# Why no gain like this?

• posted

I cannot get the expected gain from my LM358. Input signal is a 1 volt

100Hz wave fed via a 3K3 resistor to the non-inverting input. Feedback is from output to inverting input via 1.270M and then 750K from inverting input to earth. Power supply is 14VDC.

Can someone please give it quick thought and tell me what doesn't look right? I keep trying with no success.

Also, the gain increases dramatically when I put a 104 cap across the

750K, but oscillation appears.

Pardon the simple question, I am just at student level, but there is no one on duty this time of year to ask.

Eric Marlow

• posted

What's the DC bias at IN+?

```--
Regards, Joerg

http://www.analogconsultants.com/```
• posted

So you're expecting a gain of 2.7 or so?

Your resistances may be too high -- I wouldn't even use 127k and 75k on that part without at least checking it against the expected bias currents (and input impedance, if I could figure it out from the data sheet).

12k7 and 7k5 is a no-brainer, and matches your 3k3 resistor a lot better from a bias-current standpoint.

That's not surprising -- capacitance to ground on the inverting input of an op-amp generally makes Bad Things happen. It introduces a phase lag in the feedback loop, and usually makes the thing unstable.

You may see students criticized on this list. It usually happens when someone wants homework or exam questions spoon-fed to them, without them having to learn, without them having to pay whatever the going rate is for help with cheating, and _with_ helping them to cheat.

_This_ kind of student question is welcome by all but the cranks.

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Tim Wescott
Wescott Design Services```
• posted

Bias current is around 50nA one those, typically. So 1M values aren't cool but should have given him roughly the expected gain.

Especially in a dorm. "Dude, my stereo conks out when you turn that on!"

Yes, it sure is welcome. Good to know that at least some students experiment with real hardware.

Eric, not sure where you bought that LM358. When I was a student I sometimes cut corners and bought a bagful at a "bargain price". Ok, it was the LM741 in my days. Man, that has cost me hours and hours until I figured out they must have fallen off of the dumpster truck somewhere. Things like outputs only pulling up but not down, huge bias currents, and so on.

```--
Regards, Joerg

http://www.analogconsultants.com/```
• posted

It sounds like you use a single ended supply, in which case earth(-) is not the proper path for the 750k resistor. Try to get -12v as well, having a proper earth reference. Else, make a virtual ground, half the 12volt, with two 10K resistors and a 100uf cap. Then feed the -input an adjustment voltage from a 10k potmeter on 0 and 12v, with about 300k between the wiper and the minus opamp pin. then adjust until the right output voltage appears. This assumes the op amp is a proper rail-to-rail type.

• posted

Thanks for mentioning that, Joerg -- that's something that I often screw up on, even if I should know better. I've wasted days looking for design or assembly errors in boards when it was a @#\$% bad part. This generally happens on board bring-up when you _expect_ that any problem has to be either design or assembly, but it's happened to me often enough that I shouldn't miss it (but I do, and will again).

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Tim Wescott
Wescott Design Services```
• posted

If the circuit is using a single 14V supply, is the 100Hz 1V wave biased up above ground by a few volts DC ? That would make it work better and not clip at 0 volts.

Is the output of the op-amp being loaded down too much maybe ?

boB

• posted

1) check the DC bias point

2) check the resistor values with an Ohmmeter... you may be mis- reading one of the codes.

3) check the signal level at the + and - input of the opamp, they should be the same and about equal to your input signal...

My bet is on #2.. seen it happen all too often.

Mark

• posted

oh, and you ARE using a scope to look at the signals, not a meter....right?

Mark

• posted

Too many unknowns about the circuit you havent shown.

NT

• posted

What's the other amp doing? Floating inputs and saturated output can cause problems. Art

• posted

I remember those. They even claimed to be "tested" - but didn't say if they passed or failed the "test". I ended up building a simple test circuit (part dependent) to sort out the obvious bad ones.

Eventually I realized that paying for a couple of good parts from a reliable source was cheaper in the long run than a bag of "bargain" parts.

• posted

As a student I often didn't have the budget. Touching the beer kitty for that was absolutamente off limits. So I resorted to scrapping parts out of discarded mainframe boards and such. Which meant a painstaking process of finding datasheets for somewhat obscure parts (no Internet back then) and then redesigning circuits to accommodate that part. Which also hones the skills when you realize "I need another linear amp but all I have left is logic chips", and then make do with those.

I hope we haven't scared Eric away now ;-)

```--
Regards, Joerg

http://www.analogconsultants.com/```
• posted

No, but I am starting a new thread, "does electronic design really need to be so complicated?

This is a serious proposition. I am really interested in the experts' take on this.

The op amp is now working. Thanks.

Eric

• posted

It isn't. Just some simple rules:

(1) Don't panic

(2) Analyze (mathematically... it's usually just simple minded Algebra or less)

(3) If you don't understand why a circuit works, or not, post the SCHEMATIC. That will make it easier to for us to help.

Great! Do you understand WHY it now works? If not, go back and analyze. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
• posted

Nah, just takes practice. In the beginning everything looks complicated. Remember when you learned the bicycle? I do, and I vividly remember the road rash I promptly ended up with and mom had to pick gravel pieces out of the wound with tweezers. Then I knew that training wheels are not a failsafe thing. Ouch, ouch ...

What was the problem?

```--
Regards, Joerg

http://www.analogconsultants.com/```
• posted

well what did you do to fix it?

share so we all learn.,..

Mark

• posted

No, but that's what we tell the boss.

As others have said, the sorts of problems you had take practice. It's usually something that has been ignored.

Good deal.

• posted

Well, yes, it does. It's just that dealing with the complexity gets automatic as you learn it. What you're learning in school is the foundation, so that when one of us starts babbling about bias currents or node impedances you can say "ah ha!" and understand what it means. Then you learn what to check, or what component values 'make sense', and you go on to the next layer of complexity.

Do you remember how overwhelming it was when you first learned how to drive? You had to keep the car pointed down the middle of the lane, you had to read all the signs, you had to look for cars out of place all around you, you had to deal with your parent or instructor spasming and shouting in your right ear, etc. But now (assuming you learned) it's automatic -- right? Circuit design is the same thing, only more mental.

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Tim Wescott
Wescott Design Services```
• posted

uL900 series? (RTL parts that could be creatively used as linear parts.)

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