What is function of this complicated circuit?

Discussion in 'Homework Help' started by xxxyyyba, Apr 3, 2016.

  1. xxxyyyba

    Thread Starter Active Member

    Aug 7, 2012
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    6
    Hello. Here is my task:

    a) Explain principle of working of circuit below. Draw waveforms of voltage at points A, B, C, D and Q, as well as capacitors voltage waveforms.
    b) What is minimum time constant of charging of C3 (ie value of resistance R6) so that voltage waveform of point Q are rectangular pulses

    It is known: R1, R2, R3 (R2=R3), R4, R5, R6, C1=C2=C3, 5R1>>R2 and R1>>R3. Threshold voltage equals VDD/2.

    ddd.png

    Any suggestion?
    I did nothing because I have no idea how to start :( I'm beginner in Digital electronics. In analog electronics and in circuit theory, there are well known methods for solving circuits, but what to do here? I'm really stuck :(

    EDIT: This is not homework question. I don't have final result. This was on my exam. I didn't solve it on exam and I'm trying to figure out what is solution to this problem. Every suggestion is welcome, I want to pass exam next time :)
     
    Last edited: Apr 3, 2016
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    First notice that the NAND gates work here as a inverter (NOT gate) and this two NAND gates form almost a classic example of a astable generator.
    And the right part of a circuit (Two op amp + NOR gate SR flip-flop and BJT) is a slightly modified version of the famous NE555 IC.
    http://forum.allaboutcircuits.com/threads/555-monostable.40792/#post-267489

    Also this diode across R6 is wrongly connected
     
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  3. Jony130

    AAC Fanatic!

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  4. xxxyyyba

    Thread Starter Active Member

    Aug 7, 2012
    279
    6
    Hello, thanks a lot for reply. So original "complicated" circuit consists of few simple circuits conected together and to understand "complicated" circuit, I should first understand its "parts", right?
    But then it's not clear to me: Let's say I analysed first part of this circuit (astable generator) and found voltage waveform at point B. If I then connect second part to it (high pass filter - C2R4), it would "interact" with previous stage and, logically, waveform of point B I got when analyzed first stage on it's own will CHANGE?
     
    Last edited: Apr 3, 2016
  5. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Point B voltage is a GATE output which means that this voltage is load independent (C2 and R4 will not have any influence on the voltage at point B ).
    In fact the voltage at point B will be a square wave with amplitude equal to VDD because gate output will behaves just like a voltage source.
    http://forum.allaboutcircuits.com/t...entities-or-physical.59524/page-2#post-402300
    And in time domain C2 and R4 is just a simply RC differentiator
    1.png
     
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