# Decoupling capacitor for RF power amplifier

#### Mabrok

##### Full Member level 2
Hi,

I can not see any effect on the simulation results after adding decoupling capacitor. Even when i tune the capacitance value, I can not see any effect. Is this normal?

#### KlausST

##### Super Moderator
Staff member
Hi,

It depends on how realistic your simulation is.

Every battery has different parameters. Series resistance, series impedance depending on frequency and load current and aging.
It depends on manufacturer, type, size, chemistry....

When I see it correctly, then there is an ideal capacitor connected to an ideal battery with ideal wiring.
But usually there is a real battery with real wiring .... causing impedance. And to compensate for this there should be a capacitor placed on the PCB close to the load.

Klaus

#### Dominik Przyborowski

You are shorting these caps by ideal voltage sources. No behavior difference is expected.

### Easy peasy

points: 2

#### BigBoss

If you wanna see the effect of the capacitor, use s-parameters or equivalent modeled circuit of the capacitor.
You will see how a capacitor effects the PA.

Mabrok

### Mabrok

points: 2

##### Full Member level 4
To emphasize what KlausST said if you connect ANY component in parallel
with true V source, the V source controls the V of that branch its connected
to, no matter what Z is attached to that branch. Except of course in the limits,
like a short.

Same is true if you connect ANY component in series with a current source
the current source controls the current in that branch. Except of course in the limits,
like an open.

Only with sources with non ideal parameters, series R > 0 in case of V source,
series R < infinity for current source, will be affected by the addition of other
impedance's.

Regards, Dana.

#### Mabrok

##### Full Member level 2
If you wanna see the effect of the capacitor, use s-parameters or equivalent modeled circuit of the capacitor.
You will see how a capacitor effects the PA.
@BigBoss How to choose the right value of the capacitor? Based on what? Are there mathematical equation to calculate the value?