Class A amplifiers are defined as circuits in which the transistors remain on and operate linearly across the full input and output range.

Note that the transistor bias current is chosen higher than the peak signal current $I_p$, to ensure that the device does not turn off at any time during the signal excursion. Ensuring that the amplifier is always on does not necessarily imply that the PA is always linear. In figure, if $I_1 = 5 I_2$, the transistor transconductance varies considerably from $t_1$ to $t_2$ while the defination of class A comes to hold. This is where its defination becomes vague. Nonetheless, we can still assert that if linearity is required then class A operation is necessary.

Lets now compute the maximum drain (collector) efficiency of class A amplifiers. To reach maximum efficiency, we allow $V_x$ to reach $2 V_{DD}$ and nearly zero. Thus, the power delivered to the matching network is approximately equal to $(2V_{DD}/2)^2 / (2R_{in}) = V^2_{DD} / (2R_{in})$, which is also delivered to RL if the matching network is lossless. Also, the inductive load carries a constant current of Vdd/Rin from the supply voltage. Thus,

$$ \eta = \frac{V^2_{DD} / (2R_{in})}{V^2_{DD} / R_{in}} = 50\%$$

The other 50% of the supply power is dissipated by $M_1$ itself.

Assumptions leading to an efficiency of 50% in class A stages are:

1. the drain (collector) peak to peak voltage swing is equal to twice the supply voltage i.e the transistor can withstand a drain-source (or collector-emitter) voltage of $2 V_{DD}$ with no reliability or breakdown issues.

2. the transistor barely turns off, i.e the non-linearity resulting from the very large change in the trans-conductance of the device is tolerable.

3. the matching network interposed between the output transistor and the antenna is lossless.