On the design of low-noise, giga-hertz bandwidth preamplifiers for optical receiver applications

The noise characteristics of TIA (transimpedance amplifier) for optical receiver applications is conventionally described by equivalent input noise current spectral density, rather than noise figure (NF). However, since optical receivers consist of several blocks such as preamplifier, wideband amplifiers, variable gain amplifier and decision circuit, NF can be a useful tool to determine the noise characteristics of the whole optical receiver. In this paper, NF of a TIA (preamplifier) is described by using g_m and the bandwidth to cut-off frequency ratio B/f_T, showing that NF of TIA degrades quadratically with increasing bandwidth. With a given technology and the noise specification of the whole optical receiver, the maximum achievable bandwidth is obtained, facilitating the design of the TIA. As an example, a common-source TIA was designed and simulated with 0.6 μm AMS CMOS technology, showing a close match with the predicted results