bode plot vs. theory

[simply_me]

Hello,

I'm trying to design an ultrasonic range finder using only analog components. So far I've designed the Wein Bridge and one amplifier. The problem is that I'm trying to create a band-pass filter in the amp loop. While my calculations show that the filter's band is between 35.367-41.882kHz which is fine (I think ) since the incoming wave is ~38kHz, the bode plotter shows a different band:1-1MHz. Maybe one of my connections is wrong? Or the plotter is wrong? I'll be happy to hear any input you might have on the matter. 

Thank you,

A

[RLC]

S_M,

The Bode plotter shows that the middle of your passband is about  17.7 kHz. Your Bode plotter is so huge you can't see the cursor frequency at the bottom of it. See attached. Move the cursor to the center of the hump in your plot and read the frequency on the bottom of the Bode plotter. The bandwidth is generally defined by the points on either side of center that are 3 dB less tahn the magnitude at center. I think the 1-1Mhz you are referring to is the frequency range over which the Bode plotter is set to sweep the circuit. You can change that to anything you want.

[RLC]

S_M,

OK, now I see your problem. Neither the Bode plotter or AC Analysis will work correctly on anything other than a linear circuit. If I replace your

op amp with a virtual (ideal) op amp then the Bode plot works ok. The "real" op amp has non-linearities such as slew rate limit. Since the circuit you chose does not have a negative supply for the op amp I think it may have been getting into the negative limit as well. I'm attaching a modified version of your circuit with the ideal op amp. Try the Bode plot now and see if it doesn't work for you. I didn't try AC Analysis but expect it would work as well.