I was looking at a lab in Digital Circuits and there was a questions:

  1. Why do we use diodes 1N914 in the circuit in figure?
  2. What type of input signal to the circuit would make the diodes unnecessary?

So I believe that the answer should be:

  1. Diodes are use a protection for the circuit when we have high voltage and that is able to rectify AC frequencies.
  2. To use DC signals

But, I am not certain about these answers. Can someone help me with these questions?

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  • 3
    \$\begingroup\$ Those are protection diodes, just as I think you say above. \$\endgroup\$
    – jonk
    Nov 4, 2019 at 6:48
  • \$\begingroup\$ Indeed the diodes prevent the input voltage of the 7414 from going too high above Vcc and too low below GND. How low/high: about 0.7 V which is the diode's forward voltage. \$\endgroup\$ Nov 4, 2019 at 7:25

2 Answers 2


Those are external protection diodes to protect the pins of 7414 IC.

The upper diode clamps the voltage to Vcc + 0.7V, protecting the pin from much higher voltage than Vcc.

The lower diode clamps the voltage to Vcc - 0.7V, protecting the pin from voltages much lower than the ground.


You've got part a) figured out--it's to protect the input. As for b), a suitable input signal will normally be the output of another gate. Just as the output of this 7414 (which says "TTL") is suitable to connect to another gate. Logic circuits tend to be lots of interconnected gates, with an input (or output) interface only at the very ends of the logic.


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