an improved design for an optical encoder with a compensation feedback signal for compensating for effects of time and temperature on the photoemission characteristics of the encoder &# 39 ; s light source . by using a current mirror , the photodiodes which normally provide only the data signal now provide both the data signal and the compensation feedback signal , thereby eliminating the need for a separate , dedicated photodiode for the compensation feedback signal . this results in eliminating the necessity of matching the electrical operating characteristics of the data photodiodes to those of a compensation feedback photodiode . this further results in eliminating the large die area required for including a compensation feedback photodiode in an integrated circuit design .