Designing optoelectronic integrated circuit (OEIC) receivers for high sensitivity and maximally flat frequency response

This paper examines previously overlooked, but a highly effective, optimization approach to designing transimpedance OEIC receivers based on heterojunction bipolar and field-effect transistors (HBT´s & HFET´s) with high sensitivity and maximally flat frequency response. It is shown that the 3-dB bandwidth of amplifiers involving single-transistor common-emitter (CE) and common-source (CS) input stages, and the corresponding cascoded input stages can be universally expressed as f_3-dB≅1/2π√(τ´(c_μ+c_L )R_f), where τ´ is the effective transit time of the transistor including the effect of the photodetectors capacitance, c_μ is the feedback capacitance of the transistor, c_L is the capacitance in parallel to the collector or drain effective load conductance (g¯_L), and R_f is the feedback resistance of the amplifier that ensures a much greater than unity loop-gain. The gain-bandwidth product can also be universally expressed as R_f×f_3-dB≅g_m /√2πg¯_Lc_in, where gm is the transconductance of the transistor and c_in is the total capacitance appearing at the input of the respective amplifier. For the single-transistor CE and CS input stages the dependence of the optimum values of R_f on g¯_L can be expressed as R_f=2τ´(c_μ+c_L)/(g¯Lτ´+c _μ)², whereas for the corresponding cascoded input stages the same expression also applies but with c_μ=0 in the denominator. Since designing high sensitivity OEIC receivers require high values of R_f and correspondingly low values of g¯_L, thus it follows that amplifying transistors must be operated at the lowest possible current but without degradation of their high frequency performance. A low-value of the base or gate bias current also ensures a low level of shot noise thus further enhancing the sensitivity. Because of a low g_m to drain current ratio in HFET´s, receivers involving only the cascoded input stage, and not the single-transistor CS input stage, are capable of providing both high sensitivity and high bandwidth. In contrast, receivers based on HBT´s involving either the simple CE input stage or the cascoded input stage are capable of providing both high sensitivity and high bandwidth. Design examples, using HBT´s and HFET´s with intrinsic unity current gain frequency f_τ=160 GHz and very low-capacitance msm photodetectors, indicate that high-speed receivers can be realized with bandwidth f_3-dB≅20 GHz, and sensitivity as high as -28.7 dBm