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Subject: Re: 741summer gain inconsistent

Date: 06/05/02 at 11:22 PM
Posted by: John Dunn - Consultant
E-mail: ambertec@ieee.org
Message Posted:

In Reply to: 741summer gain inconsistent posted by Jim Henderson on 06/05/02 at 1:20 AM:

Hi, Jim.

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.

Good luck.

John Dunn
ambertec@ieee.org

John Dunn
ambertec@ieee.org


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