Calculating Power Ratings

To prevent component failure the specified maximum voltage, current, and power ratings cannot be exceeded at any time during the circuit operation.

This article will show you why a good method to estimate the power dissipation of the component is the determine the power dissipation at half supply biasing and multiply by 2.

The transistor shown in the circuit is a basic BJT NPN fixed bias amplifier (https://learnabout-electronics.org/Amplifiers/amplifiers12.php). This circuit was drawn with online Easy Eda software (easyeda.com). The power dissipated by the transistor at zero input signal (including JFET, MOSFET) is mostly in the output of the transistor:

Pdc = Vce * Ic

Maximum DC power dissipation occurs when collector-emitter voltage equals to Rc voltage (maximum power transfer theorem https://en.wikipedia.org/wiki/Maximum_power_transfer_theorem):

Vce = Vrc = Ic * Rc = Vcc / 2

If Vcc is 5 V then:

Vce = Vcc / 2

= 5 V / 2 = 2.5 V

Vrc = Vce = 2.5 V

Ic = Vrc / Rc = 2.5 V / 1000 ohms = 2.5 mA

The power dissipated by the transistor at zero input will equal to:

Pdc = Vce * Ic = 2.5 V * 2.5 mA

= 0.00625 W = 6.25 mW

Now the next step is to add AC power to DC power to obtain the total power dissipation.

Both DC and AC power is the square of the voltage divided by the resistance.

P = (Vpeak^2) / Rc

If RMS voltage equals to (https://en.wikipedia.org/wiki/Root_mean_square):

Vrms = Vpeak/sqrt(2)

then:

Prms = Vrms * Vrms / Rc

= (Vpeak^2) / 2 / Rc = Pdc / 2

Thus the RMS of AC power is half the DC power for the same voltage magnitude for sinusoids.

For maximum amplitude sinusoidal outputs the power total dissipation will equal to:
Pt = Pdc + Pac = Pdc + Pdc / 2 = 9.375 mW

For square wave outputs the power total dissipation will equal to:

Pt = Pdc + Pac = Pdc + Pdc = 12.5 mW

(refer to the link again: https://en.wikipedia.org/wiki/Root_mean_square)

A square wave is the maximum AC power dissipation signal and is equal to average DC power. An AC power cannot be more than DC power of the same magnitude because the AC signal is not ON all the time like the DC signal.

This is why a good method to estimate the power dissipation of the component is the determine the power dissipation at half supply biasing and multiply by 2.