# bias a signal with DC input coupling, but without attenuation?

I want to bias the analog signal present on an MCU output pin. This signal varies between 0v and ≃3V, I need to shift it down to -/+1.5V range. Most importantly, I need to keep any DC offset potentially present in the MCU output signal, and shift it down with the AC component.

I've been googling DC biasing circuits for a few days and found several circuits to do that; they generally fall into one of two groups:

In the first case, naively removing the cap just shorts the input to the output i.e. no biasing is applied. I ran SPICE simulations which (I think) confirmed what I'm saying, although I never used SPICE before so I may well have got some or all of it wrong.

Are there circuits that can bias a signal negatively without attenuation, but with DC input coupling?

Note that I'm a Software Engineer by trade and my electronics knowledge is very basic. Please forgive me for any improper or inaccurate terminology (I'm happy to be kindly corrected), and please provide answers in terms that a novice like myself can understand.

• You would use an opamp circuit. Try a non-inverting amp configuration but connect bias the inverting input at something other than ground. Jun 3, 2020 at 20:39
• What is wanted is an analog level-shift circuit that works from DC to some frequency. I don't know of any way to do that with passive circuitry. I think an op-amp will be required, but the op-amp will need a negative voltage rail (-1.5V or lower, depending on the op-amp). Jun 3, 2020 at 21:45
• @mkeith Is there any way to shift a signal 1:1 without first attenuating it? The only way I know how offset it with a non-inverting amp which requires you to arbitrarily divide it down to compensate for the fact the non-inverting amp must amplify. Jun 3, 2020 at 23:19
• @DKNguyen not that I know of. Jun 4, 2020 at 2:16
• thanks @mkeith, adding an opamp enabled me to level-shift & amplify my signal as required. Jun 22, 2020 at 12:46