I need to test some differential output circuit, eg. amplifier, OTA, and filter etc. The circuit is make up by CMOS, hence has poor driving capability. I need to build internal buffer: source follower, amplifier. But, this limits my output swing and affect the circuit's frequency response, as the internal buffer needed is very large in size, although there is another unity analog buffer outsite the chip to drive a balun, which match to 50-Ohm of the measurement equipment. I use mini-circuit balun, it is quite good.
At high speed, the above method is not efficient at all. So, I try to use opened-drain kind of output buffer. I use a source follower to drive a open-drain amplifier. And the drain of the amplifier are connected to the balun. However, the problem comes;

1.  I don't know how to determine the design spec. of the open-drain buffer, such as; the current needed, the device size, the output drain voltage, what kind of balun can be use?

2. How to simulate this condition?? if assumed I use mini-circuit ADT1-2T balun? I am using Cadence, spectre. The note from Kundert using balun to simulate the voltage output, how about the current output? use resistor load to convert? how to build the ADT1-2T balun in Cadence. ADT1-2T has turn ration of 2, and is center tapped. How to simulate the impedance matching condition of the balun??

Thanks.