



71. 



A 3 2 1 



"T" 



3 



Forward 

 volts 



-5mA 



•10 mA 



Backward 



current 



Figure 45.49 



voltage reference glow discharge tube in thermionic valve practice. This is 

 the Zener diode, a silicon diode having a voltage-current curve of peculiar 

 shape {Figure 45.49). The device behaves quite normally for applied forward 

 voltages, and for backward voltages up to a critical point. Thereafter the 

 insulating properties suddenly break down and a reverse current can flow. 

 When this happens the potential diff"erence across the diode is highly indepen- 

 dent of the reverse current. The slope of the characteristic in this region 

 corresponds to a resistance of the order of 1 ohm only. Thus, if the 

 reverse current through the diode can be defined at 5 mA or so by connecting 

 it in series with an appropriate resistance to an unstabilized supply {Figure 

 45.50), a stable voltage, which varies according to diode type between 3 and 

 8, is available. The simple earthed collector stabilizer with Zener diode 

 reference voltage has the appearance of Figure 45.51. 



To improve stabilizer performance, we can emulate hard valve stabilizer 

 practice and use an error-amplifying transistor to operate our control 

 transistor. In Figure 45.52 a fraction of the output voltage is applied to the 



706 



i 



