Hi, as in the title i have simulated the circuit below, and the gain should be equal to 10 right? Well i am powering the 741 op-amp with +2.25v and -2.25v, and the input signal is a sine wave of 400mv pk-pk. You can see a snapshot of my output graph as well in the screenshot below, i am using a program called Yenka for simulation.

This is really bugging me as its a simple circuit, trust me i have played for ages trying to get this to simulate properly, changing from voltage rails to batteries etc.

Thanks,
James

 

You have drawn the non-inverting opamp incorrectly. Take a closer look at your hookup.

hgmjr

 

The 1k in your circuit should be from the negative input terminal to ground.

hgmjr

 

It is worth noting that a 741 is specified only for +/- 15 volt rails. The op amp may not be able to swing the output to within 2 1/2 volts of either rail. A more modern micropower op amp would likely work a lot better.

 

It is worth noting that a 741 is specified only for +/- 15 volt rails. The op amp may not be able to swing the output to within 2 1/2 volts of either rail. A more modern micropower op amp would likely work a lot better.

I've tried the same circuit with +/-15 volt rails and the output doesn't change. And i don't understand how the circuit is wrong? are the resistors the wrong way round?

Thanks,
James

 

You have the hookup of the components incorrectly connected.

If you move the end of the 1k from the output of the opamp to the negative input you should get a much closer result.

hgmjr

 

You have the hookup of the components incorrectly connected.

If you move the end of the 1k from the output of the opamp to the negative input you should get a much closer result.

hgmjr

Ah yeah i can see it now, its such a stupid mistake but i just couldn't see it . Thank you.

 

And now the voltage rail values also have an effect, i get clipping when using +/-2.25v rails, even though the gain only amplifies the output upto +/-2v. I found that actually any rail voltages below +/-4v give me clipping with an input of 400mv pk-pk. And at 200mv pk-pk in, any rails below 3v give me clipping.

 

If you think about it. The reason that you got a gain of +1 before was that you had the opamp connected at a voltage follower. The 1K resistor simply acted as a load resistor having little or no effect on the gain of the opamp.

hgmjr

 

I need you to draw your circuit hookup and post it here.

hgmjr

 

Yeah i understand it just ensured the output was the same as the input. Usually i would be able to draw the circuit my own way, but as my simulation program does not let me switch the order of the input terminals, i got a little confused and ended up connecting the feedback wrong. I'm just hoping my labs have some op-amps available which can function fully with rails of +/-2.25v as my specification says to use 3 AA batteries.

 

Here you go:

Input is 400mv pk-pk, as you can see rails are +/-2.25v and clipping occurs on the output. Gain is 10, thus theoretical output is 4v pk-pk.

 

The circuit acts as though the opamp is open loop.

What happens if you change the value of the 9K resistor to 1K. This equates to a gain of +2.

hgmjr

 

I still get clipping:

 

Something must be wrong with the power rails then.

What happens if you increase them?

hgmjr

 

If i increase them to +/-2.5v or above for an input of 400mv pk-pk there is no clipping at a gain of +2. For a gain of +10 this threshold is +/-4v.

 

I simulated the same circuit with voltage rails as opposed to the batteries and i get the same situation. One thing i noted earlier is that for a gain of +10, an input of 200mv pk-pk, i require only +/-3v or above to get no clipping.

 

It is just possible that the opamp model is set to mimic the behavior of the 741 for input supplies less than 5Volts.

hgmjr

 

Sorry i don't understand. I am using a 741 op-amp in the simulation. Also when the input is 200mv pk-pk then i require voltage rails of at least +/-2.2v for a gain of +2. So it appears the minimum required voltage rails to get no clipping, reduces as the input signal reduces in magnitude.

 

Just wanted to say i started reading about Millman's theorem after seeing your profile and it does look alot quicker than the alternatives from the little that i've read, i'm definitely going to read up more about the theroem.