Alternate voltage comparator

[Grossel]

Hi. I know that is several aproach how to make an analog adder. Exept from the regular one (inverter with more than one input resistor) I found out another way to achieve the same:
**broken link removed**

It might working fine, but there is one issue I'll try to solve: Is there possible to make an non-inverting adder this way, without adding an extra inverter block? I hope that can be done in a similar way.

Reason I want to do it this way is to try avoiding need for negative voltage supply. A regular adder requires negative voltage supply because of the following inverter stage.

I tried to think of a way to overcome this and ended up with this:
**broken link removed**
But I have serious doubt that this will ever work. One reason is that input voltage have to be much higher - near power supply. And of course the output voltage can only be less than those two input voltages, unless i add an non-inverting stage.

My brain goes in saturation - Is there anybody that can think out a circuit that is somewhere between those.

Final goal is having an adder that consist of no more than two opamps (next stage have high impedance input = no need for voltage follower) but use small voltage through a pair of low-value resistor to control current so I get an output that is inputs added together.

 

[Roff]

Have you considered just summing the inputs by connecting two equal-valued resistors together, and applying the inputs to the unconnected ends? The junction of the two resistors is the sum, divided by two.
Why did you use the term "comparator" in your title?

 

[Grossel]

Have you considered just summing the inputs by connecting two equal-valued resistors together, and applying the inputs to the unconnected ends? The junction of the two resistors is the sum, divided by two.

No you're wrong. The output will be the some function almost like average value to the two inputs. If bot voltages is 1V for instance you get 1 volts out. Sum of two times one volts should become 2 volts.

Why did you use the term "comparator" in your title?

You're language is native english, I'm not. So you tell me why I won't use that word in title field.

 

[ericgibbs]

hi,

I think Ron is referring to this OPA method.

BTW: Comparator is to compare the level of two signals,

You should have named it 'Summing'

 

[Roff]

No you're wrong. The output will be the some function almost like average value to the two inputs. If bot voltages is 1V for instance you get 1 volts out. Sum of two times one volts should become 2 volts.

No, I'm not wrong. The output will be the sum, divided by two.
If you take the average of two values and multiply that number by two, you will get the sum.
Eric showed you how to multiply the average by two using an op amp configured as a noninverting, gain-of-two amplifier.

 

[Grossel]

No, I'm not wrong. The output will be the sum, divided by two.
If you take the average of two values and multiply that number by two, you will get the sum.
Eric showed you how to multiply the average by two using an op amp configured as a noninverting, gain-of-two amplifier.

Ok, I got it, thanks

Only now I have to consider the need for a buffer on each inputs. If neccesary, I actually think making a negative voltage supply would be the better solution after all.

 

[Roff]

Ok, I got it, thanks

Only now I have to consider the need for a buffer on each inputs. If neccesary, I actually think making a negative voltage supply would be the better solution after all.

How about putting a gain-of-2 buffer on each input, before the summing resistors? This will work if your supply voltage is at least two times the range of your input voltage (a little more if you don't use rail-to-rail output op amps).
You will need op amps whose outputs will go to the negative rail (ground).
What are the source resistances of your inputs?

 

[Grossel]

How about putting a gain-of-2 buffer on each input, before the summing resistors? This will work if your supply voltage is at least two times the range of your input voltage (a little more if you don't use rail-to-rail output op amps).
You will need op amps whose outputs will go to the negative rail (ground).
What are the source resistances of your inputs?

The output from the adder will go straight into a comparator's negative input, but I haven't chosen any specific part yet. Maybe I just make it out of a handfull bjt's (have a lot of them laying around).

One input is part of a feedback voltage that comes from a simple voltage divider, so yeah, it needs an buffer or high input resistance.
The other input comes from a triangle oscillator, and at this time I don't know the need for a buffer as design isn't decided yet.