In my current project setup I generate audio signals from my MCU's internal DAC. These audio signals are transferred to a speaker box with an internal amplifier via phone jack. To make the whole project more portable, I thought about giving my \$8\Omega/0.25W\$ speaker a try. Since the DAC should not output too much current I decided to add a class A audio amplifier with voltage divider biasing to the DAC's output.

The result would look something like this:


simulate this circuit – Schematic created using CircuitLab

Now my problem is that the DAC's output is always positive (between \$0.2V\$ and \$3.1V\$). Because C3 is an AC coupling capacitor, it will feed current back into the DAC when the audio signal becomes negative. Will this damage the MCU? And at which voltage level should I bias Q1?

By the way, the MCU I'm using is the STM32H743ZI.

  • \$\begingroup\$ What does the data sheet tell you about the currents that are allowable to flow in and out of the DAC output? \$\endgroup\$ – Andy aka Jun 11 '18 at 17:42
  • \$\begingroup\$ Page 97 of the datasheet claims "output current sunk by any pin" as to be 20 mA. Under DAC characteristics there's no information about current besides +/-0mA allowable current injection. \$\endgroup\$ – Overblade Jun 11 '18 at 17:56
  • \$\begingroup\$ To drive an 8ohm I’d suggest an emitter follower configuration since it has a lower output impedance \$\endgroup\$ – PDuarte Jun 11 '18 at 18:56
  • \$\begingroup\$ If you'd use a MOSFET (possibly followed by an end stage) and a high impedance bias circuit, your \$C_3\$ can be much smaller, reducing the negative current considerably. \$\endgroup\$ – HarryH Jun 12 '18 at 13:11

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