ObjectiveBalanced photodetectors are a kind of high-sensitivity, low-noise photodetectors with two optical signal receivers, and the output signal is proportional to the difference between the two input optical powers. Owing to the differential detection technique, balanced photodetectors can effectively suppress common-mode noise, greatly improving the sensitivity and signal-to-noise ratio (SNR) of the optoelectronic system. Balanced photodetectors have been widely used in the fields such as laser coherent detection, coherent optical communication, and quantum optics. The performance parameters such as bandwidth, conversion gain, and noise equivalent power (NEP) of balanced photodetectors are mainly determined by the transimpedance amplifier (TIA). OPA847 and OPA657 produced by Texas Instruments are two high-speed, low-noise operational amplifiers that are suitable for implementing TIA and are widely used in commercial photodetectors and related research. Studying the performance of the TIAs based on OPA847 and OPA657 has important reference significance for designing different types of balanced photodetectors.MethodsThis paper theoretically and experimentally studies the bandwidth and noise characteristics of balanced photodetectors based on TIAs with OPA847 and OPA657. The dependence of the −3 dB bandwidth and the NEP of the balanced photodetectors on the transimpedance gain of TIA is theoretically calculated by using the equivalent circuit and noise model of photodiodes and TIA. The performance difference between the OPA847- and OPA657-based TIAs are compared. The contributions of the voltage and current noises of the operational amplifier and the thermal noise of the feedback resistor to the NEP are analyzed in detail. The balanced photodetectors based on OPA847 and OPA657 with transimpedance gains of 2 kV/A and 200 kV/A are experimentally performed, and their frequency and noise performances are measured. The −3 dB bandwidth is obtained from the frequency response and the NEP is obtained from the noise power spectrum. The measured −3 dB bandwidth and the NEP are compared with the theoretical values. The noise figure (NF) of the secondary voltage amplifier is evaluated from the TIA and the total noise power spectra. In additional, the noise power spectra of the balanced photodetectors at different local-oscillator powers are measured.Results and DiscussionsThe theoretical calculations show that OPA847 leads to a smaller NEP for a lower transimpedance gain and a larger bandwidth, while OPA657 leads to a smaller NEP for a higher transimpedance gain and a smaller bandwidth. In experiment, for the transimpedance gain of 2 kV/A, OPA847 and OPA657 give rise to −3 dB bandwidths of 220 MHz and 130 MHz, and NEPs of $8.2\text{pW/}\sqrt{Hz}$ and $22\text{pW/}\sqrt{Hz}$, respectively. For the transimpedance gain of 200 kV/A, they give rise to −3 dB bandwidths of 9.2 MHz and 8.5 MHz, and NEPs of $4.3\text{pW/}\sqrt{Hz}$ and $2.3\text{pW/}\sqrt{Hz}$, respectively. All the four configurations of the balanced photodetectors achieve common-mode rejection ratios (CMRRs) greater than 30 dB within the −3 dB bandwidth range. A shot-noise measurement of 10 dB-higher than electronic noise is achieved within the saturation power range for these balanced photodetectors. The theoretical and experimental studies demonstrate that OPA847 is suitable for achieving large-bandwidth, low-noise balanced photodetectors, while OPA657 is suitable for achieving high-gain, low-noise balanced photodetectors.ConclusionsThis paper presents a study on the frequency and noise characteristics of the balanced photodetectors based on TIAs with OPA847 and OPA657. The dependence of −3 dB bandwidth and the NEP on the transimpedance gain of TIA is calculated, and the performance advantages of OPA847 and OPA657 at low and high transimpedance gains are analyzed. The balanced photodetectors with transimpedance gains of 2 kV/A and 200 kV/A are experimentally implemented based on OPA847 and OPA657, respectively. The frequency response, noise power spectrum, CMRR and NEP are measured. Furthermore, the noise power spectra at different local-oscillator powers are measured, and the optical power and frequency range where the shot noise is 10 dB higher than the electronics noise are given. The results show that OPA847 and OPA657 are suitable for implementing high-bandwidth and large-gain balanced photodetectors, respectively. This research provides important reference significance for selecting suitable operational amplifiers in the design of different types of balanced photodetectors.