Op-Amp Trick Questions: Formulas for Av and LED Circuit Solutions

[Femme_physics]

Homework Statement

http://img502.imageshack.us/img502/4082/vddvss.jpg

I'm asked to

1) Develop a formula for

Av = Vout/Vin

And

2) If the LED was flipped the other way, what must we do for for the rest of the circuit so current would still flow through it

The Attempt at a Solution

1) I'm really confused about this one. I thought to just do:

http://img23.imageshack.us/img23/6084/avavavv.jpg


2) You just turn the op-amp from non-inverter to inverter

 

[gneill]

Somethings fishy about your derivation for Av.

Some questions to ask yourself:

- What potential will appear at the op-amp's inverting input?
- What current will flow through R1 (and in what direction)? Where must that current come from?
- So what current will flow through R2 (and in what direction)?

Given the current flowing through R2 and the potential at the op-amp inputs, what then must be the potential at the op-amp's output? (KVL)

Note that the op-amp is powered by a dual voltage supply. What are the limits on the possible range of Vout? How might you alter the polarity of Vout without changing the basic layout of the circuit? (Hint: how does vout depend upon Vin?)

 

[Femme_physics]

Somethings fishy about your derivation for Av.

Should ignore the denominator - should be just Vin

- What potential will appear at the op-amp's inverting input?

2 volts

- What current will flow through R1 (and in what direction)? Where must that current come from?

At non-inverting?

Vout = R2 x Vin / R1 + R2 = 1.66 V
Vout = 1.66 volts

Using KVL:

1.66 -R2I1 - R1I1 = 0

I1 =0.1333 mA

The current will come from the op-amp and flow towards the ground.

- So what current will flow through R2 (and in what direction)?

Same current, I1.

The current will come from the op-amp and flow towards the ground as well.

Given the current flowing through R2 and the potential at the op-amp inputs, what then must be the potential at the op-amp's output? (KVL)

I did it via the non-inverter formula already

Note that the op-amp is powered by a dual voltage supply. What are the limits on the possible range of Vout? How might you alter the polarity of Vout without changing the basic layout of the circuit? (Hint: how does vout depend upon Vin?)

The limits are +18 volts -18 volts... I can alter the polarity by making the limits to be -8 volts and -18 volts

 

[gneill]

I think you're complicating the analysis unnecessarily If the potential at the inverting input is Vin, what is the potential across R1? Then use Ohm's law to find the current through R1 (in terms of Vin). Using that current, use Ohm's law again to find the potential across R2 (in terms of Vin). If you know the potentials across both resistors you can find the potential at Vout.

Changing the op-amps power supply values isn't going to be helpful, since Vout is trying to satisfy the requirements of the feedback network and Vin. There's a niftier way to affect the change. Consider what you might do treating Vin as something you can play with...

 

[Femme_physics]

Changing the op-amps power supply values isn't going to be helpful, since Vout is trying to satisfy the requirements of the feedback network and Vin. There's a niftier way to affect the change. Consider what you might do treating Vin as something you can play with...

Ahh I set Vin = -2 insteaf of +2

I think you're complicating the analysis unnecessarily If the potential at the inverting input is Vin, what is the potential across R1? Then use Ohm's law to find the current through R1 (in terms of Vin). Using that current, use Ohm's law again to find the potential across R2 (in terms of Vin). If you know the potentials across both resistors you can find the potential at Vout.

VR1 = 2 volts
R1 = 2k

IR1 = VR1/R1 = 2/2000 = 1 mA

 

[gneill]

Ahh I set Vin = -2 insteaf of +2

Yup. Easy-peasy.

VR1 = 2 volts
R1 = 2k

IR1 = VR1/R1 = 2/2000 = 1 mA

Keep the calculation in terms of a variable Vin. You can also keep the resistors as variables until the end if you want Av as a general formula. You can then plug in the resistor values to find Av for this particular circuit.

So, what's the expression for the current? Then, what's an expression for the potential at Vout?

 

[Femme_physics]

IR1 = Vin/R1

So, what's the expression for the current? Then, what's an expression for the potential at Vout?

http://img689.imageshack.us/img689/2848/vouts.jpg

I'm not sure how to connect all that to a single formula though

 

[gneill]

IR1 = Vin/R1




http://img689.imageshack.us/img689/2848/vouts.jpg

I'm not sure how to connect all that to a single formula though

Well, you've got Vin factored out in the first line above, simply divide both sides by Vin so that you have the expression for Vout/Vin...

 

[Femme_physics]

Vout/Vin = (1+R2/R1)

 

[gneill]

Vout/Vin = (1+R2/R1)

Yes, that looks good

 

[Femme_physics]

Thanks gneill, I appreciate the mentoring. They're going to have some funky questions at the final electronics test this year and trying to get ready for them... appreciate your time and knowledge

 

[gneill]

Thanks gneill, I appreciate the mentoring. They're going to have some funky questions at the final electronics test this year and trying to get ready for them... appreciate your time and knowledge

It's always a pleasure to help. Good luck in your test!

 

[Femme_physics]

Thanks!