# Help don't work opamp

Discussion in 'The Projects Forum' started by badabat, Dec 15, 2010.

Oct 4, 2010
3
0
Hi all;

I try to work noninverting amplifier. But I dont work it. Picture of my desing is below. Referance voltage is approximately 200 mV(on 1.1 ohm resistor). I used LM 358N for circuit. although I have changed opamp input Opamp output is not changing. I need help

2. ### #12 Expert

Nov 30, 2010
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.2611276 volts times a gain of 46 = 12.01 volts.

Does the output of the amplifier seem to be stuck at 12 volts?

3. ### Wendy Moderator

Mar 24, 2008
20,765
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This is because the power supply (to the op amp) is 12V. The plus input is 0.261VDC (R3, basic voltage divider), and the inverting input is 0V the gain from the plus input will be -46 (R2 ÷ R1 + 1). Since .261VDC is the reference signal for this op amp, there is -0.261 on the input (0VDC, R1). Gain -46 X -0.261 = 12.144VDC. Since the op amp can only put out 12V that is what you have.

If the regulator circuit was removed and 24VDC was used for the power supply it would be 12.14VDC.

Now define exactly what you are trying to do.

Side note: The LM358 is one of the few op amps that can have it's inputs that near ground and still work. Most op amps can't do this. Generally keep the middle voltage reference (think of it as ground) at ½Vcc, and work from there. Think of what you now are calling ground a -Vdd (the minus power supply). Do this and any op amp will work.

Creating a Virtual Power Supply Ground

The gain of this op amp is -R2/R1.

The gain of this op amp is R2/R1+1.

I didn't have an appropriate picture drawn so I made do, ignore C1 for the sake of argument.

Last edited: Dec 15, 2010
4. ### SgtWookie Expert

Jul 17, 2007
22,182
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Change R1 to 2.2k (2k2) and R2 to 99k, and see if you get better results.

990+22 Ohms = 1,012 Ohms; quite a heavy load for an opamp.

Your output won't change from a particular fixed voltage, as you don't have a signal source that changes levels. The maximum output voltage you'll see is around Vcc-1.5v; in your case 10.5v.

Add a 0.1uF capacitor between U1:8 and GND.

5. ### Wendy Moderator

Mar 24, 2008
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While I agree with Wookie, he is talking a practical circuit. For static DC values those caps are not as necessary, but they will prevent accidental oscillation.

6. ### SgtWookie Expert

Jul 17, 2007
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It's good practice to include bypass caps on everything.

Opamps usually have pretty good rejection of transients on the supply, but if you're near the limits of the output swing on the opamp, that kind of goes out the window; the output will be saturated and supply noise will be coupled to the output.

A resistor/Zener supply is fairly noisy when using real components. A 0.1uF cap keeps things a good bit more quiet.

7. ### Wendy Moderator

Mar 24, 2008
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Funny, the times I've used a simple zener/resistor I've had excellent results, no hiss in the audio whatsoever. They have other limitations, but for many applications they are plenty good enough. Nowdays I stick with solid state regulators such as the LM317, it is much more programmable overall.

I've noticed the AAC book doesn't seem to go through the basic equations fort these two configurations (the inverting is mentioned). I'll bring it up in the feedback and corrections. If other people can't find the non-inverting input equation I'll probably write something up.

Oct 4, 2010
3
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I change referance voltage 0.261 V to 0.100 V. But output is not changing. I will change resistor Sgtwookie says that. I will write result later

9. ### #12 Expert

Nov 30, 2010
16,283
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Output is not changing from what?

10. ### Wendy Moderator

Mar 24, 2008
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You never have said what you are trying to do. The closer you get to the power supply rail the more problems you will likely have. This is the reason I referred you to the pseudo power supply ground.

When they teach op amp theory the use a ± power supply. You can use a single power supply, but you should bias the op amp mid range (½Vcc).

11. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
The LM358 input common mode range includes ground, so he should not have problems amplifying 100mV DC, so long as the gain does not drive the output into saturation.
Virtual power supply grounds are good for AC amplifiers, but they complicate the heck out of DC amplifiers.