common emitter transistor question

[Mark S.]

Can someone please help me. On the webpage below about halfway down there is a graph plotting an a.c. current signal and the current flowing through the battery on the right of the diagram. Why are the two 180 degrees out of phase. Is it that when the input signal voltage is at maximum that no current is flowing into the base of the transistor?



thanks

Mark S.

 

[Mark S.]

I think I've cracked it. If no current flows from collector to emitter then the voltage is across the load. As input to the base increases then the voltage across the load decreases. Is that right?

 

[crutschow]

The confusion is that the bottom graph is plotted as negative current (apparently as current into the positive node of the battery). This designation is arbitrary. Notice that the current actually does increase in magnitude when the base current increases. This current flow is out of the battery and through the collector to the emitter common.

 

[Mark S.]

Thanks for the response crutschow but I'm still completely confused.

I am going to describe what i think should happen in this circuit and maybe someone can tell me where I am going wrong.

If there is no voltage signal going into the base terminal then no current flows through the speaker or V1. As the voltage increases positively into the base terminal current can flow through the speaker and V1.

I don't understand why the current in blue graph is negative. I know you tried to explain this crutschow but i still don't get it.

Thanks

Mark S.

 

[crutschow]

Your explanation is correct.

Negative and positive current simply refers to the direction of the current, not the magnitude. It's designation as positive or negative is arbitrary. Thus, in the simulation, they chose to have the current out of the battery and through the transistor be negative. Sometimes simulators use current into a node as positive, thus current out of the positive battery node, which is normal, would be negative. If you were charging the battery, the current would be positive. If you look at the graph, the maximum (negative) current occurs at the maximum input voltage as you would expect. Think of the negative current as sort of a mirror image of the positive current.

 

[Mark S.]

It's designation as positive or negative is arbitrary..

In this respect are you talking about conventional current flow and electron current flow?

And as long as there is a current flow it does'nt matter?

Thanks again

Mark S.

 

[crutschow]

You could be talking about either flow, but it this case it is current flow. In electronics the most common convention is current flow. It makes it easier since the current flow is in the same direction as the arrows on diodes and transistors.

 

[Ratchit]

Mark S.

Can someone please help me. On the webpage below about halfway down there is a graph plotting an a.c. current signal and the current flowing through the battery on the right of the diagram. Why are the two 180 degrees out of phase. Is it that when the input signal voltage is at maximum that no charge (not current) is flowing into the base of the transistor?

Although almost everyone uses the term "current flow", current means charge flow. So saying "current flow" means "charge flow flow", which is nonsense

This is a good example of the confusion that can result from not using the conventional current method. They are trying calculate values based on the real direction of the charge carriers. While this may seem intuitive, it can be confusing when both types of charge carriers exist in a circuit, which in this case are electrons and holes. Semiconductors and ammeters are marked for the conventional current method, so you can see in the fourth diagram from the top of the article that their designated current direction is opposite with respect to the arrow of the transistor, and opposite to what the ammeter says it is.

Your confusion can be relieved without using misleading and conflicting schematics. The voltage reversal comes from the topology of the circuit. It matters not whether a BJT, FET or tube is used. Think of it this way. Putting a positive voltage on the base of a NPN BJT, the gate of a FET, or the grid of a tube will increase the conduction of the any of the above devices. Ultimately a high enough positive voltage will saturate the device and bring the output voltage down to almost ground. A high enough negative voltage will reduce the conduction so the output voltage will be close to the supply voltage. Therefore, high input voltage means low output voltage and vice versa. That explains the 180 degree reversal.

Ratch

 

[MikeMl]

One of the reasons I stopped participating in AAC forums is because of their religious instance in explaining the operation of circuits using "electon current"! This does nothing but confuse folks; witness the question that started this thread.

 

[Ratchit]

MikeML,

That's correct. Amen, brother!

Ratch

 

[Mark S.]

Thanks for your help guys.

Just to make sure I get this (God I hope I'm right).

The reason they show a negative results on the graph is because they are measuring electron flow moving from the negative side of the battery to the positive side.

If they had made a graph showing conventional flow then the graph would have measured results in the positive range.

Please tell me if I am correct.


What are the AAC forums?

Thanks Mark S.

 

[Ratchit]

Mark S.

The reason they show a negative results on the graph is because they are measuring electron flow moving from the negative side of the battery to the positive side.

If they had made a graph showing conventional flow then the graph would have measured results in the positive range.

Please tell me if I am correct.

Don't bother trying to figure out such a confusing presentation.

What are the AAC forums?

All About Circuits Forum - Powered by vBulletin

You referenced this site in your link.

Ratch

 

[crutschow]

The reason they show a negative results on the graph is because they are measuring electron flow moving from the negative side of the battery to the positive side.

If they had made a graph showing conventional flow then the graph would have measured results in the positive range.

That's certainly a valid way of interpreting the results.

But as I previously noted, it may just be a result of the arbitrary circuit simulator convention in which current (flow) into a node is labeled positive. That would make current out of the battery positive node negative. No need to invoke electron flow which, as MikeML noted, often just confuses things..