Noise reduction in AC-coupled amplifiers

AC-coupled amplifiers are noisier than dc-coupled amplifiers because of the thermal noise of the resistor(s) in the ac-coupling network and the increased contribution of the amplifier input noise current $i_{n}$ . Both contributions, however, diminish if the corner frequency $f_{c}$ of the high-pass filter observed by the signal is lowered, the cost being a longer transient response. At the same time, the presence of large resistors in the ac-coupling network suggests that the use of FET-input amplifiers will be advantageous because $i_{n}$ is smaller than in bipolar amplifiers. However, the noise floor, defined as that for a dc-coupled amplifier, can be larger for FET-input amplifiers because of their higher input voltage noise. Therefore, there is no clear criterion to decide which amplifier type is quieter. In this paper, we propose the quotient between the high-pass frequency of the signal bandwidth $(f_{L})$ and $f_{c}~(K=f_{L}/f_{c})$ as quantitative design parameter to reduce the additional noise contributed by a first-order ac-coupling network as compared with dc coupling. Using this parameter noise is greatly reduced without unnecessarily lengthening the transient response that results from the use of common qualitative design criteria. We demonstrate that relatively large $K$ values yield total noise closer to the noise floor, hence it is better not to determine $f_{L}$ at the front amplifier stage but later on in the signal chain. Furthermore, a large $K$ value permits bipolar op-amps to achieve lower noise than FET amplifiers even for capacitive signal sources, provided amplifier biasing is guaranteed.