TRANSISTORS 



short-circuited) if current is taken out of the base by applying a reference 

 wave which makes the base more negative than either emitter or collector. 

 Conversely, when the reference wave goes positive, causing current to enter 

 the base, the transistor switches to high resistance, again irrespective of 

 the polarity of the input signal, provided the reference wave is large enough. 

 The transistor chopper and rectifier have respectively the simple circuits 

 of Figure 45.28a and b. The reference wave is generated by a transistor 



Output 



Output 



Ret. wave 

 in 



Ref.wave 

 in 



(a) 



(b) 



Figure 45.38 



multi-vibrator (see below), the a.c. coupled amplifier employs 5 transistors, 

 and two further transistors in earthed collector follow the rectifier and 

 filter system to secure an output impedance of only a few tens of ohms. 

 The apparatus is used to amplify the output of a thermocouple and, since 

 the latter is a low resistance device, the performance is expressed by Burton 

 in terms of voltages. The basic voltage gain of about 1,000 is reduced to 

 55 by negative feedback. Feedback as heavy as this enables Burton to 

 claim a drift equivalent to only 1 mV at the input over the ambient temperature 

 range —12°C to -\-50°C. Even allowing for the fact that 1 mV across the 

 low resistance of a thermocouple (10 Q) represents a current change of 10~* 

 amps, the figure given augurs adequate independence of room temperature. 



D.c. converters 



These are devices for producing a direct output from a direct input at 

 some other voltage. As normally used, they are arranged to produce a 

 voltage step-up. For example, a small converter described by Johnston^* 

 supplied an output of 30 V, 100 //A from an input at 3 V. The object was 

 to supersede the relatively expensive HT battery in thermionic hearing aids 

 by power derived by the converter from the relatively cheap LT cells. The 

 efficiency of conversion was 60 per cent and the unit measured only 1 f x 

 1t6 X \ in. At the other end of the scale, a converter employing a 

 power transistor^^ delivered 10 kV; 100 [jiK for an input of 12 V, 150 mA; 

 efficiency, 55 per cent. Much higher conversion efficiencies are possible. 

 Perhaps the classical British paper on d.c. converters is by Light and 

 Hooker^^, but see also Light^'. The simplest type is shown in Figure 45.39; 

 in principle it is a transistor oscillator, with feedback achieved from collector 

 circuit to base circuit by mutual induction, arranged so that the collector 

 winding, L^, forms the primary of a step-up transformer. A high alternating 

 voltage appearing across the secondary winding Lg is half-wave rectified 



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