Analysis and Design of Transimpedance Amplifiers for Optical Receivers

Appendix D

Nonlinearity

Nonlinearity and the resulting signal distortions are important in systems that perform linear signal processing (equalization, data conversion, etc.) and in applications that use higher-order modulation (4-PAM, QPSK, QAM, etc.) or multicarrier modulation (SCM, OFDM, DMT, etc.). In the following, we review nonlinear effects, such as gain compression, harmonic distortions, intermodulation distortions, and composite distortions. We show how these effects are related to the power-series coefficients of the nonlinearity and to the input signal strength.

Importance of Linearity

In Section 4.1, we introduced the linear channel as an abstraction for the chain formed by the photodetector, transimpedance amplifier, and main amplifier. How linear does this channel have to be?

If the linear channel is followed directly by a binary decision circuit, as shown in Fig. 4.1, linearity is of little concern. In this case, even a very nonlinear amplifier such as a limiting amplifier is fine, as long as it does not create undue pulse-width distortion and jitter. However, if the linear channel is followed by an analog-to-digital converter, an equalizer, or some other type of signal processing circuit, linearity becomes important.

If the linear channel is part of a receiver for 4-PAM, QPSK, or another higher-order modulation format, a certain amount of linearity becomes mandatory. For multicarrier signals, such as a SCM, OFDM, or DMT signals, linearity becomes essential. ...