# DC Biasing a signal and what prevents the signal going to the power supply?

I have seen some schematics that deals with audio inputs, with this kind of circuit to DC bias a signal. The signal enters on the left side of C and gets out on the right side of C and goes to the amplifier.

But this is the problem. What prevents the signal going up R1 to the power supply and interfering on other parts of the circuit?

And if I have left and right channel I will have other pair of resistor for DC offset of the right channel and as R1 of both channels are connected to V+ and R2 of both channels are connected to ground, if a signal could go to the power supply it could reach the other channel.

• This is what bypass capacitors are for. Apr 14, 2018 at 2:17

You are correct in your thinking that there is potential for interference from this type of arrangement. There are a few ways to solve it.

Figure 1. R6 and C4 provide some de-coupling for the supply of Q1. Source.

The circuit above isn't a great example but it shows the basic principle used in "the old days". Being powered from a battery there is a risk of interference from the output back onto the input due to the battery's internal resistance. R6 and C4 form a low-pass filter with a time constant of about 2 s and the voltage at the top of C4 will be held adequately stable for audio-frequency inputs.

The use of modern voltage regulators generally solves the problem altogether. In your example V+ would be held quite tightly by the voltage regulator.

• When your input goes negative which would tend to pull V+ down the voltage regulator would "open up" to pass more current to maintain the voltage.
• When your input goes positive which would tend to push V+ up a bit the regulator will shut down a little to maintain or regulate the voltage.

The end result is that V+ can source or sink enough current to be unaffected by the injected current.

• thanks I was suspecting that this could be a problem. Why do you say this is a solution back it the day? What kind of circuit designers use today to bias an audio signal?
– Duck
Apr 13, 2018 at 18:10
• @SpaceDog As Transistor mentioned, a regulated supply is more "modern." (Perhaps this could be made clearer?) Apr 13, 2018 at 18:17
• so you are implying that they still use the method I have mentioned today?
– Duck
Apr 13, 2018 at 18:29
• The biasing arrangement you show in your schematic is still used - typically where an alternating signal is fed into a single-rail supply amplifier stage. Apr 13, 2018 at 18:30