purpose of capacitance to tumble DC flow

[electronics_kumar]

we are using a capacitance to block the flow of DC ..if we allowed means what are the consequences that we face....

 

[elmolla]

Hi Electronics_Kumar,

I don't really get what you want, but I thikn this'll make it more clear for you.

In some applications, we prefer blocking the DC and amplify only the small signal AC signal. This is also used in multistage cascaded amplifiers, where we don't want the DC level of one of the stages to alter or affect the biasing of the following stages, i.e. isolate them and in the same time pass the AC signal without the use of a bulky transformer.

Sometimes, we even want to pass the DC and block the AC, for example, in VCO's making use of Varactors so we use a coil with high inductance or whats commonly known as a RF Choke coil.

 

[electronics_kumar]

what consequences if allowed DC to pass that is i want to know....
from the book i simply got the sentence that it will affect operating point ..how ...
is this only dire consequences we will have ..or more to be added what are they......

 

[elmolla]

Assume we have two cascaded Common Emitter Amplifiers with no coupling capactors, if the supply Vcc is +5V, and we adjusted the first stage's DC voltage alone to be 2.5 V for a symmetrical max swing (approx). On loading it with the 2nd stages base this voltage will change because of the input resistance of the 2nd stage, if we again adjust it to 2.5 V, this will cause the base voltage of the 2nd stage to be 2.5 V!, if this doesn't destroy the BJT this will cause it to be in the saturation region or on the edge of being there due to the same loading effect as above.

When using coupling capacitors, we isolate the operating point(bias point= operating point), but we allow the AC signal to pass freely.