# sine wave bandpass filter with 741

I'm generating a sine wave to apply to a transistor in the push-pull configuration.

I am using a symmetrical source with 741 operational amplifier, + the IC 555 with plym 60 Hz 50% duty clycle, + another 741 with a filter passes the band at its input.

the problem is that when I measure in real life the circuit with the negative input of the oscilloscope on the ground plus the other on pin 6 of the 741 filter, I get a square wave.

Why do I get this square wave?.

Where am I going wrong?

formulas for the bandpass filter are these.

R1 = Q / (2 * 3.14 * F * G * C)
R2 = Q [(2 * Q 2 -G) * 2 * 3.14 * F * C)
R3 = R4 = 2 * Q / (2 * 3,14 * F * C)
Q = quality factor.
c = capacitance in farady.
f = frequency. • There are several problems with your circuit. (1) Ground confusion. Where are the 555 and second op-amp's grounds connected to on the OP1 circuit? (2) There are no junction dot-connectors on the second op-amp circuit so it is impossible to tell what lines are connected. (3) It appears that you have positive feedback on the second op-amp. This will act as a Schmitt trigger and probably explains your output. Draw a proper schematic. – Transistor Mar 5 at 22:45
• You can add one in using the CircuitLab button on the editor toolbar. Double-click a component to edit its properties. 'R' = rotate, 'H' = horizontal flip. 'V' = vertical flip. Note that when you use the CircuitLab button on the editor toolbar an editable schematic is saved in your post. That makes it easy for us to copy and edit in our answers. You don't need a CircuitLab account, no screengrabs, no image uploads, no background grid. – Transistor Mar 5 at 22:45
• hello transistor, I re-cycled the circuit without the symmetrical source and tested it. I still get the square wave at 60 Hz equal to 555 :( new image in description – gabriel Mar 6 at 2:18
• You have the + and - terminals of your op-amp in the wrong order. R2 should go from the + input to the floating ground at 1/2Vcc. Do you have a design to work from? You seem to be guessing what your connections should be. – Sparky256 Mar 6 at 2:23
• sparck256, I found this schematic of bandpass filter on a portuguese site. link here: newtoncbraga.com.br/index.php/eletronica/52-artigos-diversos/… – gabriel Mar 6 at 2:59

## 1 Answer

Why do I get this square wave?. The result of the above circuit is likely to be an oscillator and not a filter. The output WILL be a square wave.

I'm not ruling out other problems too but this is the obvious showstopper.

• correct andy, the value of R4 would have to be 27k but the value went wrong in print. but given some solutions here in the forum I came to the conclusion that the problem is now in the oscilloscope itself :( I'll have to buy another device. then when I get to testo and put the result here – gabriel Mar 6 at 21:57