Baseband filters for use in wireless communication devices
View Patent ↗An embodiment of a baseband filter in a transmitter subsystem of a wireless device comprises an operational amplifier (op-amp), a pole circuit, a feedback capacitor, and an active device. The op-amp is adapted to produce an amplified signal that includes noise gain produced by the op-amp. The pole circuit is electrically coupled with an output terminal of the op-amp, and is adapted to receive the amplified signal and to attenuate the noise gain to produce a filtered, amplified signal. The feedback capacitor is electrically coupled between the first pole circuit and an input terminal of the op-amp, and is adapted to compensate for a phase shift produced by the pole circuit. The active device is electrically coupled with the pole circuit, and is adapted to amplify the filtered, amplified signal and to produce a baseband filtered output signal.
1. A baseband filter having a baseband filter input node and a baseband filter output node, the baseband filter comprising:
an operational amplifier (op-amp) having a first op-amp input terminal, a second op-amp input terminal, and an op-amp output terminal, wherein the op-amp is adapted to produce, at the op-amp output terminal, an amplified signal that includes noise gain produced by the op-amp;
a first pole circuit having a first pole circuit input node and a first pole circuit output node, wherein the first pole circuit input node is electrically coupled with the op-amp output terminal, and the first pole circuit is adapted to receive the amplified signal and to attenuate the noise gain to produce a filtered, amplified signal at the first pole circuit output node;
a first feedback capacitor having a first terminal and a second terminal, wherein the first feedback capacitor is electrically coupled between the first pole circuit and the first op-amp input terminal, and is adapted to compensate for a phase shift produced by the first pole circuit; and
an active device having an input node and an output node, wherein the input node of the active device is electrically coupled with the first pole circuit output node, and the output node of the active device is electrically coupled with the baseband filter output node, and wherein the active device is adapted to amplify the filtered, amplified signal and to produce a baseband filter output signal at the output node of the active device.
2. The baseband filter of claim 1 , wherein the active device includes a source follower that includes a common drain amplifier having a field effect transistor, with a gate terminal electrically coupled with the input node of the active device, a drain terminal electrically coupled with a voltage reference node, and a source terminal electrically coupled with the output node of the active device.
3. The baseband filter of claim 1 , further comprising:
a second feedback capacitor electrically coupled between the output node of the active device and the first op-amp input terminal.
4. The baseband filter of claim 1 , wherein the first pole filter comprises:
a first resistor having a first terminal and a second terminal, wherein the first terminal of the first resistor is electrically coupled with the first pole circuit input node, and the second terminal of the first resistor is electrically coupled with the first pole circuit output node; and
a capacitor having a first terminal and a second terminal, wherein the first terminal of the capacitor is electrically coupled with the first pole circuit output node, and the second terminal of the capacitor is electrically coupled to ground.
5. The baseband filter of claim 4 , wherein the first pole filter further comprises:
a second resistor serially connected with the first resistor, wherein the first resistor is electrically coupled with the first pole circuit input node through the second resistor, and wherein the first terminal of the first feedback capacitor is electrically coupled with a connection point between the first resistor and the second resistor.
6. The baseband filter of claim 1 , further comprising:
an additional pole circuit electrically coupled between the first pole circuit and the active device.
7. The baseband filter of claim 6 , further comprising:
an additional feedback capacitor electrically coupled between the additional pole circuit and the first op-amp input terminal.
8. The baseband filter of claim 1 , further comprising:
a resistor capacitor filter electrically coupled with the baseband filter input node; and
an additional pole circuit electrically coupled between the resistor capacitor filter and the first op-amp input node.
9. A transmitter subsystem comprising:
a digital-to-analog converter adapted to produce a complex analog signal;
a baseband filter electrically coupled with the digital-to-analog converter and adapted to receive and filter the complex analog signal and to produce a filtered baseband signal, the baseband filter including
a baseband filter input node,
a baseband filter output node,
an operational amplifier (op-amp) having a first op-amp input terminal, a second op-amp input terminal, and an op-amp output terminal, wherein the op-amp is adapted to produce, at the op-amp output terminal, an amplified signal that includes noise gain produced by the op-amp,
a first pole circuit having a first pole circuit input node and a first pole circuit output node, wherein the first pole circuit input node is electrically coupled with the op-amp output terminal, and the first pole circuit is adapted to receive the amplified signal and to attenuate the noise gain to produce a filtered, amplified signal at the first pole circuit output node,
a first feedback capacitor having a first terminal and a second terminal, wherein the first feedback capacitor is electrically coupled between the first pole circuit input node and the first op-amp input terminal, and is adapted to compensate for a phase shift produced by the first pole circuit, and
an active device having an input node and an output node, wherein the input node of the active device is electrically coupled with the first pole circuit output node, and the output node of the active device is electrically coupled with the baseband filter output node, and wherein the active device is adapted to amplify the filtered, amplified signal and to produce a baseband filter output signal at the output node of the active device;
a modulator electrically coupled with the baseband filter, wherein the modulator is adapted to receive and upconvert the filtered baseband signal to produce a radio frequency (RF) signal; and
an amplifier electrically coupled with the modulator, wherein the amplifier is adapted to receive and apply a gain to the RF signal.
