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Topic: LM358 saturates the output (Read 2713 times) previous topic - next topic

TomGeorge

Hi,
Did you look at the link I posted in #8?

Look at the magnitude of the resistors being used.
To obtain 1/2 Vcc  use two 10K, not 100K ?

The amp has some level of input impedance that influences the external input components.
The series input resistors make 4K7 or 10K, not 50K.

Thanks..  Tom... :)
PS. I have used LM358 and not had your problems.
Everything runs on smoke, let the smoke out, it stops running....

Wawa

Output clipping points of the LM358 are not ground and VCC, but (almost) ground and VCC-1.5volt.
An LM358 should NOT be biased at 1/2VCC because of that.
To get max output swing, replace the two bias resistors with a trimpot.
Adjust untill a ~4volt output signal clips symetrically.
Leo..

Yoram

DC offset on the output should be about 2volt with that 100k:150k divider at the +input of the opamp.

Leo..
The real problem is that the output DC level is not at the same level as the +Vin...
The +Vin it se with the 100K and 150K divider  at Vcc*100/150=0.4Vcc, but the Vout DC is not consistent.

So when I test and change VCC I get the following readings:

VCC     +Vin      VoDC
5          2          2
6          2.4       2.85
9          3.6       5.65
12        4.8       8.00

We can see that as I use higher VCC the Vout DC tends to climb toward the upper rail. It starts when Vcc=5v at 2V which is 0.4Vcc and ends at when Vcc=12V at 8V which is 0.75VCC...

Any logic explanation for this?

Yoram

@TomGeorge
I will take a second look at the +vin input, but as far as I remember they were consisted with the expected voltage divider.

@Wawa
To get max output swing, replace the two bias resistors with a trimpot. Adjust untill a ~4volt output signal clips symetrically.
Leo..
This is a good practical procedure, but this tuning is exactly what I want to avoid. Couldn't it be designed such that the output Dc level follows the design? see the measurements I posted above..

Wawa

Can be explained if there is someting wrong with the input caps on the pots.
If there is a DC short/leak to ground there, then the opamp tries to compensate by increasing it's output.
Did you connect the 10uF caps with negative to pot and positive to 50k resistor.
Post a picture of the setup.
Leo..

TomGeorge

#20
Jul 01, 2017, 03:02 pm Last Edit: Jul 01, 2017, 03:06 pm by TomGeorge
Hi,
Can you change the values I have highlighted, and recalculate the 75K to give the gain you need with 4K7 or 10K input resistors.

Place a 1uF cap from the junction of the two 10K providing the 1/2 Vcc, to gnd.
10uF is too high for the high values of resistors you are using.

Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

Yoram

Can be explained if there is someting wrong with the input caps on the pots.
If there is a DC short/leak to ground there, then the opamp tries to compensate by increasing it's output.
Did you connect the 10uF caps with negative to pot and positive to 50k resistor.
Post a picture of the setup.
Leo..
Hi Leo,

You were right the input capacitors were assembled the wrong way. Now the Output DC voltage follows the one of +Vi :-)  Thanks...

I changed the voltage divider to 2x100k and the output signal is centered at Vcc/2. and I can get a max peak to peak voltage  that depends on VCC as follows:

VCC        MaxVPP
5             3
16           6

So while at 5V the output can swing up to 1.5v close to each rail, at 16V it can only swing up to 5V close to each rail. Any idea why there is a bigger gap as I use higher Vccs, and why most of the voltage is not translated into higher output amplitude.

Yoram

Hi,
Can you change the values I have highlighted, and recalculate the 75K to give the gain you need with 4K7 or 10K input resistors.

Place a 1uF cap from the junction of the two 10K providing the 1/2 Vcc, to gnd.
10uF is too high for the high values of resistors you are using.

Tom... :)
Hi Tom
At this point I have a BW of 1-20KHz which is kind of important. Will this not change if I change the caps into 1UF?

Also If I change the 50K into 10K each will it not adversely affect the channel separation?

Thanks

MarkT

Quote
Also If I change the 50K into 10K each will it not adversely affect the channel separation?
No, the virtual ground at the opamp input takes care of that.

However as the pots are 50k you certain do not want to load them with 10k or 4k7, ideally they
would not be loaded at all.  With 10k you'd severely distort the linearity (or logarithmic response if
the pot is log law).  It would actually be better to use 100k for that reason, although
high impedances have issues with pickup and noise.  The existing 50k seems like a not unreasonable
compromise.

A better approach overall is to buffer pot wiper voltage with a unity gain buffer, then the summing
junction resistors can be a nice low (ie low noise) 1k or whatever.  For a low noise microphone preamp
you'd have to pay close attention to such issues, but that's not the case here of course.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Southpark

#24
Jul 01, 2017, 11:19 pm Last Edit: Jul 02, 2017, 05:57 am by Southpark
Good pickup by TG.

OP.... make sure to always include polarity symbols for polarised capacitors in circuit diagrams.

The Vac drawing (from the original diagram) isn't really AC is such..... but is a combination of dc and ac.

Also..... make sure to write the voltage levels in waveform diagrams for making things clear. DC level.... clipping level etc

TomGeorge

#25
Jul 02, 2017, 12:36 am Last Edit: Jul 02, 2017, 12:37 am by TomGeorge
Hi,
Example maximum channel separation.

From link I posted.
Have you measured the BW?

Tom..... :)
Everything runs on smoke, let the smoke out, it stops running....

Yoram

It would actually be better to use 100k for that reason, although
high impedances have issues with pickup and noise.  The existing 50k seems like a not unreasonable
compromise.

A better approach overall is to buffer pot wiper voltage with a unity gain buffer, then the summing
junction resistors can be a nice low (ie low noise) 1k or whatever.  For a low noise microphone preamp
you'd have to pay close attention to such issues, but that's not the case here of course.
Thanks for this recap.

In fact there are 3 inputs and one of those inputs is actually coming from a mic x10 preamp (transistor based). So I don't think it should not make a difference.

Yoram

Have you measured the BW?

Tom..... :)
Hi Tom,

Measured the BW ad it goes 1-25Khz

Yoram

Thanks all, with all the help and comments I have this circuit (see attached). It works fine but there is two points that I do not understand.

1.  At 5V VCC it has a nice clean 3vpp output (centered at VCC/2) before it gets into saturation. However if I raise VCC to 15V it can only output 5Vpp before it gets into saturation. Why the higher VCC increase the output voltage amplitude in a very small proportion?

2. Once it get into saturation the output signal, is not immediately clipped, but changes form sine wave into triangle ??


Anyone with an idea why this happens?

Thanks

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