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# How do I determine coupling capacitor value in Power amplifier design?

#### rd_dinesh

##### Member level 1
How do we determine the value of the coupling capacitor in a power amplifier design?

Low frequency cutoff corner in a 50 ohm system, and how
much PA-input loading costs you on the far side of it. Those
will affect your gain@freq / NF.

How do we determine the value of the coupling capacitor in a power amplifier design?
Xc = 1 / 2 PI() F C

Usually, to behave as a coupling capacitor (or a decoupling capacitor), its reactance should be abut 10x smaller than the impedance of the circuit where is used.
For example, the reactance of a 32pF capacitor at 100MHz is 50ohms. If the impedance of the circuit where the capacitor will be used is 50ohms, its reactance should be 5ohms, so its capacitance should be 320pF.

It's an arguable question.
The capacitor might be a part of matching OR it may act a simple coupling capacitor.
In any case, the capacitor should chosen very attentively. SRF, Current Handling Capacity, Voltage Handling Capacity, Temperature Characteristics, Tolerance, Case etc. all have to be taken into account.
Therefore the question has a particular answer.

It's an arguable question.
The capacitor might be a part of matching OR it may act a simple coupling capacitor.
In any case, the capacitor should chosen very attentively. SRF, Current Handling Capacity, Voltage Handling Capacity, Temperature Characteristics, Tolerance, Case etc. all have to be taken into account.
Therefore the question has a particular answer.
yes it is very difficult, particularly high capacitance values with high SRF, Is it possible to get a 1000pF capacitance with SRF close to 4GHz?

yes it is very difficult, particularly high capacitance values with high SRF, Is it possible to get a 1000pF capacitance with SRF close to 4GHz?
Yes, it's possible. For instance;
They claim they can be used up to 16GHz. I didn't check SRF but it's possible.

Is it possible to get a 1000pF capacitance with SRF close to 4GHz?
Why do you want (or need) that? You want low series impedance at your operating frequency, so check if you can reach that goal with smaller C values. Look at capacitor measured data (S21), that is more relevant for your case than the exact C value.

Why do you want (or need) that? You want low series impedance at your operating frequency, so check if you can reach that goal with smaller C values. Look at capacitor measured data (S21), that is more relevant for your case than the exact C value.
I didn't get you, can you please elaborate.

Xc = 1 / 2 PI() F C
indeed. if the capacitive reactance is less than say 0.1 ohms....you are good to go

It depends on the frequency you want to decouple, basically you need to find a value with the SRF is slightly higher than your decoupled freq and the |Z| should be less than 5ohm (1/10 of 50ohm should be good enough for decoupling).
There is a useful online tool from Murata called Simsurfing that can help you to choose the right value.