# scaling a voltage down using op amp

I have already looked and found similar things but still couldn't solve my own problem. I have an input voltage of 9V to 11V and I want to scale it to 0V to 5V. It's better if it is linear but it's not an obligation. I have tried using an inverter setup with a potential divider bridge at the inverter input but I can never go down to 0, it is always minimum 1. I am allowed to use LM741 or equivalent.

• What do you mean you're allowed to use a LM741 or equivalent? A LM358 might work, but you'll have to add a pull down resistor on the output to get it down to zero. Mar 20 '15 at 0:13
• I assumed by "allowed to use LM741 or equivalent..." is because this is homework and the OP is constrained to use that part, so any answers proposing other devices are not going to be acceptable. Mar 20 '15 at 7:06

A 741 cannot do what you ask. Find a 741 data sheet, and look for "output voltage swing". You will see that, for instance, at a power supply of +/- 15 volts it will allow an output of ~+/- 12. In other words, the lowest output voltage specified is never anywhere near the - supply. In your case, the - supply is ground, so you cannot expect the 741 to give you a zero volt output.

Just as bad, please note that the 741 is specified (typically) for a voltage supply of +/- 15 volts. It really wasn't intended for any other situation.

The simplest solution to your problem is to get a "rail-to-rail output" op amp. Also check to be sure that it will handle a supply voltage of 12 volts (some will only go to 5 volts - or less). If that doesn't appeal to you, you'll need to provide a negative Vee.

With the op amp sorted out, your circuit is not a good design. Try

simulate this circuit – Schematic created using CircuitLab

EDIT: Please disregard the previous schematic. I had misread your requirement. Your circuit still won't work. For instance, if you provide a V- of about -16 to -20 volts, at 9 volts the circuit will produce -9.34 volts, and at 11 volts the output will be -12.61 volts.

Having said that you want the 9 to 11 volts to scale to 0 to 5 volts, you don't specify which value scales to which output value. For the requirement that 9 volts produces 0 volts, and with no negative supply,

simulate this circuit

does the job, while still using the 2 volt reference.

• +1 I was thinking of two opamps, subtract 9V, then multiply by 2.5. Mar 20 '15 at 1:46
• @GeorgeHerold - See edit. Mar 20 '15 at 5:12

The 741 op-amp will not go down to zero - it's just not designed to be able to cope with signals close to either power rail. Choose a rail to rail opamp or provide the 741 with a split supply i.e. positive and negative. Also, read and understand the data sheet and you'll never likely fall into this trap again.

Outputs are typically +/-12V on a +/-15 volt rail so don't expect the output to drop as low as 1V. Also: -

Input voltage range is typically +/-13V on a +/-15V supply - this means the inputs won't work properly if taken within 2 V of either rail. Given that you have 0V as the negative rail it will fail to work correctly if any inputs approach either rail within 2 V typically.