Electronic – Class B amplifier with Op-AMP

amplifieroperational-amplifier

I have been reading the following article about Class-A and Class-B amplifers.
http://www.antonine-education.co.uk/Pages/Electronics_2/Amplifiers/Power_amps/further_page_12.htm#Question 1.

I am not sure I understand the point of having an Op-AMP in the class-B amplifier !
Why do we use a set up pull-push in the Op-AMP's feeback loop?
Why don't we just use OP-AMP by itself? And, how does OP-AMP helps to improve cross-over
distrotion?

Also, in the same article, right above question 3, it states the following:

"The op-amp holds the base-emitter voltage of the npn transistor at 0.7 V. When there is a negative signal, the op-amp changes over, dropping 1.4 V, so that it turns on the pnp transistor. The voltage gain is 1, but the power gain is almost infinity."

Could someone please explain the above in relatively easier way?

Best Answer

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

schematic

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.

schematic

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!

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