In Reply to: 741summer gain inconsistent posted by Jim Henderson on 06/05/02 at 1:20 AM:
I got your circuit okay via your e-mail. If I may describe it, there are two 10K resistors serving as inputs to the summing junction and a single 10K feedback resistor (inverting mode).
One input has 7.5 volts DC and the other has 4.5 volts AC which, if that number is in RMS, means excursions to approximatley ± 6.3 volts peak.
This means that the output wants to be -7.5 ± 6.3 volts which comes to a swing of -11.8 to -1.2 volts. With the 741 powered from +12 V and -12V, this will not work. The 741 cannot drive to -11.8 volts because there isn't enough "headroom" to the negative rail. The output pin will clip on the negative-most excursions and what will happen in consequence of that is problematical given that there are many kinds of 741-devices out there.
One possibility is that you may want to reselect your rail voltages assymmetrically, say +4.5 and -19.5 volts. Sounds really awkward, but it would center the rails equally about your -7.5 volt q-point.