2012/04/04

Figure 1: Side Coax v3 Circuit used during Run 26 (left) and Corrected Circuit (right).

Nader and Gary identified the problem. There is a reverse bias saturation current through the gate. In the circuit we were employing this run, shown in Fig. 1 (left), the current in the feedback loop flows through a coupling capacitor on the SCUHBA. Thus, the potential at the gate increases and saturates the amplifier.

This is simple to fix, as shown in Fig. 1 (right), by diverting the feedback to bias the gate of the FET. This means that the circuit needs to be changed. The SCAB boards can be reused, but a SCUHBA must be designed to incorporate the feedback resistors.