1
\$\begingroup\$

the complete circuitI designed a Sallen-key band pass filter where the gain at center frequency is 45.6dB. After feeding it with a sine wave at the center frequency, it amplifies the signal by 45.6dB. However, after coupling the output of a Gilbert cell (at 25mv max and consisting of two multiplied signals with the carrier at the center frequency), the band pass filter only amplifies it 1.6 times.

I used an op-amp to subtract the two voltages from the Gilbert cell as well as to isolate the currents. The input measured at the input of the filter is 25mv so why is the output only 40mv?
just a snippet of the bpf circuit (without the gilbert) and how the source is connected vi at the filter showing 50mVpp wave and the output at 80mVpp the ac analysis of the band pass filter showing a center frequency gain of 45.6dB

\$\endgroup\$
  • \$\begingroup\$ Where is the Gilber cell in your schematic? \$\endgroup\$ – The Photon Dec 1 '19 at 16:51
  • \$\begingroup\$ @ThePhoton I added the whole circuit and the waves \$\endgroup\$ – theroguecode Dec 1 '19 at 18:53
  • \$\begingroup\$ I don't see any of the obvious loading problems I expected to. You could check the operating point more carefully to see if all th active devices are still biased correctly after connecting the filter to the Gilbert cell. \$\endgroup\$ – The Photon Dec 1 '19 at 19:06
  • \$\begingroup\$ I don't even want to look at this mess, for now. First off, why aren't you using an MC1496 instead of all those BJTs? What's the modulation carrier frequency? (No, I don't want to look at the pictures.) What's the signal you want to isolate? Where's the detector (product, diode, or otherwise?) \$\endgroup\$ – jonk Dec 1 '19 at 19:41
  • \$\begingroup\$ NO guarantee: Increase R7 8 9 10 to 100k each. What happens? \$\endgroup\$ – Russell McMahon Dec 2 '19 at 0:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.