The input of the photoelectric coupler is the light-emitting diode. Therefore, its input characteristics can be represented by the volt ampere characteristic of the light-emitting diode. The output end is photosensitive triode, so the volt ampere characteristic of the photosensitive triode is its output characteristic. It can be seen that the photoelectric coupler has a nonlinear working area, which is directly used to transmit the analog quantity.
One solution is to use two photoelectric couplers with the same nonlinear transmission characteristics, T1 and T2, and the two emitter follower A1 and A2. If the T1 and T2 is the same as batch of photoelectric coupler, with nonlinear transmission characteristics which can think they are completely consistent, namely the K1 (I1) = K2 (I1), and the voltage gain of the amplifier G = Uo/U1 = I3R3 / I2R2 = (R3 / R2) [K1 (I1)/K2 (I1)] = / R2 and R3. It can be seen that the symmetry of the current transmission characteristics of T1 and T2 can be used to compensate their original nonlinear bu using the feedback principle.
Another solution to analog volume transmission that is to adopt a VFC (voltage frequency conversion) approach. In the field transmitter output analog signal (assuming voltage signal), the voltage frequency converter converts the voltage signal sent by the transmitter to the pulse sequence, which is sent out after the decoupling of the optical coupling. On the host side, the pulse sequence is reduced to a analog signal by a frequency voltage conversion circuit. At this time, it is the digital quantity which is quite decoupled, which can eliminate the effects of the nonlinear coupling. This is an effective and simple transmission mode of analog quantity.
Of course, linear optical coupler
can also be selected for the design, such as precision linear coupling TIL300, high-speed linear coupling 6N135/6N136. Linear decoupling is generally higher than normal, but it is easy to use and simple to design. With the decrease of device price, the use of linear Photoelectric Coupler will be the trend.