Short story: Conjugate match transfers
maximum power whereas loadline match generates
Long story: You know this already that maximum power is transferred when you do conjugate matching, and yet when we talk about power amplifiers we have the audacity to say we will be matching to a specific impedance called as loadline. If you do conjugate matching for power amplifier, you won’t get maximum output power at all. Period. Okay, but nobody tells you that its not just about maximum power transfer, it is about maximum power generation as well. And your conjugate load may not let the transistor generate maximum power.
Assume you have a big output transistor for power amplifier, its output impedance is very low (say Rlow
|| C). You conjugate load will say, ok present Ltune
is set to resonate out C, then you would be all set and have maximum power transfer. However, load that your PA will see now is very small (0.5Rlow
), and voltage swings generated across transistor will also be very small, thus underutilizing the transistor and not generating max power at all. Similarly, if your PA had high output impedance (say Rhigh
|| C), your conjugate load will be Ltune
, your PA would see a much bigger impedance (0.5Rhigh
), it can generate huge voltage but now the current drawn by the load would be too small, again limiting your power generation.
So, how do you find a load that extracts maximum power from transistor? Optimum load for maximum power generation from the transistor is simply R = Vmax
For example, for device biased in class-A, you can give optimal load as:
) / Imax
is the highest voltage your transistor can sustain reliably
is the knee voltage or lowest voltage across the transistor need to operate in saturation
is the maximum current which in class-A is 2Iq
is the quiescent current.
In actual practice, optimum load is found through LoadPull simulations which is basically a blind load impedance search. You provoide an area of Smith chart and number of points to be swept, you get simulation results for each data point and then you pick the point that gives you highest power. Looking at IV curves is good to gain some insights but real device is much complicated with parasitics and bias dependencies. Therefore, designers just rely on loadpull data either simulated or measured with LoadPull instrument with actual transistor.
Ok, what about reflections though? One of the motivation for conjugate matching was to prevent reflections. Now with this loadline match, there might be a lot of reflections, they will travel back to PA and create a mess or possibly damage the PA? Yes, that can happen or let’s say will happen. You need to insert an isolator between PA and antenna to absorb the reflections. Ok, what else do you lose by not having conjugate match? You lose a little bit of gain. Conjugate matching gets you the highest gain (how else did you think it transferred max power). So is it still worth it to maximize power generation over power transfer for PAs? Yes, otherwise this concept wouldn’t have existed. You don’t lose as much power in not having the max transferred as much you lose in not having max power generated.