TRANSISTOR CIRCUITS FOR ANALOG AND DIGITAL SYSTEMS 319 



one millivolt of ground potential during the time the transistor is in 

 saturation. Xow, it has been shown that when a junction transistor in 

 the common emitter connection is driven into current saturation, the 

 minimum voltage between collector and emitter is theoretically equal to 



— in - (25) 



q oci 



where k is the Boltzmann constant, T is the absolute temperature, q is 

 the charge of an electron ((kT/q) = 26 millivolts at room temperature), 

 and ai is the inverse alpha of the transistor, i.e., the alpha with the 

 emitter and collector interchanged. There is an additional voltage drop 

 across the transistor due to the bulk resistance of the collector and 

 emitter regions (including the ohmic contacts). A symmetrical alloy 

 junction transistor with an alpha close to unity is an excellent switch 

 because both the collector to emitter voltage and the collector and emit- 

 ter resistances are very small. 



At the present time, a reasonable value for the residual voltage* be- 

 tween the collector and emitter is 5 to 10 millivolts. This voltage can be 

 eliminated by returning the emitter of the transistor switch to a small 

 negative potential. This method of balancing is practical because the 

 voltage between the collector and emitter of the transistor does not 

 change by more than 1.0 millivolt over a temperature range of 0°C to 

 50°C. 



4.3. Automatic Zero Set of the Integrator 



A serious problem associated with the transistor integrator is drift. 

 The drift is introduced by two sources; variations in the base current of 

 the first transistor stage and variations in the base to emitter potential 

 of the first stage wdth temperature. In order to reduce the drift, the 

 input resistor R and the feedback condenser C must be dissociated from 

 the base current and base to emitter potential of the first transistor stage. 

 This is accomplished by placing a blocking condenser Cb between point 

 T and the base of the first transistor as shown in Fig. 15. An automatic 

 zero set circuit is required to maintain the voltage at point T equal to 

 zero volts. This AZS circuit uses a magnetic modulator known as a 

 "magnettor."^^ 



A block diagram of the AZS circuit is shown in Fig. 16. The dc drift 

 current at the input of the amplifier is applied to the magnettor. The 

 carrier current required by the magnettor is supplied by a local transistor 



* The inverse alphas of the transistors used in this application were greater 

 than 0.95. 



