Noise Transfer Function Optimization of the TIA in Current Mode Receivers

Direct conversion current mode receivers have earned success in the last years for their superior performance in terms of noise and linearity. Most of the advantages of this architecture are obtained thanks to the use of a passive mixer. In this paper, attention is paid on how the mixer affects the noise performance of the TIA analyzing the time-varying behavior of the mixer output admittance. An analytical model is provided to quickly determine the impact of TIA noise at the receiver input as a function of design parameters. A theoretical analysis is at first carried out, considering an ideal TIA, where the even harmonics contribute to the output noise due to folding. Subsequently, a concrete case of study is considered where the TIA bandwidth is limited by providing a simplified formula for the admittance calculation. Then, the effects of the TIA noise referred to the receiver input are analyzed, highlighting that this noise can be minimized, by changing the carrier duty cycle according to the wanted receiver gain, obtaining a noise reduction of up to 7 dB. Calculated and simulated results match very well demonstrating the correctness of the proposed approach.