TRANSISTOR CIRCUITS FOR ANALOG AND DIGITAL SYSTEMS 321 



accuracy of the circuit is limited to about ±100 millivolts. By taking 

 advantage of the properties of semiconductor devices, the transistor 

 blocking oscillator comparator can be designed to have an accuracy of 

 ±5 millivolts throughout a temperature range of 20°C to 40°C. 



5.1. General Descri'ption of the Voltage Comparator 



Fig. 18 shows a simplified circuit diagram of the voltage comparator. 

 Except for the silicon junction diode D\ , this circuit is essentially a 

 transistor blocking oscillator. For the purpose of analysis, assume that 

 the reference voltage Vee is set equal to zero. When the input voltage V, 

 is large and negative, the silicon diode Di is an open circuit and the jiuic- 

 tion transistor has a collector current determined by Rb and Ebb [Expres- 

 sion (18)]. The base of the transistor resides at approximately —0.2 

 volts. As the input voltage Vi approaches zero, the reverse bias across 

 the diode Di decreases. At a critical value of Vi (a small positive poten- 

 tial), the dynamic resistance of the diode is small enough to permit the 

 circuit to become unstable. The positive feedback provided by trans- 

 1 former Ti forces the transistor to turn off rapidly, generating a sharp 

 I output pulse across the secondary of transformer T-z . When Vi is large 

 and positive, the diode Di is a low impedance and the transistor is main- 

 tained cutoff. In order to prevent the comparator from generating more 

 than one output pulse during the time that the circuit is unstable, the 

 natural period of the circuit as a blocking oscillator must be properly 

 chosen. Depending on this period, the input voltage waveform must 

 have a certain minimum slope when passing through the reference level 

 in order to prevent the circuit from misfiring. 



I The comparator has a high input impedance except during the switch- 

 1 ing interval.* When Vi is negative with respect to the reference level, the 

 \ input impedance is equal to the impedance of the reverse biased silicon 

 i diode. When Vi is positive with respect to the reference level, the input 

 I impedance is equal to the impedance of the reverse biased emitter and 

 ! collector junctions in parallel. This impedance is large if an alloy 

 ; junction transistor is used. During the switching interval the input im- 

 ■ pedance is equal to the impedance of a forward biased silicon diode in 

 series with the input impedance of a common emitter stage (approxi- 

 mately 1,000 ohms). This loading effect is not too serious since for the 

 circuit described, the switching interval is less than 0.5 microseconds. 



The voltage comparator shown in Fig. 18 operates accurately on 

 voltage waveforms with positive slopes. The voltage comparator will 

 operate accurately on waveforms with negative slopes if the diode and 



* The switching interval is the time required for the transistor to turn off. 



