Need help understanding circuit

**broken link removed**

I understand the first resistor filters out the dc offsets. I know that than some sort of voltage-divider is then used. I'm not sure what the hell it's there for though.

Also, the book says the transistor has to remain in a bias state. Which means, partially on?

I don't understand the difference between a transistor which is partially on, and a transistor that is "fully" on.

Jeremy_ said:

I understand the first resistor filters out the dc offsets. I know that than some sort of voltage-divider is then used. I'm not sure what the hell it's there for though.
The capacitor blocks any DC component which could cause current to flow into or out of the transistor resistor biasing network or transistor base.

Also, the book says the transistor has to remain in a bias state. Which means, partially on?
When a transistor is used in a class A type amplifier, its biased so the collector voltage is approx half the supply voltage.
This is so the collector voltage can swing lower and higher than the half point voltage.

I don't understand the difference between a transistor which is partially on, and a transistor that is "fully" on
A partially on transistor is biased in the way I described.
A transistor used as a switch is either conducting, so that the collector voltage is close to 0V and all the supply voltage appears across the collector load or off, so no collector flows .

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Click to expand...

hi,
Look at this pdf

The above description is very basic but it should give you the general idea.

Thanks. I'm new to electronics, so I'm finding it difficult to understand the terminology and diagrams :X

Hmm. Okay, well. Here's my guess as to what's happening.

R1 and R2 are keeping the transistor in a bias state by creating a large enough voltage drop across R2. When a low AC signal passes through from the source input, it decreases flow from the emitter to the collector, and when it's higher, it increases flow from the emitter to the collector. I still can't see how this would effect the output though.

I *think* R3 and R4 are for another voltage drop across R3, which would control how much to amplify the source signal.

Maybe this is just an error in the schematic, but I noticed the output signal is inverse of the input signal. I.e High-Input is Low-Output.

This makes sense because as the transistor turns on, more current is passed through to ground, if it is low, less current is passed through to ground and more to the output signal.

Maybe I'm totally wrong here.

Jeremy_ said:

Maybe this is just an error in the schematic, but I noticed the output signal is inverse of the input signal. I.e High-Input is Low-Output.

This makes sense because as the transistor turns on, more current is passed through to ground, if it is low, less current is passed through to ground and more to the output signal.

Maybe I'm totally wrong here.

Click to expand...

hi,
You are correct, that configuration will invert the signal at the collector with respect to the base signal.

Generally speaking it will depend on which part of the charateristic curve you bias it to operate on. Could be the linear part , non linear part or the saturation part

Your doing well but I think you are still confused how the amplification takes place.
Think of Ohm's law and what is happening to the collector voltage/current as the input voltage/current changes.