COMPLETE PIECES OF TRANSISTOR APPARATUS 



of the input resistance, Ri^, of the amplifier the device feeds, to R. This 

 particular circuit is therefore better for thermionic amplifiers, where /?i„ is 

 high. It has been used with a valve amplifier by Fleming^", who reported 

 a drift equivalent to a signal at the input of only 100 ju\ per hour. 



A silicon diode chopper (or, more strictly, modulator) intended to feed 

 a transistor amplifier has been described by Moody^^. The circuit is shown 

 in Figure 45.36 and is seen to resemble closely the full-wave phase-sensitive 



detector discussed in Chapter 6. The zero stability claimed at room tempera- 

 ture is equivalent to a signal at the input of less than 10~^ amps. It is 

 interesting to notice that this is no better than that claimed for the 'straight- 

 forward' d.c. amplifier of Neale and Oakes. 



Transistors can be used both for phase-sensitive rectification and for 

 chopping. A phase-sensitive rectifier has been described by Sutcliflfe^^ in 

 which two transistors act as ampHfiers as well as rectifiers. A complete 

 amplifier employing a similar transistor circuit both for chopping and 

 rectification is due to Burton^^. The principle of the method is illustrated 



Refwave in 

 o 



Figure 45.37 



in Figure 45.37. We have seen that if the base of a transistor is of N germa- 

 nium, both emitter and collector are of P germanium, and this suggests 

 there ought to be some degree of reversibility between the latter two electrodes. 

 In fact this is the case. Whether the collector be made negative with respect 

 to the emitter (conventional usage) or whether the polarity be reversed, 

 the current which flows between them is always large when a large current 

 is taken out at the base, and vice versa. The only point which should be 

 made is that, in the reversed connection, /5 is about one-tenth of its proper 

 value. Thus, in Figure 45.37, whatever the polarity of the signal at the 

 input, a large emitter-collector current can flow (corresponding to output 



699 



