# AC current through a transistor amplifer - a concept I can't get my head around

#### tim04444

Joined Apr 1, 2016
6
Dear all,

I am completely stuck on understanding a certain concept about AC current and transistors and am wondering if anyone could possibly help me.

I want to build an amplifier where I first amplify the AC voltage using a common-emitter circuit with an NPN transistor. I get this bit. I then want to amplify the AC current coming from this circuit using a emitter-follower circuit. Could someone please help me understand how AC current flows through a transistor.

I know that when analysing AC, all DC sources are grounded, and that the collector current is beta times the base current. When DC current flows, it goes from base to emitter and collector to emitter. If the DC source is grounded when analysing AC, does this mean that AC current can also flow from emitter to collector (Vcc is grounded)? I am confused about which direction AC flows through a transistor.

I also want to find my AC current flowing down the path of Vout (parallel to the load resistor). I want this because this is what I presume will then go on to being amplified in the emitter-follower circuit. Do I get this by finding the output impedance of the amplifier.

I am just generally confused about what the AC current on the right side of the transistor (emitter and collector) actually is (is it the DC current modulated or is it AC current passing through the transistor???), and which direction it travels.

I know there are easier ways to create amplifiers but I would like to know how to do it via transistors.

If anyone could help me on this issue I would be very grateful. I know how the amplifiers work as a concept as a whole but would like to be able to understand what is going on piece by piece.

Thanks

Tim

#### Jony130

Joined Feb 17, 2009
5,212
Yes it's true that a amplifier "modulated" power supply energy in the rhythm of a input signal.
Because the amplifier is a device that allow as control the flow of "high power" by helps of a low power. To the Amplifier effect occurred, two things are necessary: source of energy (power supply) and a device for controlling the flow of this energy - > the amplifier.
So the AC current in properly biased BJT is a nothing more then the DC current modulated.

#### tim04444

Joined Apr 1, 2016
6
Thanks for the answer. So does this mean that the AC current that flows through Vout is actually the DC current which flows through the collector resistor modulated by the AC input, i.e. if we know the peak voltage at the collector resistor, we can then know the peak current here (which is controlled by the AC input at the base)? If we know this alternating DC current, we can then find out the current at Vout? I assume this is small because the collector resistor is large.

Thanks,

Tim

#### mcasale

Joined Jul 18, 2011
210
Do you have a schematic or sketch of your circuit? It's difficult to talk about current flow when we cannot see how the components are hooked up.

#### Jony130

Joined Feb 17, 2009
5,212
So does this mean that the AC current that flows through Vout is actually the DC current which flows through the collector resistor modulated by the AC input
Yep. But this "modulated DC" is no longer DC current (constant in time) but now we have a AC current.
assume this is small because the collector resistor is large.
And this is why we do not use this type of a amplifier to drive a heavy loads (low resistance, high current).