What is parasitic capacitace ?

[zahrein]

capacitace

Can anyone exaplain to me , what is a parasitic capacitance.. ?

What had caused it and how to eliminate them...

 

[terryssw]

You can look for a dictonary what is the meaning of "Parasitics"

Parasitics is somethings that you don't want it to be appeared, but they are usually unavoidable in nature. For example if you doing amplifier with transistors, there are always some unwanted parasitics caps that naturally appeared as the gate capacitance of the transistors, and they slow down your circuits but you cannot eliminated them.

 

[papyaki]

Hi,

Parasitic capacitance do not appear on your schematic. It is tipicaly the capacitance between two wires on a pcb, or the input capacitance of an integrated circuit, or the capacitance between two pins of a transistor or an integrated circuit or the capacitance between a pin and the metalic case of a component etc.

 

[samcheetah]

a parasite is basically an unwanted thing that "eats out" its host. like the parasites on animals. they reside on the bodies of animals and extract food from them. a good example is the leech. a leech sticks to an animals body and sucks blood from it. therefore weakening the animal.

in the electronics world parasite has simillar meanings. parasitic capacitance is that which develops between the electrodes of a device. for example the interelectrode and inter-elemental capacitance in semiconductor devices. for example the capacitance between the base and emitter terminals of a BJT. this capacitance has a parasitic effect. that is, it reduces the gain at higher frequencies. so in electronics terms the parasitic effect is the reduction of gain or something like that.

i hope that helps

 

[rudane]

Hi Zahrein,

Wouldn't it be great if everything was ideal? Well, nothing is ideal. Every electronic component, no matter what it is, contains resistance, capacitance, and inductance. When something goes wrong in our circuits because of these resistances, inductances, or capacitances we name it something bad like "parasitic capacitance", "stray inductance" or even an ESR (Equivalent series resistance, has to do with capacitors).

So to answer the question about parasitic capacitance, it's simply when the capicitance of a device starts to get in the way of us using it in a design. For example, if you drive a BJT transistor too fast, the parasitic capacitance that exists between collector and base will break down and the transistor won't work properly anymore.

If you have an understanding of the relationship between frequency and capacitive reactance then you can see that as the frequency increases, the reactance produced by the capacitance of the circuit decreases. If it gets too small, we have a problem because the circuit we want the signal to "see" no longer exists. Reactance starts to get so small that the circuit can be drastically altered.

Since every device has capactiance, they have frequency limits. After these limits, parasitic capacitance takes over. So we need better devices designed by guys like you. So get cracking eh! I hope this helps.

 

[lguancho]

Hi, my 2 cents of advice. When you simulate your RF circuit, include the parasitic of passive component such as cap, resistor and inductors. If possible, include EM models for the SMD pads of these passive devices also.

The approach is the see the effect of the parasitic, and then devise a solution to it. If you think your response has a high frequency roll off due to capacitive loading, then the solution could be some inductive compensation. For example, very often, the launching point of an SMA to microstrip interface is often capacitive ( maybe due to pin width and microstrip line width mismatch), you can a some small shunt inductor to eliminate it ( for some narrowband application).

 

[cedance]

Can anyone exaplain to me , what is a parasitic capacitance.. ?

What had caused it and how to eliminate them...

sincethere has been many useful replies, to learn more abt the other parasitics in designing the ckts and also more ompletely abt the parasitic capacitances... i recommend u learn this book...

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regards,
Arun.