Low-noise transmitter system and method
View Patent ↗A low-noise transmitter architecture and method for high linearity, high output-swing systems such as Asymmetrical Digital Subscriber Line (ADSL) systems. The transmitter uses a switched-current DAC having a current source coupled to ground, followed by a resistive transimpedance amplifier (TIA). The resistance of the current source is typically large enough so that noise from an op-amp included in the TIA is not significantly amplified at the output. The current source may be passive and may include at least one resistor connected to ground. With a passive current source, portions of a signal output by the DAC enter either the current source or the resistive transimpedance amplifier, but not both, eliminating noise in the system produced by the current source.
1. A transmitter, comprising:
(a) a digital-to-analog converter (DAC);
(b) a current source coupled to an output of the DAC and to ground; and
(c) a resistive transimpedance amplifier coupled to the output of the DAC,
wherein a gain of the resistive transimpedance amplifier is approximately equal to a feedback resistance within the resistive transimpedance amplifier.
2. The transmitter of claim 1 , wherein the current source is an active current source.
3. The transmitter of claim 1 , wherein the current source is a passive current source.
4. The transmitter of claim 3 , wherein the current source comprises at least one resistor coupled to ground.
5. The transmitter of claim 4 , wherein a resistance of the current source is determined by the amount of noise produced by an operational amplifier in the resistive transimpedance amplifier.
6. The transmitter of claim 5 , wherein the resistance of the current source is approximately 1 kΩ.
7. The transmitter of claim 1 , wherein the current source establishes a common mode level for the transmitter.
8. The transmitter of claim 1 , wherein the resistive transimpedance amplifier comprises:
(i) an operational amplifier;
(ii) a first feedback resistor coupled in parallel to a positive path of the operational amplifier; and
(iii) a second feedback resistor coupled in parallel to a negative path of the operational amplifier.
9. The transmitter of claim 8 , wherein the first and second feedback resistors are variable resistors.
10. An amplification system for amplifying a digital signal, comprising:
(a) a digital-to-analog converter (DAC);
(b) a current source coupled to the output of the DAC and to ground; and
(c) a resistive transimpedance amplifier coupled to the output of the DAC,
wherein a gain of the amplification system is approximately equal to a feedback resistance within the resistive transimpedance amplifier.
11. The amplification system of claim 10 , wherein the digital signal is an ADSL signal.
12. The amplification system of claim 10 , wherein the current source is an active current source.
13. The amplification system of claim 10 , wherein the current source is a passive current source.
14. The amplification system of claim 13 , wherein the current source comprises at least one resistor coupled to ground.
15. The amplification system of claim 14 , wherein a resistance of the current source is determined by the amount of noise produced by an operational amplifier in the resistive transimpedance amplifier.
16. The amplification system of claim 15 , wherein a resistance of the current source is approximately 1 kΩ.
17. The amplification system of claim 10 , wherein the resistive transimpedance amplifier comprises:
(i) an operational amplifier;
(ii) a first feedback resistor coupled in parallel to a positive path of the operational amplifier; and
(iii) a second feedback resistor coupled in parallel to a negative path of the operational amplifier.
18. A transmitter, comprising:
(a) a digital-to-analog converter (DAC);
(b) a resistive transimpedance amplifier;
(c) a signal path connecting the DAC and the resistive transimpedance amplifier; and
(d) a passive current source coupled between the signal path and ground,
wherein portions of a signal traversing the signal path enter either the current source or the resistive transimpedance amplifier.
19. The transmitter of claim 18 , wherein the current source comprises at least one resistor coupled to ground.
20. The transmitter of claim 19 , wherein a resistance of the current source is determined by the amount of noise produced by an operational amplifier in the resistive transimpedance amplifier.
21. The transmitter of claim 20 , wherein a resistance of the current source is approximately 1 kΩ.
22. The transmitter of claim 18 , wherein the resistive transimpedance amplifier comprises:
(i) an operational amplifier;
(ii) a first feedback resistor coupled in parallel to a positive path of the operational amplifier; and
(iii) a second feedback resistor coupled in parallel to a negative path of the operational amplifier.
23. The transmitter of claim 22 , wherein a gain of the transmitter is approximately equal to a feedback resistance within the resistive transimpedance amplifier.
24. A method of amplifying a digital signal, comprising:
(a) converting the digital signal to an analog voltage-based signal;
(b) amplifying the analog voltage-based signal using a resistive transimpedance amplifier whose gain is approximately equal to a feedback resistance within the transimpedance amplifier;
(c) outputting the amplified voltage-based signal; and
(d) driving the analog voltage-based signal with a current source connected to ground.
25. The method of claim 24 , wherein said step (d) comprises:
driving the analog voltage-based signal with an active current source.
26. The method of claim 24 , wherein said step (d) comprises:
driving the analog voltage-based signal with a passive current source.