I have the following circuit as part of a class D amplifier:


simulate this circuit – Schematic created using CircuitLab

I couldn't get the amplitude of Vsig to display, it is ~.447Vp, to simulate a line level input. The goal of this circuit is to increase the amplitude of Vsig to 2Vp to feed into a comparator for the class d amp.

The problem is, as configured, I get no output. I initially used an LM324, and tried two of the 4 op amps on the IC with the same results. I also tried an LM1458, same results. When connected to an oscilloscope, Vout is flat, there is no output signal, not even an unamplified signal.

I checked before and after C2, and still get the same result.I'm not sure how to further troubleshoot this circuit.

  • \$\begingroup\$ By the way - the way you've drawn your schematic, the negative supply rail is actually at +15 V! You have a negative voltage source pointing backwards, which is -(-15 V). \$\endgroup\$ – Greg d'Eon Apr 18 '15 at 2:19
  • \$\begingroup\$ @Gregd'Eon fixed! \$\endgroup\$ – MDMoore313 Apr 18 '15 at 3:00

Why are you putting +15V on the op amp + input? That will make the output attempt to center around +15V, but it can't output that much. Maybe you meant to ground the op amp + input.

  • \$\begingroup\$ This makes perfect sense. \$\endgroup\$ – MDMoore313 Apr 18 '15 at 2:14
  • \$\begingroup\$ And, if it comes to it, you don't need that 100 ohm resistor, either. \$\endgroup\$ – WhatRoughBeast Apr 18 '15 at 15:38
  • \$\begingroup\$ @WhatRoughBeast error in schematic, that's a much larger resistor, but thank you for the info though. \$\endgroup\$ – MDMoore313 Apr 20 '15 at 12:16
  • \$\begingroup\$ @BigHomie - You're still better off getting rid of it. You're gain is low and you're AC coupled at both input and output, so you don't need to worry about offset voltages, and the bias current for a TL081 is so small that you don't need to worry about those currents, either. \$\endgroup\$ – WhatRoughBeast Apr 20 '15 at 13:35

The non-inverting op amp input should be connected to ground, not +15V. Otherwise you saturate the output because V+ is always greater than V-.

  • \$\begingroup\$ I just had a serious Duh moment after I read this. \$\endgroup\$ – MDMoore313 Apr 18 '15 at 2:04

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