Cant simulate high frequency with µA-741

[marianna8]

Hi

Im trying to buid a very simple non-inverting circuit with a µA-741. I have to amplify 500 times a 30kHz signal. So i 'designed' a two stage system, the first block amplifies the signal 20 times and the second of it 20 (they will be in series) so, it will manage to work with the high frequency issue. I just dont seem to make it possible with any of the models available of the µA-741. Ive considered the bandwidth product in each stage. Here are some calculations ive done:

Acl=500

f=30kHz

Maximum gain per stage= 1MHz÷30kHz=33.33, choosing Acl(first stage)=24 and Acl(second stage)=25.

Acl(first stage)=25  .. (considering Vi=1mV)

k=(Acl)(Vi)=(24)(.001V)

fmax=SR÷[(2π)(k)]=31.83kHz

---> proving that the output voltage is able to follow he high frequency at the entry.

Can anyone tell me what's wrong with my simulation? (I just posted the first stage). Thanks!

[digitalguy]

Check your circuit configuration for a non-inverting amp. I think you will find it is incorrect.

[marianna8]

Thank you so much, it's embarassing that was the problem .. one last thing, it is supposed to not have frequency distortion at the output, any idea of what's wrong this time? Thanks in advance!

[marianna8]

Also, i forgot to mention, the Acl it's not well either.. :s at first stage its showing 17 and second 20 giving 340, which is far from the desired of 500. Thanks!

[digitalguy]

Hi,

The gain of the first circuit, based on your resistor values and assuming the correct circuit configuration for a non-inverting amp, should be (R4/R5) +1 or (19k/1k)+1=20. If you lower the frequency to 1kHz, this is the value you get. At 30kHz the gain value drops to around 17. This may be normal for a 741 op amp. I don't know, as the 741 is rather old. I am not very familiar with it and don't have time to look into right now. I will leave that as an exercise for the student (you) 🙂 Good luck!