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I'm trying to implement a poor man's amplitude modulation by multiplying analog signal with PWM output. Test circuit

The simulation result: The simulation result

Why I get negative voltage? How it can be?

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  • \$\begingroup\$ Couple of things. Vout is pretty much floating unless Q1 or D1 is conducting. You want pull-up on it. Moreover, you must have a resistor in series with the base of Q1. \$\endgroup\$ – winny Jul 6 '16 at 10:40
  • \$\begingroup\$ What are the voltage ranges of V2 and V1? What is Vout supposed to be? \$\endgroup\$ – Olin Lathrop Jul 6 '16 at 10:49
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There are several things going on here:

  1. The trace looks like you have that scope channel set to AC coupling.

  2. There is nothing pulling Vout up when V1 is high.

  3. It's hard to get into more detail without knowing the voltage ranges of V1 and V2, but Vout will follow V1 plus the B-E drop when V1 goes low. So if V1 goes negative by more than 700 mV or so, Vout is expected to go low.

  4. Overall you probably want to replace D1 with a resistor and have a transistor clamp Vout to ground or not. This means replace Q1 with a NPN and add a base resistor.

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  • \$\begingroup\$ Thank you! I think I have found even more simple solution. And it works pretty well on simulation. link \$\endgroup\$ – e_asphyx Jul 6 '16 at 11:14
  • \$\begingroup\$ It works well because in my overall design both inputs are connected to low-impedance buffers so I don't need additional buffer cascade at all. \$\endgroup\$ – e_asphyx Jul 6 '16 at 11:21

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