In a differential amplifier circuit, a current mirror is used to bias the input transistors and improve the performance of the amplifier. The main function of a current mirror in a differential amplifier is to provide a stable and well-defined bias current for the input stage. This helps achieve several important objectives:
Improved symmetry: In a differential amplifier, the two input transistors are designed to be identical and operate in a matched manner. A current mirror ensures that both transistors receive the same bias current, maintaining symmetry in the circuit.
Higher input impedance: A current mirror helps increase the input impedance of the differential amplifier. This is particularly crucial when dealing with signals from high-impedance sources, as it prevents loading and signal degradation.
Common-mode rejection: By providing a stable bias current to the input transistors, a current mirror helps enhance common-mode rejection. Common-mode rejection is the ability of a differential amplifier to reject noise or interference that appears equally on both inputs.
Reduced distortion: Current mirrors help minimize distortion in the output signal of the differential amplifier. Distortion is reduced because the identical biasing of the input transistors ensures similar operating conditions for both devices.
Thermal stability: Using a current mirror to bias the input stage enhances thermal stability. As both transistors receive the same bias current, their temperature-related characteristics tend to cancel out, reducing the impact of temperature changes on the amplifier's performance.
The current mirror in a differential amplifier can be realized using various configurations, such as simple bipolar transistor-based mirrors or more complex designs involving MOSFETs. Regardless of the implementation, the primary purpose remains to provide a stable and well-controlled bias current for the differential amplifier's input stage, leading to improved performance and better overall characteristics.