# How to add a DC bias?

I have 70Vrms sine wave, I rectified it with full wave rectifier. So the output is 100Vp.
I will divide it down to 1V using voltage divider (0-100V to 0-1V). How could I add a 1.4V DC bias to the output of the voltage divided ?

Briefly: $$70V_\text{rms} \Longrightarrow 0-100V_\text{p} \Longrightarrow 0-1V \Longrightarrow 1.4-2.4V$$

• I have a 5V DC I could used it in the biasing

Spehro Pefhany, it is hard to find exactly this resistors values:

I read in another answer (Note also that this circuit does not isolate V1 and V2 from each other as a summing inverting amplifier would. Each sees the other with a impedance of R1+R2. Maybe that doesn't matter, but you need to at least think about it.(

Will this circuit work? Mr. Spehro Pefhany, I had a lot of distortion in my signal after I did the voltage divider as us advise my to do it.

Here is my project When the voltage is 56Vrms, I get the following results ( the green is the signal at PWM pin1 and the yellow is at R6 -the AM demodulator- ) The voltage at pin 1 of the PWM IC (UC3526AN) have to be 1.4VDC bias with 1Vpp when te input voltage is 100Vpp but it is not. The following is the real measurements at pin 1.

• At about 20Vrms input • At about 30Vrms input • At about 68Vrms input • At about 100Vrms input • Is this a signal voltage or power? – Dzarda Apr 25 '15 at 17:50
• it is to measure the voltage output of a unit under test ( he full load current is 2A – John Alawi Apr 25 '15 at 18:34

You can just use an op-amp adder, or just a voltage divider if you can live with higher output impedance and low input impedance.

For example (voltage divider): simulate this circuit – Schematic created using CircuitLab

R1 is picked arbitrarily, and the remaining two values are chosen so that R2||R3 = 2.02K (so the output voltage changes 1.00V for a 100V change in the input) and 5V * (R1||R2/(R3+R1||R2) = 1.40V

Output impedance is R1||R2||R3 = 2.0K

Edit: Depending on the accuracy you need (which you did not state) you can use the closest E96 values of 1% resistors (eg. 7.15K, 2.80K) which would give you a gain of 100:0.996 and a bias of 1.393V (nominally). For a higher accuracy divider you could use available 0.1% resistors and combine them with 1% resistors to get very close to the desired values (or simply use something close and adjust the result digitally).

• if R2=2.7k and R3=6.8 the ration will be 100:0.996 and DC bias is 1.407V. thinks very much for the help – John Alawi Apr 25 '15 at 21:24
• @Yusuf 0.957 and 1.407V – Spehro Pefhany Apr 25 '15 at 22:31