328 



THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



voltage is equal to the negative of the input analog voltage. At this 

 instant of time the second voltage comparator generates a sharp output 

 pulse. The time interval between the two output pulses is proportional 

 to the analog input voltage if the voltage ramp is linear and has a con- 

 stant slope at all times. 



6.2. The Amplitude Selector i 



The amplitude selector increases the slope of the input voltage wave- 

 form (in the vicinity of zero volts) sufficiently for proper operation of the 

 voltage comparator. The amplitude selector consists of a limiter and a 

 dc feedback amplifier as shown in Fig. 23. The two oppositely poled 

 silicon diodes Di and D2 , limit the input voltage of the dc amplifier to 

 about ±0.65 volts. The dc amplifier has a voltage gain of thirty, and so 

 the maximum output voltage of the amplitude selector is limited to 

 about ±19.5 volts. The net voltage gain between the input and output 

 of the amplitude selector is ten. 



The principal requirement placed on the dc amplifier is that the input 

 current and the output voltage be zero when the input voltage is zero. 

 This is accomplished by placing a blocking condenser Cb between point 

 T and the base of the first transistor stage, and by using an AZS circuit 

 to maintain point T at zero volts. The dc and AZS amplifiers are identical 

 in configuration to the amplifiers shown in Fig. 12. The dc amplifier is 



50 K 



-VvV 



50 K 



:|N 



D 



1:: 



SILICON 

 DIODES 



Dp 



1.5 MEG 



Cb 



250 /ZF 



500 K 



I 



OUT 



I V^^ »— AAA^ 



50 K 1.5 MEG 



-1 



Fig. 23 — Block diagram of the amplitude selector. 



