surges when the lamp is initially off for a similar purpose the 0 68 uf capacitor connects the base of transistor 65 to ground the feedback circuit fo amplifier 63 includes a 250k ohm resistor in parallel with a series connected 50k ohm resistor and 1000 pf capacitor this latter element is used to reduce loop gain at the higher frequencies and prevent oscillation the output terminal of op amp 63 is connected serially to the inverting terminal of a high gain amplifier 67 via a 5k ohm input resistor high loop gain at low frequencies for accurate lamp intensity control is assured by means of the 22k ohm and 0 068 uf capacitor 68 in series connection which form the feedback path a 1k ohm resistor connects the output of op amp 67 to the base of the first of a serially connected pair of 2n3055 power transistors 69a 69b to control the lamp source while a in4001 diode connects the base of transistor 69a to ground to protect the first 2n3055 from reverse bias a 24 v switching power supply 70 is used to power lamp 3 this unit is typically an rmk 24 b manufactured by kepco corporation lamp 3 is a 24 v type fcs 150 watt tungsten filament bulb manufactured by general electric other lamps may be used but these should radiate relatively broad band spectral emissions power supply 70 drives lamp 3 via a 0 1 ohm load.