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Thread: General question about integrated circuits

  1. #1 General question about integrated circuits 
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    Hi all. I have a general question about ICs. I just started learning circuit design and I've been dealing with ICs. One thing I didn't realize until very recently is that certain types of ICs require negative input--I mean, are intended to have negative voltages applied to their inputs. I assumed that all ICs would require positive input.

    The problem I just ran into is that the Exclusive-Ors I'm trying to use require negative input, and the circuit I've spent weeks working on and have almost finished needs to be able to provide positive input to an Exclusive-Or gate.

    I imagine I could get an Exclusive-Or gate that requires positive input. I know there are other options, like using an IC inverter or even a PNP transitor. But I'd like for this circuit to be designed right, so I don't want to do any jerry-rigging.

    First of all, I was curious if there's any easy solution to a problem like this? Is there any way I can use this IC by applying positive inputs?

    Next, I was curious as to why some ICs require positive input, while others require negative input. What's the logic in this? And when you go about designing a circuit using ICs is it a rule of thumb that you design your circuit in such a fashion that all of the components are either positively or negatively biased? To put it another way, is it common to have circuits where you have some ICs that require positive input, and some that require negative input? Or is it generally one or the other?

    Thanks,
    Gregg


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  3. #2 Duhh!!! 
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    OK. After typing my last question I figured out what's going on. The inputs on the Exclusive-OR gate need to be grounded. I do know that you can't leave inputs on ICs free-floating. I just misinterpreted the cause and effect in my messing around with the IC.

    Though, I have another question. I have two different brands of op-amps. I reached the conclusion a while back that one required negative inputs. Was this the result of me misinterpreting cause and effect again, or are there op-amps that require negative input?

    Thanks,
    Gregg


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  4. #3 Re: Duhh!!! 
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    Quote Originally Posted by greggb
    Though, I have another question. I have two different brands of op-amps. I reached the conclusion a while back that one required negative inputs. Was this the result of me misinterpreting cause and effect again, or are there op-amps that require negative input?

    Thanks,
    Gregg
    OK, it looks like you've got your logic problem sorted out, yes, all unused inputs to gates etc. need to be either gounded or connected to the supply depending on the gates function. Don't leave them floating.

    As for op-amps, don't let the + and - symbols deceive you. this has nothing to with the voltage range of input. It simply means this...if you apply a positive signal to the '+' input, the output of the device will also go more positive. If you apply the same signal to the '-' input the output will go in the opposite (i.e. more negative) direction.

    The correct names are the non-inverting input (+), and the inverting input (-).

    ergo, if you apply the same signal to both inputs the output shouldn't move at all. Hence that's why it's called a differential amplifier, it's the difference between the inputs that is important.

    This of course is for a 'perfect' device, but in practise they come fairly close.

    Steve A.
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