Wideband Balanced Amplifier for Future ALMA Wideband Receivers

The new generations of ALMA receivers demand wider instantaneous IF bandwidths combined with 2SB mixer technology. As a result, the required larger fractional bandwidths of cryogenic IF amplifiers impose a trade-off between their noise temperature and input reflection matching. Cryogenic isolators have so far been employed in all European bands of ALMA. However, ferrite isolators are already reaching their limits in terms of fractional bandwidth and further degrade the noise temperature at the edge of the band. Balanced amplifier on the other hand are implemented in some prototype radio astronomical receivers and have the great advantage of providing larger fractional bandwidth with better overall noise performance than with an input isolator. At the workshop, we shall present the results of the latest development in terms of cryogenic balanced amplifiers for SIS receivers with enhanced IF bandwidth and the resulting performance of wideband prototypes receivers operating at 300 GHz.