A Broadband Transimpedance Amplifier with Optimum Bias Network
A novel transimpedance amplifier (TIA) based on regulated cascode (RGC) configuration was proposed, in which a bias network was added to ensure the main amplifier and the auxiliary amplifier of RGC operate at optimum condition. To achieve high bandwidth, a T passive matching network was introduced at the input. Since both amplifiers have optimum gain bandwidth product (GBW) thus low noise and high bandwidth can be obtained in our design scheme. The proposed TIA was designed based on UMC 0.18 mm CMOS process, and the simulation results demonstrated that our TIA has a transimpedance gain of 51 dBO and a 3 dB bandwidth of 14 GHz with a input capacitance of 0.3 pF. The average equivalent input noise current spectral density is about 13 pA/Hz within 3 dB bandwidth. Introduction With the rapid development of mobile Internet, cloud computing and Internet of Things, high speed data communication between board to board, chip to chip and on chip is required [1] and thus make the optical interconnection become a hot issue in recent years. On the other hand, with the continuous scaling of semiconductor technology, the peak transit frequency of the deep submicron MOSFET device has exceeded tens of gigahertz. Due to the cost effective performance with a friendly integration scheme, high speed optoelectronic integrated circuits (OEICs) based on CMOS technology have been widely explored [2 5] As a key block of optical transceiver, the transimpedance amplifier (TIA) determines the overall performance of the receive chain, such as operation speed and sensitivity. As a result, improving the bandwidth of TIA becomes one of the critical steps to achieve high speed optical receiver front end. Due to the low impedance of regulated cascode (RGC) configuration [3] which can neutralize the effects of large photodiode (PD) capacitance and the parasitic capacitance, RGC topology has been widely employed in gigabit optical receiver. Additionally, the RGC TIA also has a lower input referred noise current with respect to common gate (CG) amplifier. To obtain higher bandwidth, various bandwidth enhancement techniques have been demonstrated in RGC configuration, such as auxiliary amplifier improvement technique [4, 5] passive matching network [6 13] and polezero cancellation [14, 15] Compared with other techniques, the passive matching network is found more effective both in bandwidth enhancement and noise reduction. This paper proposed a novel TIA circuit based on RGC configuration, in which an optimum bias (OB) was added to improve the gain bandwidth product (GBW) and a T passive matching network (PMN) was introduced to further extend the bandwidth and reduce the input referred noise current spectral density. The Novel TIA with Optimum Bias As shown in Fig. 1, conventional RGC configuration is constructed by a common source (CS) auxiliary amplifier (M2 and RB) and a common gate (CG) main amplifier (M1 and RD) and the auxiliary amplifier is connected between the gate of M1 and node X. Where the PD is electrically modeled by a current source IPD shunted with a junction capacitance CPD. To achieve better 6th International Conference on Management, Education, Information and Control (MEICI 2016) (c) 2016. The authors Published by Atlantis Press 0827 performance parameters, the main amplifier is preferably selected an optimum DC bias at the expense of decreasing the gain bandwidth product (GBW) of auxiliary amplifier. To solve this problem and obtain maximal GBW, an optimum bias network consisting of C, RB1 and RB2, marked by the dotted line box, is added to the conventional RGC configuration, as shown in Fig. 2. Where the voltage divider formed by RB1 and RB2 is utilized to provide an optimum bias for M1, and the capacitance C is used to isolate the DC biases of both amplifiers and pass the AC signal. Since the DC operating points of both amplifiers are independent of each other, they can be set respectively to the optimum bias condition, and thus a larger GBW can be realized. In our design, the values of resistances (RB1 RB2) should be large enough to prevent the AC signal shunting to ground, and avoid extra noise introduced by these resistances.