Please look at the LTspice screen shot linked below.
The top amp is outputting 0.3V to 3V which seems acceptable.
However,the bottom amp will not swing any closer than about 1.2V from ground, being the "positive" rail in this instance.
What changes do I need to make to the circuit to shift this closer to 0V so I am not wasting current?
For simulation purposes, the amps are shown driving 150R resistive loads. In practice, I would probably use LM358's.
Steve Morris
Use opamps with rail-to-rail outputs.
As PimPom noted, you may need a R-R output opamp but also, the bottom opamp may be driving too much current. Moving the output closer to ground will be _increasing_ the current in the output. It may already be current limiting. 4.5-1.2/150 = 22mA
Why is the bottom load resistor connected to the negative rail, rather than ground?
I started to view your link but terminated the load since it looked obvious it was more than just an image, please provide images. I for one and I think some others may agree with me, do not like linking to items] that requires a run time load of of Java or any embedded processing to view something.
Getting back to your iusse, it's obvious you are not using a Rail TO Rail op amp which would soave this issue but may introduce others too! If this circuit is going to have its own isolated power source then you could offset that output via a DIODE on the common side, the side for each is the output common connection. This will offset the output towards the (-) side.
Or, you could simply create a virtual ground circuit and operate the Ops with a dual supply. again, this only works if your supply is isolated, for example a Battery or a wallwort.. etc..
Jamie
Oops ... that was the problem. Both outputs are now symmetrical at 0.3V off OV. Thanks.
Steve Morris
Not sure what you are referring to. The file is most definitely just a .png image (42.5KB).
Steve Morris
To avoid mistakes like that, instead of connecting up the voltage sources as you've done, just plop two voltage sources on the sheet somewhere and connect both negative terminals of the voltage sources to ground.
Label the positive terminal of one (using "label net") as Vcc or V+, and set it to output a positive voltage. Label the other Vee or V-, and have it output a negative voltage by entering e.g. -4.5 in the box. Then only use the nets to connect the supply pins of the op amps, rather than running wires directly, and there's no room for confusion.
.png image (42.5KB). "
Well it's not showing up here. All I get is a bank page. Same with IE.
Showed up fine for me - Steven had clearly grabbed an LTSpice display.
-- Bill Sloman, Sydney
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