Class A Efficiency

[electroRF]

Hi,
Could someone please write down the formula for class-A PA efficiency?
I saw on the web different formulas.
Thank you.

 

[languer]

Class A efficiency has a theoretical maximum of 50% - that is because the 1/2 the power will always be dissipated on the bias resistor (at least). It is the most linear of all the amplifiers; so it is used when distortion has to be kept to a minimum (e.g. Low Noise Amplifiers). Realistic efficiencies are more like 20-25% (and those are pretty good). Basic equation -> eff = P_RF / P_DC

You can check some basic info on the following site: **broken link removed**

 

[electroRF]

Hi,
Thanks!

I'm familiar with the collector efficiency Eq. of P(RF)/P(DC).
However, as for Class-A PA, I don't know that's the correct P(RF) and P(DC).

 

[electroRF]

For example, I read here that Class A Efficiency is 25%, not 50%:
**broken link removed**

 

[audioguru]

For example, I read here that Class A Efficiency is 25%, not 50%:
**broken link removed**

Note that the efficiency is 25% at full output power. Its efficiency is less at a lower output power.
When it has no output power then the class-A amplifier is cooking at the same temperature as when it is at full output power.

 

[languer]

If your load dissipates dc power as well as ac power (or rf power); then the maximum efficiency (and this is ideal) is 25%. This is the case for resistor-biased / dc-coupled transistors.

If the load only dissipates ac power; and you bias the amplifier so there are no resistive elements on the bias path; then the maximum efficiency is 50% (again ideal). This is the case for inductive-biased (through inductors, transformers, transmission lines) / ac-coupled transistors.

This is mostly mute; as you never really get close to any of these number. Heat loss, mismatch, device non-linearities (e.g. Vce to Vds ≠ 0) takes care of that.