# Why do we have 2 representations for load (shown below) in a power amplifier?

OR

I get confused when i see two different ways to connect load resistor in a power amplifier.

Why do we have two such representations (RL in series with collecor Vs RL connected across collector and ground)?

The closest answer I find is, for an AC signal both representations are equivalent. If so, can we connect (any) load in any of these ways? Is there any other consideration i'm missing out?

Any help would be appreciated.

As you can see in the litte graph included in your first schematic there is a current offset ("bias level") for the load current.

I.e.

• current is always positive and
• even if there is no input signal there will be some quiescent current.

If your load is e.g. a speaker you don't want to have any current going through the speaker if there is silence. That can be accomplished by AC coupling the speaker like shown in the second schematic.

That way

• current is centered at 0A (it may become positive and negative)
• if there is no input signal there will be no current through the load (no quiescent current through the load)

Adding to the good answer of Curd: notice that from a signal (AC) point of view, $R_C$ and $R_L$ are in parallel, since the supply act like a short for AC. This means that, if $R_L$ is your "real load", it must have a much lower impedance than $R_C$, which is used only to set the quiescent point of the BJT.

Moreover, in some cases you have a load that needs a DC component: take as a simple example an LED used for lighting, assuming the BJT is used as an amplifier and not as a switch, for instance using it as a current regulator (current sink in this case; inefficient but simple -- it's essentially your first schematic).

Note however that the load is not ground referenced, unless you use a PNP BJT as a current source on the high side of the load (i.e. connected "above"). Some loads may need one of their terminals connected to ground, so this is a distinct disadvantage in this case.

To sum up, there are different design decisions to be made when choosing where to place the load in an amplifier. The fact is that in most textbooks they stick to the very basic case of an AC coupled, class A, small signal amplifier for low frequency (the second schematic you posted). That's just to keep thing simple for the learners.