Class J RF Power Amplifier

Hmm, never heard of a Nullor before. Go figure. Sometimes it is easier to think of these things in terms of control systems. The amplifier tries to set the two input voltages equal to each other. One input voltage is the current command. The other one is the voltage across R1. So what it tries to do is set the voltage across R1 to the input voltage. The voltage on R1 comes from the current flowing through R1. The amplifier will increase the supply to the load until the voltage across R1 is equal to the input. In this case, Iload = Vin / R1. This circuit is called a transconductace amplifier because the gain is actually Iload / Vin = 1 / R1, which has the units of conductance.

Let's start our with a single op amp and work out way toward a class AB design. Consider the voltage follower.

^{simulate this circuit – Schematic created using CircuitLab}

In this case \$V_{in}=V_{out}\$. Why? Negative feedback. Negative feedback forces the inverting pin's voltage to match the non-inverting pin. In other words, the op amp will do whatever it takes with its output to make \$V_{NI}=V_{INV}\$.

Let's take that a step further. We need more power to drive a low impedance speaker. Well, the average op amp will only have a few tens of milliamps of drive capability. That is where we add the power stage.

^{simulate this circuit}

By tying the inverting pin to the output of the power stage, we have created a voltage follower. \$V_{in}=V_{out}\$, but now the circuit has the ability to deliver much more current than the op amp output ever could. Because the op amp has negative feedback, \$V_{NI}=V_{INV}\$. The cross over distortion is eliminated by the op amp doing whatever it takes to satisfy that relationship. As an exercise, build this up on a bread board, put a sine wave into \$V_{in}\$, and observe the output of the power stage and output of the op amp. The two will not look anything alike!