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Astable vibrator with IC 555

Keivan Karimi Hashjin
Prodigy 10 points

Hello,

I have 2 questions and need help with that.

1- If the comparators have a propagation delay of 10nS, find out how the frequency of oscillation is affected.

2-If the comparators have an input offset voltage of 2mV (but zero delay), how the frequency of oscillation is affected?

Regards,

Keivan

 

  • 0
    TI__Mastermind 34380 points

    Hi Keivan,

    Generally the E2E forums are for helping TI customers to design with TI products. The circuit in your post bears resemblance to a common 555 timer, which is a TI product - but I suspect the questions are coming from your homework assignment, where the 555 circuit is represented as a composition of several simpler abstractions you've already learned to work with. I'm happy to give you some hints to guide your thinking, but no outright answers. These questions are thought experiments you need to learn how to think through on your own.

    1- Think about the resulting inputs to the S-R NOR latch. Pick a few frequencies with periods less than, close to, and greater than the comparator propagation delay, and draw out a timing diagram for the system at some of the labeled net locations.

    2- Try structuring the problem as a set of two equations, one for each of the states in the astable vibrator, describing the voltage at the VC2 node as a function of time. Then solve for the time spent in each state for a case with no input offset voltage, and a case with 2mV input offset voltage. Keep in mind that you can leave the R1, R2, C, and VCC values as variables, since the solution for time in each state can be expressed proportional to those variables.

    This is also a great circuit to simplify and simulate in a SPICE program - once you have arrived at your own answers, a SPICE program can help visualize e.g. voltage vs. time at the relevant nets, to help confirm or refute your solutions.

    Regards,

    Derek Payne