# optimal load resistance for Class AB Power Amplifier

#### ZionWi1

##### Newbie
Hello all,
I have been studying Class A and Class B power amp., and according to my lecture script R_opt_load_classA&B = (V_max-V_knee)/I_max applies. I can see this in the output characteristic of the transistor:

Furthermore it is said that the optimal load for Class AB is different: "at Class AB, the Optimum loadline is smaller than R_opt,classA= R_opt,classB". However no formula for an optimal loadline is given. Does a formula for R_opt,classAB depending on the quiescent Current / the conduction angle exist? Or does it need to be determined in a simulation with varying Load resistance?
The circuit of the Power Amp in Class AB operation:

#### BigBoss

Because AB-Class Power Amplifier's operating Point is not defined. It's between $180^o<\theta<360^o$
Therefore there is no formulae for that.If you shift $V_{ds,max}$ to left, you will see this effect.

### ZionWi1

Points: 2

#### ZionWi1

##### Newbie
Thanks! So you mean when choosing a quiescent current in the Class AB regime, the fixed point (V_dd, I_q) in the output characteristic I_d (V_ds) moves down (compared to Class A), and when our loadline is supposed to connect (V_knee,I_max) and (V_dd, I_q) we get a steeper load line ~ 1/R, and therfore a smaller resistance?
(refering to " If you shift Vds,max to left")

#### JohnBG

##### Junior Member level 3

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2.- Note that while RL just resistive one can built a ‘triangle’ and approximate Rout, then the Load Line impedance just has to be Rout for max power transfer. But the load of the MOSFET circuit closing your question has a capacitor, therefore the load curve to overlap on transistor(s) I(V) is no longer a straight line. The relevant graphs are included in lecture available from above link.

3.- Simulate the circuit with a simulator:

Employers hardly pay any more a dime for pen and paper electronics design work. It's a long distance, why walk if one can drive.

The lecture in link above from Santa Barbara’s Uni uses Keysight simulator ADS
as college/uni student, the chances are that you may qualify to a free software licence, through your learning institution.r

ZionWi1

Points: 2

### c_mitra

Points: 2

#### BigBoss

Conduction Angle should be preferred rather than Quiescent Current.At least, terminology is so..
Yes exactly, when you move $V_{ds,max}$ to left, Load Impedance will be smaller.
BUT !
However, Load Line Theory is an Simplified Approximation and it diverges in general case from the Reality.Therefore Load Pull Technique has to be absolutely applied.

ZionWi1

### ZionWi1

Points: 2

#### ZionWi1

##### Newbie
Thanks! But how can then R_opt_load_classB = R_opt_load_classA be explained? As we have said load impedance decreases with decreasing conduction angle in Class AB. What is the relationship between the used load resistance R_load and the impedance we talked about in the output characteristic ? Is the difference that the load impedance applies for the summed up draincurrent of all harmonics and R_load is the loadline for the fundamental?

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#### JohnBG

##### Junior Member level 3
ZionWi1, this is JohnBG2012.

The reason why RLL_A=RLL_B is shown in slides 32 33 of the referred lecture, have a look.

As one can appreciate, for both A and B the triangle is the same, Load purely resisitve only.

It doesn't matter that for A just one transistor pumping current, or for B there are 2 transisitors because when one is pumping the other one is off.

Do not mix Power with impedance for max transfer: the triangle is the same so from load point of view, does it really matter if there's one transistor pumping in? or 2 taking strick turns?

Hope it helps.

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### ZionWi1

Points: 2

#### ZionWi1

##### Newbie
Dear John,
Thank you for putting in so much work to help me out, I appreciate it! I will have a look at the lecture slides and the simulation tool now.

#### JohnBG

##### Junior Member level 3
Hi ZionWil this is JohnBG2021. Actually the moderator has accepted all my comments for your question. I have removed slides 32 33, because with the circuit with Id(Vg) graphs it was enough.

Have you noticed that the moderator uses Keysight logo?

I have asked permission for reproduction to Mr Buckwalter and when reproduction is for educational-only, copyright law has exeption:

students couldn't use the internet if copy right rules were applied to educational-only purposes.

Thanks anyway