How do you want to combine them? Do you want to add the voltage of both 
signals? You have to put your problem in mathematical terms before you 
can think about implementing a solution.

I actually want to add them. I tried to solve the problem with an OPAMP, 
however I need a fast switching circuit. The slew rate of the high 
voltage OPAMPs are very low.

Ersin Oezalp wrote:
> I actually want to add them. I tried to solve the problem with an OPAMP,
> however I need a fast switching circuit. The slew rate of the high
> voltage OPAMPs are very low.

For integrated opamps, this may be true.

If you need more than 2 MHz GBW, you will have to check for discrete 
designs.
Fast video final amps will do so.

The good ol tekronix 465 (80s design) already solved this for the 
vertical final amp (1 GHz GBW). Check the service manual, to get some 
idea how to design this. Both discrete and ic solution.

ht,
Andrew

Thanks a lot.

I found two OPAMPs from the company Cirrus Logic, PA69 and PA78. They 
are expensive but I think they will solve my problem. Does anybody has 
any experience with these type of OPAMPs.

Another way to fix this problem is to use two equivalent voltage 
dividers to scale down the input voltages. Then you are able to use a 
common OPAmp. After the adding stage you need a output amplifier stage 
with a gain factor (reciprocal to scaling factor). I believe that's a 
verry fast and precission way.

I hope you have the possibility for a spice simulation.

You can get a good spice tool from texas instruments (Tina TI).

Alexander