Can someone suggest me a suitable architecture to convert a differential signal into a single ended output ........if possible with a variable gain as in PGA ????
You can use the unbalanced differential amplifier (the single-ended output differential amplifier). You can vary the gain by varying the current value, for example.
U mean to see use just a simple open loop differential amplifier and vary gain by changing it's tail current......but don't u think it will be a bit crude way of doing thing since it might not give me exact gains also if diff amp is open loop it's o/p might be out of range for large gains.
Well, this method is used mainly for high frequencies, specially for high frequency dividers. The problem is at high frequency, closed loop amplifiers are hard to design and stablize.
If you're working at low frequency, then you can apply negative feedback on a fully differential amplifier, then discard one of the differential outputs. You'll lose half of the gain however.