Two differential amplifier configuration

摘要

An apparatus is disclosed for providing a common mode voltage to the inputs of a first differential amplifier which outputs the difference between two signals. A second differential amplifier receives the output of the first differential amplifier, and the output of the second differential amplifier is fed back to the inputs of the first differential amplifier as a common mode voltage. Since both inputs of the first differential amplifier receive the fed back common mode voltage, the first differential amplifier still outputs only the difference in the two signals, but the presence of the common mode voltage allows the first differential amplifier to operate with lower noise if the voltage levels of the inputs to the first differential amplifier vary. The second differential amplifier may be of significantly lower quality and cost than the first differential amplifier, without affecting the performance of the first differential amplifier.

主权项

1. A circuit comprising:
a first differential amplifier, comprising:
a first operational amplifier having an inverting input for receiving a first part of a differential signal represented by the difference between two voltages and a non-inverting input for receiving a second part of the differential signal, and an output for outputting a single ended signal representing the difference between the two parts of the differential signal;a first resistor coupled to the output of the first operational amplifier and the inverting input of the first operational amplifier so as to create a negative feedback loop around the first operational amplifier; anda second resistor coupled to the non-inverting input of the first operational amplifier and to a ground; a second differential amplifier, comprising:
a second operational amplifier having an inverting input and a non-inverting input, the non-inverting input coupled to the ground;a third resistor coupled to the output of the first operational amplifier and to the inverting input of second operational amplifier; anda fourth resistor coupled to the output of the second operational amplifier and the inverting input of the second operational amplifier so as to create a negative feedback loop around the second operational amplifier; a fifth resistor coupled to the output of the second operational amplifier and the inverting input of the first operational amplifier in a first feedback loop; and a sixth resistor coupled to the output of the second operational amplifier and the non-inverting input of the first operational amplifier in a second feedback loop which is independent of the first feedback loop.