SEMICONDUCTOR DIODE GATES 1139 



input may be indistinguishable. When a certain combination of potentials 

 is impressed on the controls an output appears at the output terminals. 

 In the case of a ''linear" gate the input signal has simply passed from 

 input to output, so the output signal is approximately a replica of the 

 input signal. In the case of a "switching" gate the output is a pulse 

 which may have no resemblance, except in duration, to the pulses and 

 potentials which are impressed on the controls. 



There is essentially only one kind of linear gate. When a signal ap- 

 pears at its input, the potentials then present on the controls determine 

 whether it shall pass to the output or be blocked. Switching gates, in 

 spite of their apparent complexity in some particular applications, are 

 constructed of two basic types — or circuits and and gates. A typical 

 OR circuit is shown in Fig. 1. When a positive pulse is impressed on either 

 of the input terminals (terminal 1 in the illustration) it drives the cor- 

 responding diode conducting and the pulse appears at the output (3). 



n 



Fig. 1 — Diode or circuit. 



The signal pulse drives the other diode non-conducting, blocking that 

 path. 



An and gate is shown in Fig. 2. The biases are so adjusted that, in 

 the rest condition, the output voltage is zero, the diodes are conducting 

 and the bias current from the bias current source (B) flows through the 

 diodes and the control terminal resistors. In this case, when a positive 

 pulse is impressed at one of the control inputs the corresponding diode 

 is cut off and bias current ceases to flow in it. However, there is very 

 little resulting change at the output because the bias can still flow in 

 the second, low impedance control diode and resistance. If a second 

 pulse is simultaneously impressed on the second control input, the 

 second diode is also cut off and the bias current is forced to flow in the 

 load, producing an output pulse of magnitude determined by the mag- 

 nitude of the bias current and the magnitude of the load resistance. 



There is a third fundamental gating concept which must also be 

 introduced. That is the inhibiting control. Its purpose is to prohibit 

 an output, whatever may be done to the other controls. It is illustrated 

 in Fig. 3. As far as control 1 is concerned, this is like the and gate. The 

 output, in the rest condition, is zero and the diode is conducting. There 



