# How to add variable dc offset to ac signal

#### alibarghi

##### Member level 1
I want to generate an ac signal 300Vp-p Which is added with a dc voltage (0-20 V) With a maximum current of 4 amps.
Nobody has an idea for the circuit.

#### KlausST

##### Super Moderator
Staff member
Hi,

Two (isolated) power sources, one AC, maybe with a transformer and a DC source connected in series.
But mind:
* the DC source need to be able to source and sink current. (At least sink into a big capacitor)
* and the AC source need to be able withstand DC current...so a standard transformer will be critical.

Klaus

d123

### d123

Points: 2

#### FvM

##### Super Moderator
Staff member
The source can be implemented with a PWM (class D) power stage. We need specifications of signal frequency and quality requirements.

##### Super Moderator
Staff member
Suppose you start with bipolar DC supplies, +330V -330V, to power a half-bridge. Then raise or lower bias voltages to obtain desired AC output at the load. (The bias signal needs to sink and source 12 W peaks.)

By placing NPN at top, it avoids shoot-through and power loss. Positive output matches (follows) bias voltage to the NPN.
Negative output matches (follows) bias voltage to the PNP.

This arrangement is efficient if your signal is a square wave. Or else you can preserve efficiency if you apply bias as PWM. In other words create class D operation (per post #3).

#### Easy peasy

A buck converter in series with your AC will work

#### alibarghi

##### Member level 1
A buck converter in series with your AC will work
[/QUOTE
Hi,

Two (isolated) power sources, one AC, maybe with a transformer and a DC source connected in series.
But mind:
* the DC source need to be able to source and sink current. (At least sink into a big capacitor)
* and the AC source need to be able withstand DC current...so a standard transformer will be critical.

Klaus

Thanks for the replies
How can I implement the DC source can be able to source and sink current?

#### Easy peasy

if the DC voltage in series has to act like a large battery ( i.e. source & sink power ) then it would be more ideal to have an active electronic load ( could be a lash up ) to absorb power above the output voltage of the buck converter, this could be as simple as a TL431 controlling a large P fet or xtor - which would allow it to be adjustable ....
Either that or a fully active buck ( synch buck fet rather than diode ) fed from a supply that also has an active load on it - this works because the buck is now bi-directional ...

#### FvM

##### Super Moderator
Staff member
You didn't yet tell what the AC source is - a mains transformer or an artificial source. In the latter case, I would consider to generate the whole combined signal at once by a four quadrant pwm source.

Bidirectional (regenerative) DC sources are e.g. made for battery testing, but are not so common yet. The AC current through the DC source can be buffered by a big capacitor. If there's still an average DC current to be sink by the source, a ballast resistor in combination with a regular (single quadrant) DC source might do the job.

#### Akanimo

W
I want to generate an ac signal 300Vp-p Which is added with a dc voltage (0-20 V) With a maximum current of 4 amps.
Nobody has an idea for the circuit.
What is the frequency of the AC signal?

Last edited:

#### dick_freebird

It sounds like a "bias T" only perhaps not for RF
(who does 300V amplitude RF outside from

Low (like AC line) frequency would need
humongous L and C components for a "bias T"
type solution, and the DC feed would not be
at all AC-stiff.

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