# Range of input voltages for linear range in this opamp

Discussion in 'Homework Help' started by Kevin2341, Mar 31, 2013.

1. ### Kevin2341 Thread Starter Member

Nov 1, 2009
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0

"the opamp shown above is LM107 without it's supply voltages. The LM107 is obsolete, but nonetheless the maximum supply voltage can be researched. Suppose that v1=v2=v3, and R1=R2=R3=R4 = 25k ohms, and R5 = R1||R2||R3||R4 which equals 6250 ohms.

What is the range of input voltages for the circuit to be in the linear range?"

I don't understand what to do with this problem. My teacher vaguely brushed over opamps in class, told us what they did, and gave us equations. Obviously he also covered the linear range graph (where usually the VCC + and - are the limits).

I'm not sure where to even begin considering I don't have any voltages, except I know they are all equal to each other.

I've searched the book up and down for help, as well as the internet, and I'm completely lost. Can someone please help me.

2. ### JoeJester AAC Fanatic!

Apr 26, 2005
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You can start by getting the datasheet for the LM107.

Then you can address the problem correctly ... How did you arrive at the R7 value?

Can you tell us the function of this circuit?

Last edited: Mar 31, 2013
3. ### Kevin2341 Thread Starter Member

Nov 1, 2009
19
0
My apologies, I was probably working on a different circuit before I drew up that one, so the resistor names are all screwy.

The resistors that are valued at 25k ohms SHOULD be labeled as resistors 1-4, and the resistor from the positive terminal to ground should be R5.

I have looked at the datasheets, and they tell me either 0-30V, or +\- 15v.

4. ### JoeJester AAC Fanatic!

Apr 26, 2005
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Your datasheet shows Vcc being 0 to 30V or +/-15V?

Attached is the datasheet I'm viewing

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5. ### Kevin2341 Thread Starter Member

Nov 1, 2009
19
0
I suppose so? But I don't know which to pick, or why?

Edit: with some reasoning, considering that the voltage sources go to the negative terminal of the opamp, I would presume that it is safe to say -\+ 15V?

6. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,865
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The voltage sources I was concerned with were (Vcc and Vdd) are connected to the power + and power - pins on the op amp. The input voltages can be +/- 15V ... or 0 to 30.

Do you the common name for that circuit topology?

7. ### Kevin2341 Thread Starter Member

Nov 1, 2009
19
0
Yeah, that's what we use in my class, all of my research online thus far has called them V+ and V- so I was thinking that was the more common name of those. VCC+ and VCC- is what we usually use, but I imagine VCC and VDD are the same pins of interest.

Does an opamp require that those pins be used in practical applications for it to work? Most of my work with them thus far have been in the ideal applications and don't involve them, I always thought they were just a limiter based off what my teacher described them as in class.

8. ### JoeJester AAC Fanatic!

Apr 26, 2005
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Yes, you need V+ and V- for the circuits to work. According to that datasheet I posted, the supply voltages can be +/- 22V max. Your input voltages, are connected to the input resistors that are connected to the - input terminal of the op amp.

Do you know the name of that circuit? Can you derive the expected output?

9. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,865
1,712
Kevin,

Read up on "summing amplifier". Therein lies a clue to your problem.