OpAmp Input Distinction

Hello,

I have a question regarding operational amplifier. How does exactly the inverting and non-inverting terminals make a difference in calculations, the output voltage for example? Lets say I ignored the fact there's inverting and non-inverting inputs, so no signs provided, how should my calculations be affected? I'm asking this because I have not noticed anything that takes into account the sign of input terminals in performing calculations.

Thanks.

Maybe my question is unclear. Let me reward it this way:

Where do we take into consideration the inverting/non-inverting input in an OpAmp terminal behavior analysis/calculations?

Let me put it this way. Calc the formula for a simple inverting amplifier. Then just redo the calculations for the same amplifier but this time switch the inverting/non-inverting terminals (only the sign) no connection changes. How does this make any difference?

The names of the inputs are a clue to the answer to your question. The inverting input _inverts_ the input and the non-inverting input, doesn't. So, a rising voltage on the inverting input of an ideal opamp will induce a falling voltage on the output or Vout = Vin * -a where 'a' is the open loop gain of the amp [that's Vin _times_ negative 'a']. A rising voltage on the non-inverting input will induce a rising voltage on the output or Vout = Vin * a [note the lack of a negative sign].

It's that simple -- it's merely a matter of the sign.

Thanks but hmmmm... :)

What I'm asking is when deriving these formulas for a certain configuration, how does switching the - and + of the input terminals change the way we do the math (KVL/KCL, nodal analysis, ...etc.)? For me, I could not find any difference, so I assume so far it's just a convention that has no meaning in studying the terminal behavior of the OpAmp. However the real physical element distinguishes between the two input terminals by + and - which makes me suspicious about my assumption.

I looked at how basic circuit analysis calculates the output voltage for different configurations, but the input signs have no rule. What am I missing here?

I'm not quite sure where your confusion lies.

In a negative feedback closed loop configuration, the op amp functions to force both input terminals at the same voltage. In that configuration, a rising voltage applied through an impedance to the V- terminal will send the outout more negative and a rising voltage applied to the V+ terminal will send the outout more positve.

In an open loop configuration, the op amp output is the V+ minus the V- times the gain. So a slight voltage on either terminal will send the output to the positive or negative rail respectively. The "sign" of the input definately matters.

Yeah but my question is where does the "sign" of the input appear in applying KVL/KCL/nodal analysis in calculating a certain amplifier configuration output voltage?

You don't need all of that _analytical firepower_ to analyze a typical opamp configuration.

It's very simple. It's just positive gain in terms of the non-inverting input and negative gain in terms of the inverting input. Or put another way, the output voltage = *A(V1-V2)* where *A* is the open loop gain of the amp, *V1* is the non-inverting input, and *V2* is the inverting input. Or how about like this:

*Vout = A(V1 + -V2)* [*V2* is negative because it's the _inverting_ input]

Let me again put my question in a different way.

Given a problem to analyze an OpAmp amplifier circuit using KVL/KCL/Nodal methods, where the input signs are not shown, how would you solve it? And if they are given, how would you solve it? Do they really matter? or just a convention based on the final output? But then again why it's shown on the OpAmp IC component?

Sorry but my brain cannot comprehend it.

If the input signs are not shown, then you can't solve it. The input signs are there to indicate the phase relationship between that input and the output. Have a look at the attached circuit. It's a differential amplifier, the basic circuit used in an opamp (the actual circuit is typically far more complex, but this demonstrates the basic principle). Notice that there are two inputs and two outputs. The typical IC opamp has only one output because, arbitrarily, only one of the outputs is selected as the opamp output. Lets choose the minus output (collector of Q2) so the Vin+ and Vin- designations correctly indicate the phase in our case.

Now, in an IC opamp, Re [that's the resistor at the emitters] is usually replaced by a constant current sink. Doing so makes the analysis easier, so consider it done. With a constant current divided between Q1 and Q2, it acts like a see-saw. If Vin+ rises, it turns Q1 further on, which allows more current to flow through Q1. That leaves less current for Q2, thus Q2 turns further off and the output voltage of our simple opamp goes up.

Did you notice how a rise in the Vin+ voltage caused a rise in the output voltage? That is the action of a non-inverting input, thus the '+' sign.

Conversely, if the voltage at the Vin- input rises, Q2 turns further on, which causes the voltage at Q2's collector to fall. There you have an inverse effect which is the action of an inverting input, thus the '-' sign.

The sign indicates the phase relationship between the input and the output. If all you are interested in is _magnitudes_ in your KVL/KCL/Nodal analysis, then the sign, in some cases, is unimportant. But, if you also want to include phase (or direction of current flow or polarity of resultant voltages) then the sign is imperative.

And, in the case of an opamp circuit with no phase designation on the inputs, there is no way to determine what type of circuit it is, without the + and - signs, and without that knowledge, there is no way to analyze the circuit with any confidence.

I got it! Thanks for the information.

Another way to think about it is "forced polarity" that affects the polarity of the connected elements to a specific node.