Amplifier with common-mode feedback circuit
A common-mode feedback circuit is provided. An amplifier with a common-mode feedback circuit is compensated by adding a compensating unit so that the amplifier totally has two poles and one zero in its frequency response. Accordingly, the gain of the amplifier is not sacrificed, and both the stability and the phase margin of the circuit are improved.
1. An amplifier, comprising:
an operational amplifier having a first output terminal and a second output terminal for amplifying an input signal and generating an output signal;
a common-mode detector coupled between the first output terminal and the second output terminal for detecting a common-mode output voltage of the output signal; and
a common-mode feedback amplifier, coupled to the common-mode detector, for generating a control signal to the operational amplifier in accordance with the common-mode output voltage and a reference voltage, comprising:
a first transistor for receiving the common-mode output voltage;
a second transistor for receiving the reference voltage;
a first compensating capacitor for compensating the amplifier; and
a first compensating resistor for compensating the amplifier.
2. The amplifier according to claim 1 , wherein the common-mode detector comprises:
a first resistor coupled to the first output terminal;
a second resistor coupled between the second output terminal and the first resistor;
a first capacitor coupled to the first output terminal; and
a second capacitor coupled between the second output terminal and the first capacitor;
wherein resistance values of the first resistor and the second resistor are substantially equivalent and capacitance values of the first capacitor and the second capacitor are substantially equivalent.
3. The amplifier according to claim 1 , wherein the first compensating resistor is coupled to a drain of the first transistor and the first compensating capacitor is coupled between the first compensating resistor and an operating voltage.
4. The amplifier according to claim 1 , wherein the common-mode feedback amplifier further comprises:
a second compensating capacitor for compensating the amplifier;
wherein the first compensating resistor is coupled between the first compensating capacitor and the second compensating capacitor, and the first compensating capacitor and the second compensating capacitor are respectively coupled to drains of the first resistor and the second resistor, and wherein capacitance values of the first capacitor and the second capacitor are substantially equivalent.
5. The amplifier according to claim 4 , wherein the common-mode feedback amplifier further comprises:
a second compensating resistor coupled to the drain of the first transistor; and
a third compensating resistor coupled between the drain of the second transistor and the second compensating resistor;
wherein resistance values of the second resistor and the third resistor are substantially equivalent.
6. The amplifier according to claim 3 , wherein the common-mode feedback amplifier further comprises:
a second compensating resistor coupled to the drain of the first transistor; and
a third compensating resistor coupled between the drain of the second transistor and the second compensating resistor;
wherein resistance values of the second resistor and the third resistor are substantially equivalent.
7. The amplifier according to claim 1 , wherein the common-mode feedback amplifier further comprises:
a first current source coupled to both the first transistor and the second transistor for providing a current to the common-mode feedback amplifier.
8. The amplifier according to claim 1 , wherein the common-mode output voltage is substantially equivalent to the reference voltage.
9. The amplifier according to claim 1 , wherein the operational amplifier is a fully differential operational amplifier.
10. The amplifier according to claim 1 , wherein the output signal is a differential signal.
11. The amplifier according to claim 1 , wherein the first transistor and the second transistor are metal-oxide-semiconductor (MOS) transistors.
12. The amplifier according to claim 1 , wherein the first transistor and the second transistor are bipolar junction transistors (BJTs).
13. The amplifier according to claim 1 , wherein the reference voltage is generated by a reference voltage generator.
14. The amplifier according to claim 1 , wherein the reference voltage is generated according to a voltage division.
15. The amplifier according to claim 1 , wherein the amplifier has two poles and one zero in frequency response.