Why no gain like this?

[Eric Marlow]

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

 

[Joerg]

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.

What's the DC bias at IN+?

 

[Joerg]

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).

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

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.

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

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.

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.

 

[Sjouke Burry]

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

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.

 

[Artemus]

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

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

 

[Dennis]

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.

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.

 

[Joerg]

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.

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 ;-)

 

[Eric Marlow]

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

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

 

[Joerg]

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

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 ...

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

The op amp is now working. Thanks.

What was the problem?

 

[[email protected]]

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

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

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

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

The op amp is now working. Thanks.

Good deal.

 

[Dennis]

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.

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

 

[Jamie]

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

Vg = (750k+1.270)/750k = 2.693

So with your 1 Volt in you should be getting ~2.69 output..

If you are getting less, apply a DC of 1 Volt in and then if
you do get this gain correctly, you maybe running into a BandPass
issue with that large loop back R you're using..

There is a small amount of Cap in an Op-amp to help reduce the
chance of oscillation, something that you get with comparators.

Also, you may want to look for "Miller Effect"

http://ece.wpi.edu/~mcneill/529c/webdocs/Op-Amp_Design_Overview.pdf

Get down to ~page 30 and see the second on miller effect..

Jamie.

 

[Joerg]

Joerg wrote:
[...]

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.

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

I am not _that_ old

It was mostly CMOS stuff, CD-series. Nice thing is that the quiescent
current isn't too high when mid-biased, as long as you kept the supply
around 4-5V. Of course, there wasn't any bandwidth to write home about.