Explaining Transistor Operation in a Signal Cycle

[Lunat1c]

Would someone be kind enough to explain what happens to the operation of both transistors in both the positive cycle and the negative cycle of the input signal please? I'm having a hard time understanding how this works exactly.

What I know is that with no input signal, both transistors have a quiescent current going through them. Since they're complementary they will have the same voltage drop across them and in between them (i.e. the point where RL is connected) will be at Vcc/2. Now, once the input signal is applied, the positive cycle is supposed to be applied across TR1. TR2 would have a base voltage greater than the collector voltage and therefore it will go into cutoff.

When on the other hand there's the negative cycle, TR1 goes into cutoff which is equivalent to an open circuit. How will TR2 maintain the quiscent current given that now there's an open circuit instead of TR1?

 

[vk6kro]

There is a mistake in that circuit. The bottom supply rail would have to be negative, not earthed, if you have the load connected like that.
If it was a speaker and with a single supply, there would be a large electrolytic capacitor in series with the load.

Each transistor acts as an emitter follower with the load alternately getting power from the two transistors but at opposite polarity.
It doesn't matter that the non operating transistor is cut off while the operating one sends a lot of current into the load resistor.

When there is a signal, neither transistor gets quiescent current. This is just a small current that flows when there is no signal.

 

[Lunat1c]

if we were to consider the loadlines of both transistors.. would it be correct to say that the quiescent voltage of the first one is Vcc/2 and that of TR2 to be -Vcc/2 for all values of collector current?

that would mean that the signal is applied to both transistor. however the first one clips the negative cycle while the other one clips the positive cycle and my problem would be solved ^^

 

[vk6kro]

No, unfortunately not.

Each transistor turns on for slightly more than half of an input cycle, but each on alternate half cycles. That is what Class B means.

 

[DeeNos]

Sure, I'd be happy to explain the operation of transistors in a signal cycle.

First, it's important to understand that transistors are electronic devices that act as switches or amplifiers. They consist of three layers of semiconductor material, known as the emitter, base, and collector. The flow of current through the transistor is controlled by the voltage applied to the base.

In a signal cycle, an input signal is applied to the base of the transistor. Let's start with the positive cycle. When the input signal is positive, it will cause the voltage at the base of TR1 to increase. This increase in voltage will cause the transistor to conduct more current from the collector to the emitter. This current will then flow through the load resistor (RL) and create an output signal. At the same time, the voltage at the base of TR2 will decrease, causing it to conduct less current. This creates a complementary action between TR1 and TR2, where one transistor is conducting more and the other is conducting less.

Now, let's look at the negative cycle. In this case, the input signal will be negative, causing the voltage at the base of TR1 to decrease. This will cause the transistor to conduct less current from the collector to the emitter, and the voltage at the base of TR2 will increase, causing it to conduct more current. Again, we see a complementary action between the two transistors.

In terms of the quiescent current, or the current that flows through the transistors when there is no input signal, this is determined by the biasing circuit. This circuit is designed to provide the necessary voltage and current to keep the transistors in the active region, where they can amplify the input signal. When there is no input signal, both transistors will have a quiescent current flowing through them, and this is maintained by the biasing circuit.

I hope this explanation helps to clarify the operation of transistors in a signal cycle. If you have any further questions, please let me know.