TRANSISTORS 



a constant-voltage generator it introduces a lot of distortion, whereas if 

 it is fed from a constant-current source it does not. With control of the 

 Figure 45.29 type, the source impedance seen by the load is low at low-gain 

 settings, whereas with the Figure 45.30 type, it is almost constant. The 

 latter arrangement therefore gives lower distortion when the gain setting 

 is reduced. 



COMPLETE PIECES OF TRANSISTOR APPARATUS 



A.c. coupled amplifiers 



An audio-frequency amplifier for driving a loudspeaker, recommended by 

 Mullard Ltd., is shown in Figure 45.31. Two R-C coupled stages drive a 



NFB 



56 k 



i^A 



R.M.S. for 

 full output. 



all 



Speaker 



Input o 



Coupling transformer : Gilson, W0780/6V 

 Output transformer : Gilson, W0781/6V 



Figure 45.31 



pair of output transistors in class B push-pull. The circuit is largely self- 

 explanatory; the starred 56 kQ resistor introduces negative feedback 

 round the output and penultimate stages. The maximum audio output 

 power is 215 mW, and, with a 5 in. loudspeaker is sufficient to fill a large 

 room with sound. The writer has used this amplifier, with an additional 

 R-C coupled transistor at the front and the feedback removed, to demon- 

 strate muscle action potentials via a concentric needle electrode. 



A transistorized myographic amplifier has been described by George*^. 



For detailed design procedure for audio amplifiers, Jones and Hilbourne 

 may be consulted^. Units with output powers ranging from 20 mW to 

 20 W are described. 



Direct-coupled amplifiers 



As with valves, we distinguish between 'straightforward' amplifiers and 

 carrier amphfiers. The phototransistorized and transistorized galvano- 

 meter amplifier remains to be announced. 



'Straightforward'' d.c. amplifiers — The sensitivity of meters may be improved 

 by the addition of a single transistor in a simple circuit such as Figure 45.32, 



696 



