# Where does the output current of an op amp come from? I am trying to build a circuit that will amplify a 3.3V signal to 5V, but I also need to have an output current of 100mA. The source of $V_i$ (an FPGA) only has an output current of around 15mA. Where does the output current come from in a non-inverting setup; the voltage supply or $V_i$?

If the current comes from $V_i$ (and therefore wouldn't be able to supply the necessary current to $V_o$), then would it just be better to have a setup using transistors to increase the current and voltage?

• Whats the input impedance of the opamp? From that, you can tell that almost no current flows through the inputs, therefore the output current must come from somewhere else. – Wesley Lee Oct 31 '16 at 2:37
• Here is the datasheet for the one I am looking at: link The input resistance and capacitance is on page 4 – Cody495 Oct 31 '16 at 2:52
• What is your actual load type? linear ? or? output AC only or with DC. split supply? – Tony Stewart Sunnyskyguy EE75 Oct 31 '16 at 3:34
• @Cody495 -- It was a rhetorical question. If it has 5GOhm input resistance, how are you going to put 15mA through it? – Wesley Lee Oct 31 '16 at 3:38

The output current comes from the supply pins. The input impedance is extremely high and effectively does not load the input.

Also, your above diagram shows an inverting configuration, with a load impedance on the input of Ri.

• Ah yes, thank you for noticing. I fixed the picture. So if I use a power supply with say a 1A output, then that should work to send a signal with 100mA, correct? – Cody495 Oct 31 '16 at 2:56
• You need to look at how much current your op amp can actually source and sink. For instance, an LM358 can source 20mA and sink 10mA. It all depends what you're trying to do here, but you probably want to look at using a power transistor. Are you switching the voltage or controlling the voltage? Also bear in mind that most op amps have a maximum output voltage which is less than the supply rail voltage. – Ian Bland Oct 31 '16 at 3:14
• I'm looking to use an ADA4891-4ARZ-R7 from analog devices, it has an output current of 125 mA. What do you mean by switching and controlling voltage? – Cody495 Oct 31 '16 at 3:44

For crude requirements of adding a buffer to an op amp, this is one way. In some cases a DC load on OA output is needed to reduce crossover noise. Normally supply lines to OA are not shown for simplicity.

• But that's where the current comes from.