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Basic Practical COncepts???


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Hi guys!

well! i want to know how the following components work in a circuit, and why we use them, and how to determine their values for a particular circuit??

1) Pull Down Resistor
2) Pull Up Resistor
3) Decoupling Capacitor

I shall be very thankful to u for this act of kindness.

Take carez

Good Bye!

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A decoupling capacitor is a supply bypass capacitor and provides a low impedance to ground for the supply, so the voltage doesn't jump all over the place when the load current changes, and to supply momentary high supply current when needed.
Without a supply bypass capacitor, many analog circuits oscillate and have distortion, and many digital circuits have extra pulses.
Its value is calculated from the impedance of the supply (a battery's internal resistance rises when it runs down) and the frequency of the circuit (a small ceramic disc cap has an impedance much lower than an electrolytic cap at high frequencies).

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You have to remember all wires and circuit traces have inductance ~20nH/inch. If you need to supply large amounts of current fast, the voltage will dip. Many components (active) can't tolerate dip's below certain levels, so a "local current source", in the form of a capacitor, is added.

Pull-up resistor... the perfect example is a open collector comparator. If the output transistor isn't on, it's collector is just floating in space.

Pull-down resistor... it's like the pull-up, but it connects to a lower potential, like ground or even a negative voltage. It insures that if a node isn't "forced" to a particular level it will "tend" to go towards the pull-down potential.

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