Inverting Op Amp

[bluesteel]

Hi All

ive had a look around but cant seem to find any answers for what im trying to do

i have a signal that basically goes from 5v to 0v and i need to invert it to 0v to 5v

i have used a simple inverting op amp circuit based on an lm741

the actual input voltages are 4.8v to 0.9v and i cant quite get the output to reflect that properly.
i have changed the input resistor to change the high output side, and that is now up to 4.5v and that is good.
i cannot however get the low output side to drop under 1v

attatched is a diagram with what i have
**broken link removed**

any help would be awesome

cheers

 

[bychon]

Your first problem is that a 741 amp can not function with the input at zero and the negative power supply at zero. Think of it as head room, or in this case, foot room. You need to supply the amp with a negative supply voltage so it can get to zero on its output. Either that or use a different amp chip. Second, the inverting configuration inverts polarity, not the range of voltages. To get a range inversion, you need to pump in a current to force the range to change. For this, we need to know the impedance of the supply voltage, so we don't mess up your incoming signal, and, "how accurate do you need this?" The current injector will need to know how accurate to be.

 

[MikeMl]

Search for rail-to-rail input, rail-to-rail output operational amplifier on Digikey, TI, National, or Fairchild's web sites. The 741 is very far from being a rail-to-rail amplifier.

 

[crutschow]

As Mike ML stated, you need a rail-to-rail op amp, not a 741, if you only have one supply voltage. And to invert 0V-5V to 5V-0V you need to bias the amplifier as bychon noted. The easiest way with only one power supply is to connect the non-inverting input to +2.5V. This can be done with a two equal resistors in a voltage divider from the +5V.

 

[MRCecil]

You say you can't find answers to what you are trying to do. So what are you really trying to do? How is the entire circuit actually configured?

Your query requires a bit more information to give a proper response. Is the signal digital (1010) or analog (sinusoidal)? If it is a digital signal going from 0V to 5V, I would suggest using a CMOS inverter with Vdd=5V OR an open collector inverter to shift the high level to 5V with a pull up resistor to 5V. If it is an analog signal, then as your schematic shows a Vcc of 12 volts, you need to employ a rail-to-rail op amp as MikeMI has stated. The layout would be changed such that both resistors would be of equal value for a gain of 1 with the inversion. If you have a negative supply available also, then perhaps the 741 will work within certain parameters. But...?

Bottom line is not enough information has been supplied, without making improper assumptions, to lead you to a solution to your issue.