# Where does the current go?

Discussion in 'General Electronics Chat' started by EngIntoHW, Sep 4, 2010.

1. ### EngIntoHW Thread Starter Member

Apr 24, 2010
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In the following circuit,
How does the current 'I' return to ground, if Vout is connected to nothing?

2. ### KMoffett AAC Fanatic!

Dec 19, 2007
2,644
251
The output transistors inside the opamp will sink or source the current.

Ken

3. ### marshallf3 Well-Known Member

Jul 26, 2010
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Vout is meant to be connected to something - whatever the circuit is meant to be driving be it a light bulb, relay, speaker or the next stage of another circuit.

4. ### EngIntoHW Thread Starter Member

Apr 24, 2010
128
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But what if it remains unconnected?
Or connected to a cap?

Then it sinks the current, right?

5. ### marshallf3 Well-Known Member

Jul 26, 2010
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202
Voltage doesn't have to "go anywhere", think of a battery sitting on the shelf.

If that circuit you illustrated were hooked to a capacitor, and the signal were AC, then it would induce some current through the capacitor.

It's also possibly a tiny amount is used internally in the IC as a feedback or reference of some sort but just because it's there doesn't mean it's doing anything.

Think back to the basics - voltage is just a potential, current is the actual flow of electrons.

6. ### EngIntoHW Thread Starter Member

Apr 24, 2010
128
0
Hey Marshallf3,

I was talking about the current I = I1 + I2 + I3.
Vo = - I * RF

How do 'I' return to the ground, if the output is connected to nothing?

7. ### KMoffett AAC Fanatic!

Dec 19, 2007
2,644
251
The opamp's output is not connected to nothing. The output is a pair of push-pull transistors inside the opamp. Rf is connected to the junction of these transistors. The internal circuitry drives one or the other of these to conduct to establish a voltage out that will cause current (I) to flow through Rf. The current is passed to ground (or +V) through one of these transistors. If there is a load the output transistors have to deal with both the feedback current and the load current to establish the output voltage.

You need to look the internal circuit of an opamp.

No more time...got to go have breakfast with my wife.

Ken

8. ### Jony130 AAC Fanatic!

Feb 17, 2009
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1,188
"I"current will be sink or source by opamp output stage.

9. ### Ghar Active Member

Mar 8, 2010
655
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One point which doesn't really change anything here is that current doesn't return to ground, it returns to its source.

In this case for example current from V1 will go through R1, RF, through the op-amp output stage to ground and back up from ground to V1. Some V1 current will also go through R2 and R3 and back through ground.

10. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
202
Didn't really notice the formula they have written above Rf.

The internals of the IC will allow the input currents to eventually return to their sources and the output voltage available is all I was looking at. What you have there is a representative circuit of what's called a summing amplifier, often used to add the magnitude of two or more input voltages.

11. ### EngIntoHW Thread Starter Member

Apr 24, 2010
128
0
Thanks a lot guys!

Its indeed a summing op amp.

If I like to decrease voltages, shall I just connect an inverter op-amp to each desired input?

12. ### DonQ Active Member

May 6, 2009
320
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I've actually seen this circuit frequently in real life. I guarantee you it will never work until you hook the op-amp to a power supply. Once your schematic shows that needed component, the return path will become apparent.

It will also keep you from building actual circuits this way.

13. ### DonQ Active Member

May 6, 2009
320
14
No, just realize where the current is coming from. It comes as a result of the input voltages pushing it through the input resistors. Bigger resistors mean less current in the same way that less voltage would.

Look at the gain equation for a summing amp. Things like Vx/Rx ...

14. ### EngIntoHW Thread Starter Member

Apr 24, 2010
128
0
Hey,
Sorry for my mistake,
I meant if I like to subtract voltages, shall I use an inverter op amp at each desired input?

15. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
202
Inverting op amp the voltages will still equal the sum, polarity will just be negative.
+1 and +2 and +3 = -6

16. ### DonQ Active Member

May 6, 2009
320
14
Just summing and inverting on one amp, then 'adding' this voltage to a summing input of a second amp, has the result of subtracting the sum of the first amps voltage from the other inputs of the second amp (divided by their respective scaling resistors)

Depending on exactly what you're doing, it may be possible to do what you want on only one amp. Search "differential" amplifier.