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In the following circuit, in order to obtain negative feedback, shouldn't we need A2 to be positive? Also, is the correct feedback topology current-current? enter image description here

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  • \$\begingroup\$ "shouldn't we need A2 to be positive?" --- According to the specs given, \$G_m\$ for \$A_2\$ is positive. I'm not sure what the \$-/+\$ on the \$A_2\$ symbol means but, regardless, the transconductance (ratio of output current to input voltage) is positive so, I assume, if the input voltage increases, the output current increases. Is that what you mean by "need A2 to be positive"? \$\endgroup\$
    – Hal Hollis
    Commented Jun 21, 2017 at 0:22
  • \$\begingroup\$ @HalHollis What I meant was rather that since the overall loop gain had to be positive for a negative feedback (unless I'm mistaken), I think the sign of A2 should be positive too. I was asked to determine its sign so that negative feedback is enabled. However, online I see that it's actually a two-stage op amp with gain unit and buffer at the end and that the sign of the gain of the middle stage (normally a CS) is usually NEGATIVE! What do you think? \$\endgroup\$
    – peripatein
    Commented Jun 21, 2017 at 6:33
  • \$\begingroup\$ Anyone? Should I close this thread or is there still a chance to get some help? \$\endgroup\$
    – peripatein
    Commented Jun 21, 2017 at 15:26
  • \$\begingroup\$ For this example, A2 should be positive. \$\endgroup\$
    – G36
    Commented Jun 21, 2017 at 17:31
  • \$\begingroup\$ @G36 According to what rationale? Why should it be positive, as I supposed myself? \$\endgroup\$
    – peripatein
    Commented Jun 21, 2017 at 17:36

1 Answer 1

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Have a look at this example: enter image description here

The negative feedback tends to the previous state (to equilibrium).

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  • \$\begingroup\$ yet you noted A2 as negative, instead of positive as you asserted earlier it ought to be. \$\endgroup\$
    – peripatein
    Commented Jun 21, 2017 at 18:19
  • \$\begingroup\$ Because I change A1 inputs \$\endgroup\$
    – G36
    Commented Jun 21, 2017 at 18:27

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