An amplifier that won't amplify!

  • Thread starter lucy_b14
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  • #1
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Hi

I've built what is meant to be a non-inverting dc amplifier using a 741 op-amp (shown in attached diagram). Resistor values are: R0A= 470k, R0B= 47k.

I am feeding the op-amp a supply voltage of 12v and an input voltage to the non-inverting input of 0.5v. According to the resistor values I would expect to get a gain of 11, giving an output of about 5.5v.

However, the output is about 11.3v, and will not change with the input voltage; it seems to be jumping to around the +ve rail and just staying there - not a very effective amplifier!

I've tried changing the op-amp, resistors, wires.... everything, to no avail. I'm really confused and can't figure out why I can't get such a simple circuit to work.

Any suggestions would be much appreciated.

Thanks!
 

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  • #2
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If I'm not mistaken, 741's are not rail-to-rail op amps. That means it will do funny things if the outputs get close to a rail. I think it will be a problem if the input is close to a rail too. I could be wrong about that last bit though. Try giving it a -12V supply rail on the negative side instead of ground. See if that helps.

Maybe someone else has a better answer. Op amps are not my strong suit.
 
  • #3
1,758
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Hi

I've built what is meant to be a non-inverting dc amplifier using a 741 op-amp (shown in attached diagram). Resistor values are: R0A= 470k, R0B= 47k.

I am feeding the op-amp a supply voltage of 12v and an input voltage to the non-inverting input of 0.5v. According to the resistor values I would expect to get a gain of 11, giving an output of about 5.5v.
Your gain is not 11 but 57. (560k +10K)/10k

However, the output is about 11.3v, and will not change with the input voltage; it seems to be jumping to around the +ve rail and just staying there - not a very effective amplifier!
Your output is as positive as it can go.

I've tried changing the op-amp, resistors, wires.... everything, to no avail. I'm really confused and can't figure out why I can't get such a simple circuit to work.

Any suggestions would be much appreciated.

Thanks!
What is the range of input voltages that the amplifier needs to amplify? What is the corresponding range of output voltages? The (output range)/(input range) should be your gain. The ratio of resistor values R0A:R0B is (Gain-1):1. To get a gain of 11 your resistors should have a ratio of 10:1.
 

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