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    What's the purpose of R1 in this CMOS Logic Probe Circuit?

    Hi,
    Here I attached a CMOS Logic Probe Circuit...What I would like to know here is what is the purpose of R1 and why the value of 2.2 M is chosen here?
    Rina

    •   Alt14th September 2006, 14:44

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    Re: The purpose of R1

    What is the purpose of R1?
    4000 series logic incorporates two mos transistors (one p-channel and one n-channel) connected in complementary fashion to function as an inverter. Because of the symmetry of these two transistors, appropriate negative feedback around the complementary pair will cause the pair to self bias itself to approximately 1/2 of the supply voltage. When the probe input is open, feedback resistor R1 causes N1's output to go to vcc/2, which will turn off both leds (or at least dim them to the point where they appear to be off).

    Why is the value 2.2 M chosen here?
    If R1 is too high, N1 will not have enough negative feedback to function as a linear amplifier and it will oscillate when the probe is open. When the probe is connected, however, R1 is the primary source of probe input impedance seen by the probed circuit; if R1 is too low, probe measurement accuracy will be degraded due to external circuit loading. It is likely that 2.2MΩ was selected because this is the highest resistance that will reliably enable N1 to operate in its linear region.



    •   Alt14th September 2006, 15:02

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    Re: The purpose of R1

    Hi Rinaishlene,

    The purpose of the 2M2 resistor is to keep the output of the first gate
    at approx. half the supply voltage in order to make sure none of the
    leds will light when nothing is connected to the input probe. The value
    of this resistor is not critical on condition it will not interfere with the
    normal functioning of the gate. In other applications, like crystal
    oscillators, you will find similar resistors when the designer wants the
    gate to operate as an (inverting) amplifier.

    on1aag.



    •   Alt14th September 2006, 15:06

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    Re: The purpose of R1

    well thank you to both of you.



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    Re: The purpose of R1

    N1 is the same as N2? I think the desig idea is to provide a dc point before N2.



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