inverting a negative voltage with an amplifier

Thread Starter

jsnx

Joined Mar 17, 2022
21
Hello,

I'm having trouble thinking of a way to convert a negative voltage to a positive one with an inverting OPA. I have an input voltage range of [-5 to -1]V and I need to convert it into a positive voltage to compare it with a reference voltage ( regulation for my charge pump) and I only have a positive supply voltage of 5V.

I studied the one stage OTA shown below and the minimum input voltage is \[ V_{icmin} = V_{gs1} + \frac{V_{g5}-V_{th5}}{n_{5}} \] So I can't have a negative input in my OTA.

I was wondering if there is a way to be able to use the OTA for a negative input voltage or if there is another way to convert it into a positive voltage where I won't be needing a regulation.

Thank you
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crutschow

Joined Mar 14, 2008
29,810
A rail-rail type op amp will do that for you.
You just configure it as an inverter with two resistors.
What gain do you want?
 

Thread Starter

jsnx

Joined Mar 17, 2022
21
A rail-rail type op amp will do that for you.
You just configure it as an inverter with two resistors.
What gain do you want?
I want a gain of one. But what is the transistor level circuit of a rail-rail op amp? is it different from a classic one stage OTA?
 

crutschow

Joined Mar 14, 2008
29,810
I want a gain of one. But what is the transistor level circuit of a rail-rail op amp? is it different from a classic one stage OTA?
Yes, it's different.
You can look at the schematic in the spec sheet of a rail-rail op amp to see how they do it.

Are you trying to build you own op amp?
 

Thread Starter

jsnx

Joined Mar 17, 2022
21
Yes, it's different.
You can look at the schematic in the spec sheet of a rail-rail op amp to see how they do it.

Are you trying to build you own op amp?
I'm doing a design of a circuit where I'll be needing to invert a negative voltage and everything has to be in one chip.
 
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