Differential amplifiers typically have a low input impedance, which is related to their matching resistance. The impedances of the two inputs are also asymmetrical, which can lead to matching problems between the signal source and the amplifier. For the input impedance calculation at the input terminal, we can consider VI- and VI+ separately and ignore the influence of the other input. In practice, however, mismatch of the impedances of the two pins of a signal source connected to a differential amplifier may reduce the common-mode rejection ratio (CMRR).
Disadvantage 2 of differential amplifiers: low common-mode rejection ratio
The most perfect state of a differential amplifier is that the two R1s are exactly equal and the two R2s are exactly equal in the circuit in Figure 1, however, we cannot find two resistors with exactly the same resistance, and the common resistors have a 1% error. This results in a mismatch of resistors, which will greatly reduce the common-mode rejection ratio CMRR.
The ideal state for a differential amplifier is that the matching resistors at the two inputs are exactly equal, but this is difficult to achieve in practice. Even common resistors have a certain amount of error, which can lead to resistance mismatch and thus reduce the common-mode rejection ratio. A reduction in the common-mode rejection ratio means that the differential amplifier is less capable of suppressing common-mode interference, which can introduce additional noise in practical applications.
In summary, despite the advantages of differential amplifiers, their drawbacks such as low input impedance and low common-mode rejection ratio still need to be considered during the design process. By adopting an optimized circuit design and precisely matched components, these problems can be overcome to a certain extent, and the performance and stability of the differential amplifier can be improved.