Variable gain amplifiers with fine attenuation step control and flat signal-to-noise ratio versus attenuation
Variable gain amplifiers (VGAs) with fine attenuation step control and flat signal-to-noise ratio (SNR) versus attenuation are provided. In certain embodiments, a VGA includes an input that receives a radio frequency (RF) input signal, a segmented amplification circuit including multiple amplification cells that operate in parallel to amplify the RF input signal to generate multiple amplified RF signals, an impedance ladder including multiple taps each connected to a different node of the impedance ladder, and switches that control routing of the amplified RF signals to one or more selected taps of the impedance ladder. Accordingly, the VGA uses the switches to connect the outputs of the segmented amplification circuit to the selected tap(s) of the impedance ladder. By changing the tap selection, the attenuation step of the VGA is controlled.
1 . A variable gain amplifier (VGA) comprising:
an input configured to receive a radio frequency (RF) input signal;
a segmented amplification circuit including a plurality of amplification cells configured to amplify the RF input signal to generate a plurality of amplified RF signals at a plurality of outputs;
an impedance ladder including a plurality of taps each connected to a different node of the impedance ladder; and
a plurality of switches connected between the plurality of outputs of the segmented amplification circuit and the plurality of taps of the impedance ladder, the plurality of switches configured to control routing of the plurality of amplified RF signals from the plurality of outputs of the segmented amplification circuit to one or more taps selected from the plurality of taps of the impedance ladder.
2 . The VGA of claim 1 , wherein the impedance ladder is configured to provide an RF output signal to an output, wherein the plurality of taps provide different amounts of impedance through the impedance ladder to the output.
3 . The VGA of claim 1 , wherein the impedance ladder includes a plurality of resistors.
4 . The VGA of claim 1 , wherein the impedance ladder is segmented into a plurality of impedance ladder segments each connected to a corresponding one of the plurality of amplification cells through the plurality of switches.
5 . The VGA of claim 1 , wherein the plurality of amplification cells comprises a plurality of transconductor circuits configured to receive the RF input signal as an RF input voltage and to output the plurality of amplified RF signals as a plurality of RF output currents.
6 . The VGA of claim 1 , further comprising a control circuit configured to select the one or more taps to control the VGA to operate in a selected attenuation setting chosen from a plurality of attenuation settings.
7 . The VGA of claim 6 , wherein all of the plurality of amplification cells amplify the RF input signal for each of the plurality of attenuation settings.
8 . The VGA of claim 1 , wherein the plurality of amplification cells, the plurality of switches, and the impedance ladder are implemented differentially.
9 . A method of variable gain amplification, the method comprising:
receiving a radio frequency (RF) input signal at an input of a variable gain amplifier (VGA);
amplifying the RF input signal using a plurality of amplification cells of a segmented amplification circuit of the VGA to generate a plurality of amplified RF signals at a plurality of outputs of the segmented amplification circuit;
selecting one or more taps from a plurality of taps of an impedance ladder of the VGA; and
routing the plurality of amplified RF signals from the plurality of outputs of the segmented amplification circuit to the one or more selected taps of the impedance ladder using a plurality of switches of the VGA, the plurality of switches connected between the plurality of outputs of the segmented amplification circuit and the plurality of taps of the impedance ladder.
10 . The method of claim 9 , further comprising providing an RF output signal from the impedance ladder to an output of the VGA, wherein the plurality of taps provide different amounts of impedance through the impedance ladder to the output.
11 . The method of claim 9 , further comprising choosing a first selection of the plurality of taps for a first attenuation setting of the VGA using a control circuit of the VGA, and selecting a second selection of the plurality of taps for a second attenuation setting of the VGA using the control circuit.
12 . The method of claim 11 , further comprising amplifying the RF input signal using all of the plurality of amplification cells for both the first attenuation setting and the second attenuation setting.
13 . A transmitter comprising:
a radio frequency (RF) digital-to-analog converter (DAC) configured to convert a digital input signal into an RF input signal; and
a variable gain amplifier (VGA) comprising:
an input configured to receive the RF input signal;
a segmented amplification circuit including a plurality of amplification cells configured to amplify the RF input signal to generate a plurality of amplified RF signals at a plurality of outputs;
an impedance ladder including a plurality of taps each connected to a different node of the impedance ladder; and
a plurality of switches connected between the plurality of outputs of the segmented amplification circuit and the plurality of taps of the impedance ladder, the plurality of switches configured to control routing of the plurality of amplified RF signals from the plurality of outputs of the segmented amplification circuit to one or more taps selected from the plurality of taps of the impedance ladder.
14 . The transmitter of claim 13 , wherein the impedance ladder is segmented into a plurality of impedance ladder segments each connected to a corresponding one of the plurality of amplification cells through the plurality of switches.
15 . The transmitter of claim 13 , further comprising a control circuit configured to select the one or more taps to control the VGA to operate in a selected attenuation setting chosen from a plurality of attenuation settings.
16 . The transmitter of claim 15 , wherein all of the plurality of amplification cells amplify the RF input signal for each of the plurality of attenuation settings.
17 . The transmitter of claim 13 , wherein the impedance ladder is configured to provide an RF output signal to an output of the VGA, wherein the plurality of taps provide different amounts of impedance through the impedance ladder to the output.
18 . The transmitter of claim 17 , further comprising an output impedance matching circuit connected to the output.
19 . The transmitter of claim 13 , further comprising a low pass filter electrically connected between the RF DAC and the VGA and operable to filter the RF input signal.
20 . The transmitter of claim 13 , wherein the plurality of amplification cells comprises a plurality of transconductor circuits configured to receive the RF input signal as an RF input voltage and to output the plurality of amplified RF signals as a plurality of RF output currents.