I encountered a problem while solving an analog circuit. I just need to know the purpose of the last op-amp (K4) at this circuit:

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while solving I understand it just amplify the output voltage from the Instrumentation Amplifier, but there is known purpose for it beside to amplify Vout3.


  • \$\begingroup\$ Is this from EEG or EKG or EMG sensors? \$\endgroup\$ – analogsystemsrf Dec 10 '18 at 13:31
  • \$\begingroup\$ it just a problem from my homework. I know this kind of circuits very common so possibly it used in these sensors. \$\endgroup\$ – Michael Astahov Dec 11 '18 at 21:23

K4 produces amplification for AC signals whilst providing a DC amplification of 1 (so that DC levels are maintained from the InAmp output to whatever stage follows K4). It might also be used to share the burden of AC signal gain between InAmp and itself thus minimizing slew rate problems in the InAmp.

  • \$\begingroup\$ So - the last stage acts as a sort of highpass. \$\endgroup\$ – LvW Dec 10 '18 at 13:10
  • \$\begingroup\$ @LvW are you asking or saying! \$\endgroup\$ – Andy aka Dec 10 '18 at 13:12
  • \$\begingroup\$ @Andy aka...there is no question mark. Hence, it is not a question. To me, the transfer function [1+sR2C/(1+sR1C)] is a highpass with a fixed gain of unity at low frequencies. Why are you asking? \$\endgroup\$ – LvW Dec 10 '18 at 14:51
  • \$\begingroup\$ @LvW it's always best to be careful!! \$\endgroup\$ – Andy aka Dec 10 '18 at 16:08

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