Broadband driver with extended linear output voltage
Modern modulator drivers must be capable of delivering a large output voltage into a tens of ohms modulator, while minimizing the amount of distortion added by the driver. The driver should deliver the output voltage without exceeding a maximum distortion while minimizing the DC power consumption. Accordingly, a modulator driver includes a final stage amplifier with auxiliary transistors that turn on when the conventional differential pair of transistors approaches their maximum voltage of the linear region of their transfer function, thereby providing a more linear transfer function, in particular at large input voltages.
1. A transmitter driver:
a first input and a second input for inputting a differential input voltage;
an amplifier stage for receiving the differential input voltage, including:
a first transistor and a second transistor forming a first differential pair;
a first output connected to the first transistor, and a second output connected to the second transistor for outputting a differential output voltage; and
a first auxiliary transistor connected to the first transistor, and a second auxiliary transistor connected to the second transistor,
a first degeneration resistor extending between the first transistor and the second transistors configured to extend a linear voltage range of the amplifier stage;
wherein the first auxiliary transistor and the second auxiliary transistor are configured to turn on in response to the differential input voltage exceeding a set value, for increasing the differential output voltage, thereby extending a linear region of a transfer function of the amplifier stage.
2. The transmitter driver according to claim 1 , further comprising a first degeneration resistor extending between the first transistor and the second transistors configured to extend a linear voltage range of the amplifier stage.
3. The transmitter driver according to claim 1 , further comprising a buffer stage; wherein the buffer stage comprises a first buffer transistor connected to the first transistor, and a second buffer transistor connected to the second transistor.
4. A transmitter driver comprising:
a first input and a second input for inputting a differential input voltage;
an amplifier stage for receiving the differential input voltage, including:
a first transistor and a second transistor forming a first differential pair;
a first output connected to the first transistor, and a second output connected to the second transistor for outputting a differential output voltage; and
a first auxiliary transistor connected to the first transistor, and a second auxiliary transistors connected to the second transistor,
wherein the first auxiliary transistor and the second auxiliary transistor are configured to turn on in response to the differential input voltage exceeding a set value, for increasing the differential output voltage, thereby extending a linear region of a transfer function of the amplifier stage;
a buffer stage comprising a first buffer transistor connected to the first transistor, and a second buffer transistor connected to the second transistor; and
wherein the buffer stage includes a third buffer transistor connected to the first buffer transistor and the first auxiliary transistor, and a fourth buffer transistor connected to the second buffer transistor and the second auxiliary transistor.
5. The transmitter driver according to claim 4 , further comprising a first shift resistor connected to the third buffer transistor, and a second shift resistor connected to the fourth buffer transistor, for shifting the differential input voltage applied to the first auxiliary transistor and the second auxiliary transistor, wherein the first auxiliary transistor and the second auxiliary transistor are configured to turn on in response to the differential input voltage exceeding the set value, and output a current that is added in parallel to the differential output voltage, thereby extending the linear region of the transfer function of the amplifier stage.
6. The transmitter driver according to claim 5 , further comprising a first capacitor and a second capacitor in parallel with the first shift resistor and the second shift resistor, respectively, configured to increase current from the third buffer transistor and the fourth buffer transistor at increased frequencies.
7. The transmitter driver according to claim 2 , further comprising a second degeneration resistor connected to the first auxiliary transistor, and a third degeneration resistor connected to the second auxiliary transistor.
8. The transmitter driver according to claim 4 , wherein the first buffer transistor and the second buffer transistor are configured to be respectively connected to a bias voltage.
9. The transmitter driver according to claim 4 , further comprising a respective current source connected to each of the first buffer transistor, the second buffer transistor, the first transistor, and the second transistor.
10. The transmitter driver according to claim 9 , wherein the respective current source connected to the first buffer transistor and the second buffer transistor, each comprise a variable current source for controlling a turn-on voltage of the first auxiliary transistor and the second auxiliary transistor.
11. The transmitter driver according to claim 10 , further comprising a feedback loop configured to: sense a bias voltage of the first buffer transistor and the second buffer transistor respectively, compare the bias voltage to a reference voltage, and control the respective current sources connected to the first buffer transistor and the second buffer transistor to turn on the first buffer transistor and the second buffer transistor in response to the differential input voltage exceeding the set value.
12. The transmitter driver according to claim 1 , further comprising a first cascode transistor and a second cascode transistor connected to the first transistor and the second transistor, respectively.
13. The transmitter driver according to claim 1 , wherein the set value is about a maximum differential input voltage of the linear region of the transfer function of the amplifier stage.
14. A transmitter comprising:
a transmitter driver comprising:
a first input and a second input for inputting a differential input voltage;
an amplifier stage for receiving the differential input voltage, including:
a first transistor and a second transistor forming a first differential pair; and
a first output connected to the first transistor, and a second output connected to the second transistor for outputting a differential output voltage; and
a first auxiliary transistor connected to the first transistor, and a second auxiliary transistors connected to the second transistor,
wherein the first auxiliary transistor and the second auxiliary transistor are configured to turn on in response to the differential input voltage exceeding a set value, for increasing the differential output voltage, thereby extending a linear region of a transfer function of the amplifier stage;
a laser for generating an optical signal; and
a modulator for modulating the optical signal in accordance with the differential output voltage.
15. The transmitter according to claim 14 , further comprising a buffer stage;
wherein the buffer stage comprises:
a first buffer transistor including a first terminal connected to the first input, and a third terminal connected to a first terminal of the first transistor; and
a second buffer transistor including a first terminal connected to the second input, and a third terminal connected to a first terminal of the second transistor.
16. The transmitter according to claim 15 , wherein the buffer stage includes:
a third buffer transistor including a first terminal connected to the first terminal of the first buffer transistor, and a third terminal connected to a first terminal of the first auxiliary transistor; and
a fourth buffer transistor including a first terminal connected to the first terminal of the first buffer transistor, and a third terminal connected to a first terminal of the second auxiliary transistor.
17. The transmitter according to claim 16 , further comprising a first shift resistor connected to the third terminal of the third buffer transistor, and a second shift resistor connected to the third terminal of the fourth buffer transistor for shifting the differential input voltage applied to the first terminals of the first auxiliary transistor and the second auxiliary transistors, whereby the first auxiliary transistor and the second auxiliary transistor are configured to turn on in response to the differential input voltage exceeding the set value, and output a current that is added in parallel to the differential output voltage, thereby extending the linear region of the transfer function of the amplifier stage.
18. The transmitter according to claim 14 , wherein the set value is about a maximum differential input voltage of the linear region of the transfer function of the amplifier stage.
19. The transmitter driver according to claim 3 , wherein the buffer stage includes a third buffer transistor connected to the first buffer transistor and the first auxiliary transistor, and a fourth buffer transistor connected to the second buffer transistor and the second auxiliary transistor.
20. The transmitter driver according to claim 4 , wherein the set value is about a maximum differential input voltage of the linear region of the transfer function of the amplifier stage.