TRANSISTOR CIRCUITS FOR ANALOG AND DIGITAL SYSTEMS 



309 



when operating into an external load resistance equal to or greater than 

 50,000 ohms. A p-n-p transistor is used in the second stage and n-p-n 

 transistors are used in the first and third stages. This circuit arrangement 

 makes it possible to connect the collector of one transistor directly to 

 the base of the following transistor without introducing appreciable 

 interstage loss. ''Shot" noise" and dc drift are minimized by operating 

 the first stage at the relatively low collector current of 0.25 milliamperes. 

 The 110,000-ohm resistor provides the collector current for the first 

 stage, and the 4,700-ohm resistor provides 3.8 milliamperes of collector 

 current for the second stage. The series 6,800-ohm resistor between the 

 xcond and third stages, reduces the collector to emitter potential of the 

 second stage to about 4.5 volts. 



The loop current transmission is shaped by use of local feedback ap- 

 plied to the second stage, by an interstage network connected between 

 the second and third stages, and by the overall (3 circuit. The 200-ohm 

 resistor in the collector circuit of the second stage is, with reference to 

 Fig. 6(a), Zi . The impedance of the interstage network can be neglected 

 since it is small compared to 200 ohms at all frequencies for which the 

 local feedback is effective. The interstage network is connected between 

 the second and third stages in order to minimize the output noise voltage. 

 ^^'ith this circuit arrangement, practically all of the output noise voltage 



iE 



250 K 



IN 



+ 33V 



5MUf 



Hf- 



20on 



n-p-n 



250 K 

 2.4 K 200 n 



0.01/U.F 



p-n-p 



■llOK 



100 K POT. 



MANUAL 



ZERO SET 



I 



+ 33V 



I 



+ 4.5V 



OUT 



5>UH 



-45V -27V +33V 



Fig. 8 — ■ DC summing amplifier. 



