A high open loop gain common mode feedback technique for fully differential amplifiers

Fully differential amplifiers usually need a common mode feedback circuit to stabilize its common mode voltage. The open loop common mode feedback circuit needs to fulfill certain conditions to guarantee fast settling and stability. If these conditions are not met with an applied feedback circuit, commonly the gain of the feedback circuit is reduced and therefore also the speed, which may enable stability. This results in smoother settling behavior, but at the cost of worse accuracy. The proposed technique just reduces the speed, but not the open loop gain. This is possible with nested feedback capacitors, offering the possibility to stabilize the common mode and differential loop independently, enabling stability and smooth settling without decreased accuracy. An amplifier with a unity gain bandwidth of 3.5 GHz applying this technique has been manufactured in a 250 nm BiCMOS technology and measurement results prove the theory.