The first trick to low noise system design is to apply as much gain as possible in the first stage. The figure below shows two examples of an amplifier front end with a gain of 10. It can be seen that applying all the gain to the first stage is much better than distributing the gain between two stages. Please note that sometimes the requirement for the best bandwidth performance may conflict with the requirement for the best noise performance. For bandwidth, we want each gain stage to have similar gain, while for noise, we want the first stage to have all the gain.

　　The second tip is to pay attention to the source impedance. There are two reasons for this: first, the larger the source impedance, the more noise there will be in the system; second, the amplifier must be well matched to the source impedance, and if the source impedance is high, the current noise characteristic may be more important than the voltage noise characteristic.

　　The third tip is to pay attention to the feedback resistor. If you choose an ultra-low noise op amp but use a large feedback resistor, it is impossible to achieve a low noise circuit. In the same phase (Figure 1) or inverting configuration, pay attention to the feedback resistor, which is equivalent to the noise source referred to the output. The other resistors are equivalent to the voltage source at the input, or more accurately, the voltage source at the input of the inverting configuration. As mentioned earlier, when designing a low noise system, the first stage application has a high gain, in which case Rg noise dominates.