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  1. I know that the first 741 essentially adds m(t) and c(t).The gain should be 1 so why do we have 3.9kohms resistance ? What use is it ?
  2. The second 741 circuit is a band pass filter . I think there is a high pass filter first (R7 and C1 ) and then a low pass filter (C2 and R9) and an amplifier at last. What use are R6 and R8 ? How do they contribute to gain ?
  • \$\begingroup\$ If I were to tell you that e.g. which resistors reduce the gain (compared to a short) and which increase it, would that be the answer you want? \$\endgroup\$ – Dmitry Grigoryev Mar 28 at 7:52
  • \$\begingroup\$ Get a free sim tool and simulate it and find out. \$\endgroup\$ – Andy aka Mar 28 at 8:36
  • \$\begingroup\$ Reconsider your first statement: "The first 741 essentially adds m(t) and c(t)." Does it add? Look how D1 and D2 are connected in the feedback loop. \$\endgroup\$ – JRE Mar 28 at 8:41
  • \$\begingroup\$ Modulation != addition. \$\endgroup\$ – JRE Mar 28 at 8:41
  • \$\begingroup\$ @JRE Thank You ! I read more about my problem and figured out many things like bandwidth and center frequency of the band pass filter. The output of switching modulator is 2m(t)c(t) and I learned about it too. Should I answer my question or edit the question and update it ? \$\endgroup\$ – govind Mar 28 at 19:32

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