To: File arc_detect.wpd

From: Jon Hagen

Date: Feb 12, 2004

Subject: Operation of S-band Arc Detector "Fiber Repeater" Circuit

This circuit uses photo diodes to detect light transmitted through short plastic fibers from waveguide elbows near the klystron windows. The detected signals are amplified and converted back to light for transmission via glass fibers to the control cabinet. Transmission of the output via light fibers is done to ensure isolation from electrical transients.

Operation of the circuit is linear; the output light signal is proportional to input light signal. The photo diode acts as a current generator. Its current will be proportional to illumination provided it sees a low impedance. In this circuit, the low impedance provided by connecting the diode directly across the input terminals of an op amp. The op amp runs in its linear region (i.e. it is not saturated), so, because its voltage gain is very high (maybe a million), the voltage difference between the plus and minus input terminals is forced to be nearly zero.

The circuit, shown below, can be analyzed in four steps:

1. All the diode current, id, flows downward through R (because the op amp terminals draw no current) so the voltage at the positive input of the amp is idR.

2. The voltage at the negative input must therefore also be equal to idR.

3. Ohm's law then requires that the current in the other resistor R be equal to id.

4. The currents at node "A" must sum to zero: (Vout - idR)/Rf = 2 id.

or Vout = id (2 Rf+ R)