TRANSISTORS 



there is no detectable change in output voltage for a 10 per cent change in 

 input voltage to this device: nor was any ripple apparent in the output. 



Brown and Stephenson^^ have published a versatile mains-driven unit 

 delivering to 30 V d.c. at up to 1 amp. This is interesting in that the error 



Auxiliary 

 negative supply 



- o 



Stabilized 



supply 



out 



+ o 



o_ 



Unstabilized 

 supply in 



o ♦ 



Figure 45.53 



amplifier uses a long-tailed transistor pair, which should reduce the depen- 

 dence of the output voltage on temperature effects. The output characteristic 

 is given as an output resistance of less than 0-04 Q. at zero frequency, and as 

 an impedance, seen looking into the output terminals of the stabilizer, of less 

 than 0-2 Q. at all frequencies up to 100 kc/s, A 5 per cent change in mains 

 voltage produces a 0-2 per cent change in output voltage. The ripple in the 

 output is less than 1 mV, 



Transistor current stabilizers 



Power supplies delivering substantially constant currents of up to a few 

 amperes are possible by making use of the very high collector resistance r^ 

 of transistors in the earthed base mode. In Figure 45.54 the emitter battery 



♦ o- 



Stabilized 

 current 

 out 



I 



-0 + 



V, 



-=- /" Unstabilized 

 ~^~* supply in 



Figure 45.54 



drives a current EfRy into the base, defining a collector current a . {EfRy) 

 which is highly independent of fluctuations in Rj^. The relationship between 

 possible limits of current output, of load resistance, and of input voltage 

 used, may be derived by constructing a working region on the earthed base 

 collector characteristic, and drawing in the load line in the normal manner. 



Some other transistorized devices 



A cardiotachometer has been described by Molyneux^*. To achieve a high 

 input resistance the electrodes feed an earthed collector first stage, followed 



708 



