# Basic Transistor Amplifier Circuit help

#### NanyBany

May 15, 2014
12

Okay can someone please explain what is going on here. I have tried to figure it out myself, what I get out of this is that current should be stopped once C1 is charged and when the current alternates the same effect for C2. I am a nooby and any help will be greatly appreciated.

Edit: Okay I think I got a little more of it. C1 is getting charged on both sides so current is aloud to flow. But where the ground meets the resistor does the current split to the bottom output line. If so then C2 is also getting charged on both sides so current is aloud to flow. if not then that's where I am stuck at right now, assuming I got it right so far. If I am right, then why use capacitors here? Why not just leave them out?

Last edited:

#### Ratch

Mar 10, 2013
1,099
View attachment 13001

Okay can someone please explain what is going on here. I have tried to figure it out myself, what I get out of this is that current should be stopped once C1 is charged and when the current alternates the same effect for C2. I am a nooby and any help will be greatly appreciated.

Edit: Okay I think I got a little more of it. C1 is getting charged on both sides so current is aloud to flow. But where the ground meets the resistor does the current split to the bottom output line. If so then C2 is also getting charged on both sides so current is aloud to flow. if not then that's where I am stuck at right now, assuming I got it right so far. If I am right, then why use capacitors here? Why not just leave them out?

NanyBany,

I discern that you don't know what a capacitor does or how it operates. So let's start at the beginning. What do you mean when you say a capacitor is "charged"? Charged with what?

Ratch

#### NanyBany

May 15, 2014
12
Charged with electrons. From what I learned, unless I got it wrong. One of the plates in a capacitor gets charged with electrons, when the plate is fully charged then no more current flows. When the Supply Voltage is removed the Capacitor plates hold the supply voltage between them. Then the electrons transfer to the other plate till the plates reach equilibrium.

#### Ratch

Mar 10, 2013
1,099
Charged with electrons. From what I learned, unless I got it wrong. One of the plates in a capacitor gets charged with electrons, when the plate is fully charged then no more current flows. When the Supply Voltage is removed the Capacitor plates hold the supply voltage between them. Then the electrons transfer to the other plate till the plates reach equilibrium.

Would you believe that a capacitor with a high voltage across it has the same number of charges (electrons) as an identical capacitor with no voltage across it? If a zillion charges are added to one plate, an equal zillion charges are removed from the opposite plate so as to give a net charge change of zero. So the plates have an unequal charge balance between them. Anytime the charge density is different between two points, a voltage will exist between those points. It takes energy to change the charge density between the two capacitor plates. Therefore, the capacitor is charged with energy, or "energized" when the plates are not balanced. If there is no conduction path between the plates, the charge imbalance will remain, and the cap will stay energized. By the way, current does not flow. Charge flows and current is charge flow. Current flow means "charge flow flow". You should say charge flows or current exists. Next question.

Ratch

#### NanyBany

May 15, 2014
12
Would you believe that a capacitor with a high voltage across it has the same number of charges (electrons) as an identical capacitor with no voltage across it? If a zillion charges are added to one plate, an equal zillion charges are removed from the opposite plate so as to give a net charge change of zero. So the plates have an unequal charge balance between them. Anytime the charge density is different between two points, a voltage will exist between those points. It takes energy to change the charge density between the two capacitor plates. Therefore, the capacitor is charged with energy, or "energized" when the plates are not balanced. If there is no conduction path between the plates, the charge imbalance will remain, and the cap will stay energized. By the way, current does not flow. Charge flows and current is charge flow. Current flow means "charge flow flow". You should say charge flows or current exists. Next question.

Ratch

You say "If there is no conduction path between the plates, the charge imbalance will remain, and the cap will stay energized.", So how does charge flow on the opposite plate. It is my understanding that a cap has an insulating material between the two plates called the dielectric, with the dielectric in the way of the conduction path how does charge continue to flow? Thank you.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Charged with electrons. From what I learned, unless I got it wrong. One of the plates in a capacitor gets charged with electrons, when the plate is fully charged then no more current flows. When the Supply Voltage is removed the Capacitor plates hold the supply voltage between them. Then the electrons transfer to the other plate till the plates reach equilibrium.

Sure, one plate will have an excess of electrons and the other a deficit (that exactly matches -- hence Ratch's comment about the total number of electrons not changing).

But don't think of a capacitor like a tank that fills to a certain capacity then stops. Think of it more like a balloon that holds more if you blow harder. And yes, just like a balloon, if you blow hard enough it will pop. The voltage rating on a capacitor is like the pressure limit for a balloon.

In your case, what matters is that as long as the voltage is changing, charge will flow in and out of the leads of the capacitor. You see that as current flowing "through" the capacitor.

#### Ratch

Mar 10, 2013
1,099
NanyBany,
You are asking how excess charge on one plate flows to the other plate containing a defciency of charges? One way is leakage through the dielectric. A dielectric is supposed to be an insulator, but nothing is perfect. The other way is through the circuit itself. I hope you understand that with a perfect dielectric, no charge flows and no current exists through the capacitor. There is current in the leads of the cap, but that is due to the charges accumulating onto one plate and depleting from the other plate.

Ratch

#### NanyBany

May 15, 2014
12
NanyBany,
You are asking how excess charge on one plate flows to the other plate containing a defciency of charges? One way is leakage through the dielectric. A dielectric is supposed to be an insulator, but nothing is perfect. The other way is through the circuit itself. I hope you understand that with a perfect dielectric, no charge flows and no current exists through the capacitor. There is current in the leads of the cap, but that is due to the charges accumulating onto one plate and depleting from the other plate.

Ratch

Thank you very much, I guess my last two questions are, how does the circuit above amplify the input, and why is the output an inverse of the input?

#### Ratch

Mar 10, 2013
1,099
NanyBany,

The current in the emitter and collector are almost equal. If R3 is greater than R4, than it will have more voltage across it for the same current and the signal will be larger. The input voltage is measured at the top of R4, and the output at the bottom of R3. Therefore their polarities will be opposite. By the way, the two caps keep the DC bias voltages out of the signal part of the circuit.

Ratch

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