A module including a bias circuit that generates gate bias voltages by resistance dividers creates a problem in that the values of the resistances constituting the bias circuit must be finely adjusted, and accordingly extra trimming tasks are required. In one aspect, the interbody spacer includes upper and lower convex surfaces defined by an arc extending from a center point offset from the spacer midpoint. Furthermore, an encapsulating part for at least part of the drive means is positioned with at least part of its surface near a second outer side of the drive means, and said rail or plate part and said encapsulating part are connected to each other in a heat-conducting connection.