# How can i calculate the power gain in mosfet amplifiers

#### Ghina Bayyat

Joined Mar 11, 2018
112
Can u plaease help me to know how can i calculate the power gain of a mosfet amplifier without having the current gain ??
The power gain formula is Ap = Av * Ai but the current gain of the mosfet is almost infinte and there is no formula to calculate it so how can i find Ap ? Should i just calculate the power dilivered to the load by ohm's low that P= Id ( Rd // Rl ) ? But if the question was find Ap would it be right to find the power on the load ?? I don't think it's right but how can i know Ap ?

#### drc_567

Joined Dec 29, 2008
731
Are you able to measure the current that is going to the load element? Also the load voltage? P=V*I ... same idea at the input node. What waveforms are involved? Maybe you want to look at RMS current and voltage.

• Ghina Bayyat

Joined Jan 15, 2015
4,582
The power gain formula is Ap = Av * Ai but the current gain of the mosfet is almost infinte and there is no formula to calculate it so how can i find Ap ?
Power as expressed in Watts is V * I which is what you have, Voltage multiplied by the Current. In the case of an amplifier and when we state Power Gain I would expect to see the gain expressed as db (decibels). With the basic V * I formula knowing only V or I you can't calculate power so there needs to be more to this?

Just as an example:
Assume that the input voltage is 10 mV (+10 dBm) and the output voltage is 1 V (1000 mV, +60 dBu). The ratio will be 1000/10 = 100, and the voltage gainwill be 20×log 100 = 40 dB. Reference voltage V0 = 1 Volt. Power gain (dB) = 10×log (RF output power / RF input power). The above link will demonstrate power gain.Your formula is how we solve for power and not gain.

Ron

• Ghina Bayyat