Electronic – Study A Class Amplifier Schematic

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!

OK, here is a simple schematic of a transformer coupled amplifier;

Here are the voltages;

Notice before the sin starts the collector is at Vcc, 12V in this case. This is because for DC the inductor is a short. When the sin starts, it can only go as low as VE, which I decided to make about 2V.

The input voltage is multiplied by 100 so it can be seen, otherwise it would look like a flat line at 0V.

Here is the collector current;

I was trying to get the bias current to be 50mA. This is close enough.