Automatic gain control method and circuit for use in burst-mode transimpedance amplifier
Provided in the present invention is an automatic gain control method for a burst-mode transimpedance amplifier. A transistor is connected in parallel at either end of a feedback resistor of a transimpedance amplifier. A gate-source voltage of the transistor is controlled by detecting and then reversely amplifying an output voltage of the transimpedance amplifier. The present invention also provides a circuit implementing the method, obviates the need for support from any particular process, and is implementable using conventional components.
1. An automatic gain control circuit for a burst-mode transimpedance amplifier, comprising a transimpedance amplifier, a common-source amplifier, and a transistor;
wherein the transistor is connected in parallel with a feedback resistor of the transimpedance amplifier, a drain of the transistor is connected to an input terminal Iin of the transimpedance amplifier, a source of the transistor is connected to an output terminal Vout of the transimpedance amplifier; an input terminal of the common-source amplifier is connected to the output terminal Vout of the transimpedance amplifier, an output terminal of the common-source amplifier is connected to a gate of the transistor;
when an input current of the transimpedance amplifier is zero, the transistor is turned off; when the input current of the transimpedance amplifier is gradually increased, an output voltage of the transimpedance amplifier is gradually decreased, and an output voltage of the common-source amplifier gradually rises, so that a gate-source voltage of the transistor is gradually increased; when the gate-source voltage of the transistor increases to exceed Vth, the transistor is turned on, and an equivalent resistance of the transistor and the feedback resistor decreases, so that the output voltage of the transimpedance amplifier no longer continues to decrease as the input current increases;
wherein the common-source amplifier includes an NMOS transistor (M 2 ) and a resistor (R 1 ); a gate of the NMOS transistor (M 2 ) serves as the input terminal of the common-source amplifier, a drain of the NMOS transistor (M 2 ) serves as the output terminal of the common-source amplifier; and the resistor (R 1 ) is connected between the drain of the NMOS transistor (M 2 ) and a power supply voltage Vdd.
2. The automatic gain control circuit as claimed in claim 1 , wherein the common-source amplifier is a common-source differential amplifier; the common-source differential amplifier includes an NOMS transistor (M 4 ), an NMOS transistor (M 5 ), a resistor (R 2 ), a resistor (R 3 ), and a constant current source (I 1 );
wherein a gate of the NOMS transistor (M 4 ) serves as an input terminal of the common-source differential amplifier, a gate of the NOMS transistor (M 5 ) is biased at a fixed reference voltage Vref; a drain of the NOMS transistor (M 4 ) serves as an output terminal of the common-source differential amplifier;
the resistors (R 2 ) and (R 3 ) are connected between the drains of the NOMS transistor (M 4 ) and the NMOS transistor (M 5 ) and a power supply voltage Vdd, respectively, sources of the NOMS transistor (M 4 ) and the NMOS transistor (M 5 ) are connected and grounded through the current source (I 1 ).
3. The automatic gain control circuit as claimed in claim 2 , wherein the transistor is an NMOS transistor (M 3 ).
4. The automatic gain control circuit as claimed in claim 3 , wherein the transimpedance amplifier includes a feedback resistor (R 2 ) and a cascode.
5. The automatic gain control circuit as claimed in claim 4 , wherein the cascode includes an NMOS transistor (M 1 ), an NMOS transistor (MO), and a resistor (R 0 ); wherein a gate of the NMOS transistor (M 1 ) serves as the input terminal Iin of the transimpedance amplifier, a gate of the NMOS transistor (MO) is connected to a bias voltage Vbia; a drain of the NMOS transistor (M 1 ) is connected to a source of the NMOS transistor (MO), a source of the NMOS transistor (M 1 ) is grounded; the feedback resistor (R 2 ) is connected between the gate of the NMOS transistor (M 1 ) and a drain of the NMOS transistor (MO); the resistor (RO) is connected between the drain of the NMOS transistor (MO) and the power supply voltage Vdd.
6. The automatic gain control circuit as claimed in claim 2 , wherein the transistor is an NMOS transistor (M 3 ).
7. The automatic gain control circuit as claimed in claim 6 , wherein the transimpedance amplifier includes the feedback resistor (R 2 ) and a cascode.
8. The automatic gain control circuit as claimed in claim 7 , wherein the cascode includes an NMOS transistor (M 1 ), an NMOS transistor (MO), and a resistor (R 0 ); wherein a gate of the NMOS transistor (M 1 ) serves as the input terminal Iin of the transimpedance amplifier, a gate of the NMOS transistor (MO) is connected to a bias voltage Vbias; a drain of the NMOS transistor (M 1 ) is connected to a source of the NMOS transistor (MO), a source of the NMOS transistor (M 1 ) is grounded; the feedback resistor (R 2 ) is connected between the gate of the NMOS transistor (M 1 ) and a drain of the NMOS transistor (MO); the resistor (R 0 ) is connected between the drain of the NMOS transistor (MO) and the power supply voltage Vdd.