Decoupling Capacitors

[Sanjayan Vinayagamoorthy]

Hello,


I was told that there were two uses for decoupling capacitors. The one
that I know of is when there is a transition in the IC in TTL or CMOS
logic, there is a could be a lot of current consumed, which in turn
would cause spikes in the power supply lines. This is avoided by
placing capacitors on those power lines.

However I am not sure of another use. I know of one other example,
especially used to make sure sure that DC bias points from one common
source amplifier are not passed to another. I.e. one would you
resistors to bias the gate/base of a transistor and then a capacitor
from the base to the ac input. Therefore the DC voltage of the ac
signal doesn't affect the biasing conditions of the amplifier. I am
not sure if this capacitor is also called a decoupling capacitor or is
it something else? Like a Bypass capacitor instead?

Regards,
Sanjay

 

[Rich Grise]

Hello,

However I am not sure of another use. I know of one other example,
especially used to make sure sure that DC bias points from one common
source amplifier are not passed to another. I.e. one would you
resistors to bias the gate/base of a transistor and then a capacitor
from the base to the ac input. Therefore the DC voltage of the ac
signal doesn't affect the biasing conditions of the amplifier. I am
not sure if this capacitor is also called a decoupling capacitor or is
it something else? Like a Bypass capacitor instead?

Actually, in that app, it's called a coupling capacitor. It couples the
signal from one stage to the next, and blocks the DC. So, it is sometimes
(or was a thousand years ago) called a "DC blocking" capacitor.

Hope This Helps!

Rich