10. The transmitter subsystem of claim 9 , wherein the active device includes a source follower that includes a common drain amplifier having a field effect transistor, with a gate terminal electrically coupled with the input node of the active device, a drain terminal electrically coupled with a voltage reference node, and a source terminal electrically coupled with the output node of the active device.
11. The transmitter subsystem of claim 9 , further comprising:
a second feedback capacitor electrically coupled between the output node of the active device and the first op-amp input terminal.
12. The transmitter subsystem of claim 9 , wherein the first pole filter comprises:
a resistor having a first terminal and a second terminal, wherein the first terminal of the resistor is electrically coupled with the first pole circuit input node, and the second terminal of the resistor is electrically coupled with the first pole circuit output node; and
a capacitor having a first terminal and a second terminal, wherein the first terminal of the capacitor is electrically coupled with the first pole circuit output node, and the second terminal of the capacitor is electrically coupled to ground.
13. The transmitter subsystem of claim 9 , further comprising:
an additional pole circuit electrically coupled between the first pole circuit and the active device.
14. The transmitter subsystem of claim 13 , further comprising:
an additional feedback capacitor electrically coupled between an output node of the additional pole circuit and the first op-amp input terminal.
15. A wireless device comprising:
a baseband filter adapted to receive and filter a complex analog signal and to produce a filtered baseband signal, the baseband filter including
a baseband filter input node,
a baseband filter output node,
an operational amplifier (op-amp) having a first op-amp input terminal, a second op-amp input terminal, and an op-amp output terminal, wherein the op-amp is adapted to produce, at the op-amp output terminal, an amplified signal that includes noise gain produced by the op-amp,
a first pole circuit having a first pole circuit input node and a first pole circuit output node, wherein the first pole circuit input node is electrically coupled with the op-amp output terminal, and the first pole circuit is adapted to receive the amplified signal and to attenuate the noise gain to produce a filtered, amplified signal at the first pole circuit output node,
a first feedback capacitor having a first terminal and a second terminal, wherein the first feedback capacitor is electrically coupled between the first pole circuit input node and the first op-amp input terminal, and is adapted to compensate for a phase shift produced by the first pole circuit, and
an active device having an input node and an output node, wherein the input node of the active device is electrically coupled with the first pole circuit output node, and the output node of the active device is electrically coupled with the baseband filter output node, and wherein the active device is adapted to amplify the filtered, amplified signal and to produce a baseband filter output signal at the output node of the active device;
a modulator electrically coupled with the baseband filter, wherein the modulator is adapted to receive and upconvert the filtered baseband signal to produce a radio frequency (RF) signal;
an amplifier electrically coupled with the modulator, wherein the amplifier is adapted to receive and apply a gain to the RF signal; and
an antenna electrically coupled with the amplifier and adapted to receive and transmit an outgoing RF signal over an air interface.
16. The wireless device of claim 15 , further comprising:
a balun, wherein the antenna is electrically coupled with the amplifier through the balun.
17. The wireless device of claim 15 , wherein the active device includes a source follower that includes a common drain amplifier having a field effect transistor, with a gate terminal electrically coupled with the input node of the active device, a drain terminal electrically coupled with a voltage reference node, and a source terminal electrically coupled with the output node of the active device.
18. The wireless device of claim 15 , wherein the first pole filter comprises:
a resistor having a first terminal and a second terminal, wherein the first terminal of the resistor is electrically coupled with the first pole circuit input node, and the second terminal of the resistor is electrically coupled with the first pole circuit output node; and
a capacitor having a first terminal and a second terminal, wherein the first terminal of the capacitor is electrically coupled with the first pole circuit output node, and the second terminal of the capacitor is electrically coupled to ground.
19. The wireless device of claim 15 , further comprising:
an additional pole circuit electrically coupled between the first pole circuit and the active device.
20. The wireless device of claim 19 , further comprising:
an additional feedback capacitor electrically coupled between an output node of the additional pole circuit and the first op-amp input terminal.
21. The wireless device of claim 15 , wherein the wireless device is a device selected from a group of devices that includes a cellular telephone, a radio, a personal data assistant, and a computer.