# 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.
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. Mar 5, 2019 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. Mar 5, 2019 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 Mar 6, 2019 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.
– user105652
Mar 6, 2019 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/… Mar 6, 2019 at 2:59