TRANSISTOR CIRCUITS FOR ANALOG AND DIGITAL SYSTEMS 



325 



0.7 



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0.6 



R, = 30,000 OHMS 



20°C 



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u:o.5 



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o 

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0.3 



2 3 4 5 6 



DIODE CURRENT, Ip, IN MICROAMPERES 



Fig. 20 — Volt-ampere characteristic of a silicon junction diode. 



mentally it is found that Yh-e for germanium junction transistors varies 

 by about 20 millivolts throughout the temperature range of 20°C to 

 40°C. Consequently, the variation in Yi at which the circuit switches is 

 ±5 millivolts. 



It is apparent from Fig. 20 that the accuracy of the comparator in- 

 creases slightly for critical values of R\ greater than 30,000 ohms, but 

 decreases for smaller values. For example, the accuracy of the comparator 

 is ±10 millivolts for a critical value of U\ equal to 5,000 ohms. In gen- 

 eral, the critical value of R\ should be chosen between 5,000 and 100,000 

 ohms. 



5.3. A Practical Yoltage Comparator 



Fig. 21 shows the complete circuit diagram of a voltage comparator. 

 The circuit is designed to generate a sharp output pulse* when the input 

 voltage waveform passes through the reference level (set by Yee) with a 

 positive slope. The pulse is generated by the transistor switching from 

 the "on" state to the "off" state. To a first approximation the amplitude 

 of the output pulse is proportional to the transistor collector current 

 during the "on" state. When the input voltage waveform passes through 

 the reference level with a negative slope an undesirable negative pulse is 

 generated. This pulse is eliminated by the point contact diode D2 . 



The voltage comparator is an unstable circuit and has the properties 



* For the circuit values shown in Fig. 21, the output pulse has a peak amplitude 

 of about 6 volts, a rise time of 0.5 microseconds, and a pulse width of about 2.0 

 microseconds. 



