# Op Amp Confusion!

Discussion in 'Homework Help' started by ThePerk, Feb 7, 2009.

1. ### ThePerk Thread Starter New Member

Feb 7, 2009
9
0
Alright, I've got this homework problem that I cannot figure out for the life of me, I'm terrible at electronics so any help is much appreciated!

The problem is stated as follows:

Develop a schematic of a single stage opamp amplifier based on the LM324 chip. Note that LM324 chip has four opamps, use just one. The opamp is to have a single supply voltage of 5 volts and operate in an inverted configuration. The input resistance must be at least 1k ohm and the gain must be 10. The output should be 2.5 volts at 0 volts input.

I understand that the gain of 10 is achieved by having the feedback resistor 10 times that of the input resistor, right?

I'm not familiar with the LM324 chip so throwing that into the mix confuses me even more!

Also, how do I get a 2.5 volt output with a 0 volt input?! I DON'T UNDERSTAND! I thought 0 input was 0 output!

Thanks for any help!

2. ### beenthere Retired Moderator

Apr 20, 2004
15,808
294
Get the data sheet for an LM324. Given the gain figure, determine how much current would have to come through the input resistor to force a 2.5 volt output. Rig an arrangement of resistors to supply that input current.

LM324's are not the latest and greatest.

3. ### hgmjr Retired Moderator

Jan 28, 2005
9,030
218
In keeping with the general philosphy of forum members to avoid blurting out the answer to every homework question, I would recommend you spend some time reviewing the material in the All About Circuits ebook on the subject of operational amplifiers. The section contains material that should clarify this aspect of operational amplifier behavior. Read it all if you what to strengthen your understanding of these very flexible design building blocks.

We will be here for you if you encounter anything in the material that is unclear.

hgmjr

4. ### Skeebopstop Active Member

Jan 9, 2009
358
9
There are two ways to implement this. One is pretty standard and might suffice, however I don't like it as it requires you use a negative voltage into the op-amp but you only have a +5V rail.

The other solution requires no negative voltage but defies 'conventional' op-amp inverting amplifier configuration.

If you think the latter applies, I could see some hints being required.

5. ### ThePerk Thread Starter New Member

Feb 7, 2009
9
0
Ok, so...

The output needs to be 2.5 Volts when the input is 0, does this mean that I want a voltage offset of 2.5 volts?

I've been reading all the stuff on the opamps posted here on the site and I still don't feel too knowledgeable. I'm a mechanical engineering major and it seems that for most of us, electricity/electronics is a struggle

So far, this is what I think is going on...

1. I will have an inverting configuration (+5V on - input and a ground on + input.

2. There will be a resistor (R1) on the inverting input > 1kohm

3. There will be a negative feedback loop from the output back to the inverting input with a resistor (R2) in line that is 10x that of R1 to achieve a gain of 10.

4. This is all to be constructed in a LM324 chip that contains 8-pins.

Does that seem correct?

Still having trouble trying to understand the 0V input giving a 2.5V output.

Thanks a bunch for all your guys help.

6. ### mik3 Senior Member

Feb 4, 2008
4,846
69
What signal do you want to amplify?

From where it comes?

7. ### ThePerk Thread Starter New Member

Feb 7, 2009
9
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Hey mik3,

The problem statement is up above, that's all the information that was given to me. Thanks.

8. ### leftyretro Active Member

Nov 25, 2008
394
5
Well the answer is all around this thread. You are designing a simple inverting op-amp with a gain of 10. Normally an inverting op-amp with no output offset requirement just grounds the + input of the op amp. If you connect a 2.5vdc reference voltage to the + input (use two equal value resistors wired in series between +5 and ground and connect the junction of the resistors to your + amp input. With no voltage on the input to your op-amp the amp will have to raise it's output voltage to 2.5v so as to balance out the 2.5v on the + input. And any change on the input to the amp will add or subract X10 to the output. If you couple the input with a capacitor the AC signal will be amplified on the output but be centered at 2.5 volts which is half the value of the rail to rail voltage, 5vdc. If you then couple an output capacitor then you have created a X10 AC amplifier without requiring dual polarity voltages for the op amp power.

