single supply op-amp tends to like -V

[wip]

Hi everyone,

I am designing a very simple preamp based on this circuit:
**broken link removed**

It's a single supply op-amp (using opa2134). It's working great, BUT using my oscilloscope I can see the waveform going towards the -V more than +V. A picture will help:

**broken link removed**

Yes I am saturating the op-amp (this is what I want to see the V peak-to-peak). Is there any reason for it to not be +1150mV / -1150mV?
Thanks!

 

[MikeMl]

This very topic is being discussed on the "LM358 booster" thread running in parallel with this one.

Your problem is not uncommon. The LM358's output can pull down to within mV of its Vss pin, but can only pull up to Vcc-1.6V

If it bothers you, buy a modern rail-to-rail output opamp. There are hundreds of them...

 

[crutschow]

If you look at the opa2134 data sheet (gasp!) it will tell you the maximum output voltage swing.

 

[wip]

Thanks for the reply, reading the datasheet I see:
"... In addition, the OPA134’s wide output swing, to within 1V of the rails..."

I'm single supplying the OPA2134 with +5V, so swinging at +2.5V with a 3V headroom = +1.5V / -1.5V. Why it's working for the -V (clipping at -1.5V) but not for the +V side (clipping at 800mV)?

 

[crutschow]

I don't understand "clipping at -1.5V". With a single 5V supply the voltage can't go below 0V. Are you AC coupling the output signal (scope set to AC input)? It should be DC coupled.

 

[wip]

Yes, no choice since I'm using a single supply I need to remove the DC component / offset from the output using an electrolytic capacitor (10uf). The photo above is showing my scope in DC mode. Should I try to set it to AC?

I'm currently checking this video that will help me understanding the setting I should set on my scope:

EDIT:
That was it! It's not perfect, but setting the coupling to AC on my scope and keeping the swing in the available range = perfect symmetry (+V / -V). When pushing the saturation, I still see more -V than +V (not as bad as before).

crutschow / MikeMI thanks for the help!
cheers

 

[audioguru]

The MINIMUM single supply for the OPA2134 is 5V. All its specs are with plus and minus 15V (30V total). The inputs are limited to +2.50V (not higher and not lower) and the output barely swings from +1.2V to +3.5V when the load is 2k ohms. The output swing is a little more when the load is 10k ohms. What is your load resistance to ground?

The 'scope input should be set to DC coupling and connected to the output pin of the opamp to see the clipping voltages of the opamp.

 

[crutschow]

You do have a choice. Why do you want to remove the DC component if you want to see the clipping levels? AC coupling does not give accurate clipping levels since apparent clipping points now depend on the waveform duty-cycle. You want to DC couple directly to the oscilloscope, set on DC as AG stated.

 

[wip]

Ok, I think I understand: I need to set my scope to DC and probe at the output of the op-amp and not at the output of the AC coupling capacitor. This way I will see what is happening before the AC coupling.

The output swing is a little more when the load is 10k ohms. What is your load resistance to ground?

I am connecting this preamp to a line-in level - 16k ohms.

 

[audioguru]

Your load of 16k ohms is a fairly high resistance and will not have much affect on the output swing of the opamp.