Can somebody explain me the purpose of this circuit

[ku637]

Hi ,

I have a circuit with 2 input NAND gate. Its receiving its input from one NAND gate and its output is going to another AND gate.
But it has a 100k connected across its output and input.

Anybody can help me to understand what exactly is the purpose of this resistor.

 

[Dominik Przyborowski]

The resistor provides a negative feedback, and in general you have a following consequences:
1. linearization of response
2. limiting the output swing
3. duty cycle correction.

 

[ku637]

Will it change the NAND logic?

Can you please help me on the three points you highlighted.

1. Linearization of response - How it is w.r.t a logic device. I was thinking for a NAND gate only like a Logic high or logic low.Is the output is non linear..Can you please elaborate
2.Output Swing- When 0V is present at input i suppose 5V will be output .The 100k offers a path to input 0V will it reduce the amplitude also?
3.Duty cycle- just make sure if I'm understanding correctly ..suppose if I'm not using a feedback resistor and I'm supplying input with HIGH-LOW square wave pulses .. the inherent issues with gate can delay high and low transitions and cause deviations in duty of output square wave form and this resistor will help to eliminate it..Is it like that?

 

[KlausST]

Hi,

The number and the symbol say it´s a NAND gate.
Therfore the output is inverting and it generates a negative feedback.
Can you show ALL that is connected to the input and the output?

****
A similar circuit but with non_inverting gate (OR, AND...) could be used as bus keeper for the input signal.
Then it has positive feedback.

****
I saw some logic gate circuits that "misused" the logic gates as analog amplifiers.

Klaus

 

[FvM]

It's pointless to guess about the function of the 100k resistor without seeing the complete circuit - what's else connected to the input node?

 

[ku637]

Sorry i made a mistake the full circuit is like below..On deeper look i understoof that its actually two nand gates in series (forgive the paint drawing..tried to draw as quickly as possible) The output is going to a select pin of an ic.
The switch connects either 5V through a resistor or to ground (0V) the 4.75k resistor.

 

[Dominik Przyborowski]

So, this changing the point so much.
The whole circuit working as Schmitt trigger. Resistors provides a positive feedback for hysteresis needed to avoid any switching due to noises (due to switching the mechanical switch).