How current can flow out of the op amp if none flows in?

[jtucker]

Hello folks,
This is my first post here, I hope I am posting my question in the correct place/manner? I am a mechanical engineering student taking an electrical class. We are studying op amps. The thing that is confusing me is current flow in the circuits. I understand that there is infinite input impedance going into the op amp, and zero impedance out. I wonder how current can flow out of the op amp if none flows in? Am I correct in understanding that inside of an amplifier there is some sort of dependent voltage source? If there is no current flowing into the op amp how can the source inside depend on anything? Thanks in advance for any help somebody can give me in understanding op amps, I will appreciate it.

 

[jtucker]

Sorry, I just realized I am not supposed to ask coursework questions in here, I will ask my question in the coursework forum

 

[meBigGuy]

The opamp always has a power source that it can draw from to drive the output. It generally is just assummed to be there even though it is seldom drawn.

 

[RBTO]

I wouldn't think this is so much a coursework question as a general OP amp question (and a good one at that).

Think of an OP amp as having two boxes inside. On the left, a box connects to the inputs and looks at the signal coming in. On the right, a completely separate box develops the output voltage based on information the first box gathered. The box on the right is a voltage or current generator in the ideal OP amp, but in the real world it has some limitations.

In a typical differential input OP amp (a 741 for example). The left box measures two voltages (relative to a reference - usually ground), and subtracts one from the other. In the ideal OP amp, no current flows when that measurement is taken. (In a real OP amp, there is a minute current flow which needs to be taken into account for engineering reasons, but is usually insignificant for practical reasons.) That voltage difference value is what the second (right) box looks at to decide what to produce as an output. For our typical OP amp, this difference is simply multiplied by a really large value (e.g., 100,000) to get the desired output. Of course if the result is 500 volts, the OP amp will never get that unless it has a supply voltage greater than 500 V. The actual output in a real world OP amp is limited by the supply voltage(s) and its output transistor construction (there are exceptions in some OP amps which actually generate their own internal supply voltage which are higher than the supply voltages). meBigGuy stated it properly - the output voltage and current ultimately come from the supply mains to the OP amp.

This is where feedback comes into play. Usually some external circuit is employed which will cause the difference between the two input voltages (going to our first box) to decrease as the output voltage (coming out of our second box) increases, and they meet at an equilibrium point. The final difference voltage may be a few microvolts which will cause the output voltage to be a few volts which will cause the input voltages to differ by the few microvolts that cause the output voltage to be a few volts...... etc., etc., etc. You now have a closed loop with feedback.

Hope this helps.

 

[sophiecentaur]

He has moved to / repeated the question in the homework forum. You can find the thread there.