318 



THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



linearity of the voltage ramp slightly due to their finite reverse resistances 

 and variable shunt capacities. If the diodes have reverse resistances 

 greater than 1000 megohms, and if the maximum shunt capacity of each 

 diode is less than 10 micromicrofarads (capacity with minimum reverse 

 voltage), then the diodes introduce negligible error. 



As stated earlier, the integrator generates a voltage ramp in response 

 to a voltage step. This step is applied through a transistor switch which 

 is actuated by a square wave generator capable of driving the transistor 

 well into current saturation. Such a switch is required because the 

 equivalent generator impedance of the applied step voltage must be very 

 small. A suitable circuit arrangement is shown in Fig. 15. For the par- 

 ticular application under discussion the switch *S is closed for 5,000 

 microseconds. During this time, the voltage E = —217 appears at the 

 input of the integrator. At the end of this time interval, the transistor 

 switch is opened and a reverse current is applied to the feedback con- 

 denser C, returning the output voltage to —4.0 volts in about 2500 micro- 

 seconds. An alternate way of specifying a low impedance switch is to say 

 that the voltage across it be close to zero. For the transistor switch, con- 

 nected as shown in Fig. 15, this means that its collector voltage be within 



FIRST STAGE 



OF DC 



AMPLIFIER 



10.5V 



50 K 150 K 



' — WV-HVW 



RESIDUAL 

 VOLTAGE BALANCE 



(TO AZS) 



Fig. 15 — Input circuit arrangement of the integrator. 