This make sense?
Lefty

9. ### ThePerk Thread Starter New Member

Feb 7, 2009
9
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Lefty, I appreciate your help. Not sure if this is what you are describing but here's my attempt at understanding.

10. ### mik3 Senior Member

Feb 4, 2008
4,846
69
The top of R1 has to be connected to Vcc.

11. ### Skeebopstop Active Member

Jan 9, 2009
358
9
1. +5V on + 'rail' and a ground on - 'rail'

2. Correct

3. Correct for one solution but maybe a bit different for a more graceful one (the more graceful one is tricker but would probably warrant bonus points!)

4. Correct

You are on the right track. In the configuration you have, see your posted circuit, there seems to be a capacitor between the input and '-' port of op-amp. This means you will have no signal as you are dealing with DC.

Also, you have your '+' terminal able to be adjusted __with__ the feedback, so you'll just end up saturating. Consider isolating this.

Think about these things and try to draw up another circuit so we can help you along.

12. ### ThePerk Thread Starter New Member

Feb 7, 2009
9
0
Skeebopstop,

Thanks. What do you mean by isolating? Should I put a capacitor in line with the feedback resistor to block the dc voltage from circulating to the + input?

Also, I'm still confused about how I can put in 0V input and still get 2.5V out?

13. ### leftyretro Active Member

Nov 25, 2008
394
5
Not quite. The top of R1 goes to your +5volt power source.
Right side of Rf goes directly to the op amps output. Input C is only needed if you are going to input AC voltage (0-.178 RMS AC) as your input signal to be amplified, if your input signal is just a variable DC voltage that ranges from -.25 to +.25 volts then just wire it to the left of Rin.

The output cap is only needed if you just wish to pass the AC voltage being amplified without the +2.5vdc offset. If you require just the amplified DC voltage output (0-5vdc) then remove that C.

Lefty

14. ### Audioguru Expert

Dec 20, 2007
10,788
1,212
The LM324 has 14 pins, not 8 pins.
The teacher asked for a simple DC circuit so remove the capacitors.
If R1=R2 and the opamp has a DC gain of 10 then the output will be saturated high (it will try to go to +25V) when the input is 0V.

Calculate values for R1 and R2 so that the output is +2.5V when the input is 0V.

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15. ### Skeebopstop Active Member

Jan 9, 2009
358
9
Are you allowed to use negative voltages as inputs?

The circuit outlined here demonstrates the concept. Do not copy any inverting amp formulas out of the book as this is a bit of a special situation. Work it out from first principles.

16. ### Audioguru Expert

Dec 20, 2007
10,788
1,212
Hint#1:
1) Take my circuit and connect the input to 0V.
2) Reduce the voltage at the (+) input until the output voltage is +2.5V.
3) Guess what the voltage is at the (+) input of the opamp?

Hint#2:
An active opamp with negative feedback tries to keep its input voltages the same.
The (-) input voltage is 0.22727V.
Make a voltage divider with R1 and R2 so that the (+) input is also 0.22727V.

17. ### Skeebopstop Active Member

Jan 9, 2009
358
9
Aww you gave it away

My implementation was to hold + at 0.25V, but then you have to adjust R2 to 9k instead of 10k get 2.5V at the output.

18. ### ThePerk Thread Starter New Member

Feb 7, 2009
9
0
I've come to the conclusion that I am a complete electronically challenged retard. Far as I can tell you pretty much gave away the answer above yet I still have zero comprehension of what is going on.

Thanks for the help.

19. ### Skeebopstop Active Member

Jan 9, 2009
358
9
We tend to jump around a lot and that makes it confusing. Just follow AudioGurus posts as he goes through it all and in the end demonstrates a viable solution.

20. ### ThePerk Thread Starter New Member

Feb 7, 2009
9
0
Alright guys, I'm hoping this is right because I'm really tired of dealing with this and it's due soon.