| 摘要 |
A current tail circuit and method for a differential transistor pair affords the capability of sensing an input differential signal having a low common mode voltage when using, for example, an NMOS differential transistor pair. A current source device and a capacitor may be employed to provide at the common node of the differential transistor pair what appears to be a constant current source connected to a "negative voltage." In one embodiment particularly useful when using an NMOS differential pair, one terminal of a capacitor is precharged to VDD and the other terminal is precharged to VSS (i.e., ground). When the amplifier needs to sense its differential input signal, a control signal turns off precharge transistors and couples the capacitor terminal previously precharged to VSS to the common-source node of a differential transistor pair. The capacitor terminal previously precharged to VDD is driven toward VSS, preferably with a controlled current source, which couples the common-source node of the differential pair from VSS toward a voltage below VSS. As soon as the common-source node voltage is low enough for at least one side of the differential pair to conduct a current substantially equal to the controlled current source, the common-source node voltage is substantially clamped at that voltage. The actual voltage resulting on the common-source node depends on the transistor characteristics, the particular voltages present on the gates of the differential pair, and the magnitude of the controlled current source. For some operating conditions, this voltage may be above ground rather than below ground, but the "tail" current of the differential pair nonetheless remains equal to the magnitude of current through the device driving the other terminal of the capacitor from VDD toward VSS.
|
