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Good day Guys,

I have a question regarding output signals of a specific stage getting distorted when connecting the next stage. In my scenario, I am working on an AM modulator and demodulator of a 10kHz square wave with a carrier frequency of 850Khz, currently, my Modulation is working and producing a nice modulated signal. Before implementing my demodulator I run my modulated signal through a Gain stage ( Operational Amplifier ) to amplify the modulated because of its low peak values that are influencing the accuracy of the Demodulator. When I send the demodulated signal through the gain stage (without the gain stage output connected to the demodulator) I get a nice amplified signal. However, my problem comes in when I connect my demodulator to the output of the Amplifier stage, the output from the Amplifier stage changes to a very weird looking signal. The modulated, amplified (without demodulator stage connected) and amplified (with demodulator stage connected) can be seen in the figure below,Signals

So my question is why does this distortion happen, and how can it be fixed? I am terribly sorry if it is due to noobies mistake!

If have attached a screenshot of my Circuit below, LtSpice Circuit

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  • \$\begingroup\$ Are you using LTSpice??? If so, do you a model for your diodes??? Or try right click and the pick new diode a select a diode that fits your application. \$\endgroup\$ – Kvegaoro Feb 22 '17 at 21:07
  • \$\begingroup\$ I did this, it, however, didn't have any effect. \$\endgroup\$ – 0x436f72647265 Feb 22 '17 at 21:23
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The problem seems to be in how you are powering your LT1468. You have powered as a single ended source since V4 is no way referenced to ground. Use 2 Voltage sources reference to ground, one positive or one negative. Additionally you can change your amplifier setup to work as a single-end mode.

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  • \$\begingroup\$ I have accepted your answer because you answered first, however, @EnricBlanco your answer, that helped me. is the same as @Kvegaoro! Thank you very much, guys! I didn't realize that has such a massive influence, will remember this in future projects! \$\endgroup\$ – 0x436f72647265 Feb 22 '17 at 21:59
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You're biasing the OpAmp in a rather strange way. V+ should come from a +5V DC voltage source connected to ground. The same goes for V- and -5V to ground. Try this before anything else.

If after trying that it keeps not working work, try also the following in addition to it:

  1. Reduce the voltage gain of the stage before the demodulator. If required, split your total gain of 1,000 in 2 or 3 stages, with the last one having less gain than the others.
  2. Use +/-15V supply voltages.
  3. Use another OpAmp, maybe a more general-purpose one.
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  • \$\begingroup\$ Please see the comment on Kvegaoro's answer! \$\endgroup\$ – 0x436f72647265 Feb 22 '17 at 21:59

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