In the Friday night davening, I have noticed that after the chazan says the bracha "קונה שמים וארץ", the congregation always goes immediately on to saying "מגן אבות" without responding אמן to the bracha. This seems to be the case in shuls of all nusachim (as far as I can remember).
What is the reason for this? The only explanation that I can think of is that this is a "long bracha" (matbe'a arokh) which ends with "מקדש השבת". Is this the case? If not, why is Amen not said after the first bracha?