In low-
voltage circuits, there is often insufficient
voltage to use a
current source to bias a
transconductance amplifier stage. This is particularly true in mixers where a switching circuit must be stacked on top of the
transconductance input stage. One way around this problem is to get "double-duty" out of the input differential pair, using it both for
gain stage and for
DC bias. This is done by AC
coupling in a high-frequency input
signal, while using a low-frequency, DC-coupled circuit to establish the proper bias level. One common technique is to use a simple
current mirror scheme to establish the DC level. Proper biasing using this technique requires good matching of resistance. In some implementations of
transconductance amplifiers, particularly those that use inductors as degeneration elements, series resistance of the
inductor and interconnect resistance can cause significant errors in the bias current. This invention addresses that problem by using an
operational amplifier with a current-sensing
resistor and a low-frequency
feedback loop to compensate automatically for any resistance errors. The
operational amplifier drives the feedback
voltage (generated in accordance with the sensed voltage at the current-sensing
resistor and applied to one input of the
operational amplifier) towards a reference voltage that is applied to the other input of the operational
amplifier to bias the
transistor(s) in the transconductance
amplifier for desired operating conditions.