Active analog front-end
This disclosure provides an active envelope detector to generate an output voltage based on an input radio-frequency (RF) signal. The active envelope detector includes a plurality of transistors configured to operate in a sub-threshold mode and generate an output voltage based on the input RF signal. A delta-modulation analog-to-digital converter (ADC) and a sigma-delta modulation ADC are provided. Both ADCs include an implementation of the active envelope detector to receive input RF signals.
1. A delta-modulation analog-to-digital converter (ADC) comprising:
an active envelope detector configured to receive a radio-frequency (RF) input signal, and comprising:
a first transistor configured to operate in a sub-threshold mode and to generate a reference voltage;
a variable current source configured to generate an output current; and
a second transistor configured to operate in the sub-threshold mode and to generate an output voltage based on the RF input signal and the output current;
a comparator configured to generate a result signal based on a comparison between the reference voltage and the output voltage;
an integrator configured to generate an integrator output signal based the result signal; and
a digital-to-analog converter (DAC) coupled to the integrator and configured to vary the output current of the variable current source based on the integrator output signal.
2. The delta-modulation ADC of claim 1 , wherein the output voltage is exponentially related to the RF input signal.
3. The delta-modulation ADC of claim 1 , wherein the variable current source comprises:
a third transistor configured to vary the output current based on a voltage generated by the DAC.
4. The delta-modulation ADC of claim 3 , wherein the variable current source further comprises:
a fourth transistor configured to generate a fixed current, wherein the output current is based on a current of the third transistor and the fixed current generated by the fourth transistor.
5. The delta-modulation ADC of claim 1 , further comprising:
a multiplier configured to scale the result signal to generate a scaled result signal.
6. The delta-modulation ADC of claim 1 , wherein the comparator is configured to compare the reference voltage and the output voltage from the second transistor in response to a clock signal.
7. The delta-modulation ADC of claim 6 , wherein a frequency of the clock signal is at least ten times greater than a bandwidth of the RF input signal.
8. The delta-modulation ADC of claim 6 , wherein the integrator is configured to generate an output signal of the delta-modulation ADC based on an average of a magnitude of a current result signal and magnitudes of one or more previous result signals.
9. The delta-modulation ADC of claim 1 , wherein the first transistor is a diode-connected field effect transistor configured to generate the reference voltage.
10. A sigma-delta modulation analog-to-digital converter (ADC) comprising:
an active envelope detector configured to receive a radio-frequency (RF) input signal, and comprising:
a first transistor configured to operate in a sub-threshold mode and generate a first reference voltage;
a variable current source configured to generate an output current; and
a second transistor configured to operate in the sub-threshold mode and to generate an output voltage based on the RF input signal and the output current;
an analog integrator configured to generate an error signal based on a difference between the first reference voltage and the output voltage;
a comparator configured to generate a result signal based on the error signal and a second reference voltage; and
a digital low-pass filter configured to filter the result signal and to generate an output signal of the sigma-delta modulation ADC.
11. The sigma-delta modulation ADC of claim 10 , further comprising:
a digital-to-analog converter (DAC) configured to generate a DAC voltage based on the output signal of the sigma-delta modulation ADC.
12. The sigma-delta modulation ADC of claim 11 , wherein the variable current source comprises:
a third transistor configured to vary the output current based on the DAC voltage.
13. The sigma-delta modulation ADC of claim 12 , further comprising:
a fourth transistor configured to generate a fixed current, wherein the output current of the variable current source is based on a current generated by the third transistor and the fixed current generated by the fourth transistor.
14. The sigma-delta modulation ADC of claim 10 , wherein the first transistor is a diode-connected field effect transistor configured to generate the first reference voltage.
15. The sigma-delta modulation ADC of claim 10 , wherein the comparator is configured to compare the error signal and the second reference voltage in response to a clock signal.
16. The sigma-delta modulation ADC of claim 15 , wherein a frequency of the clock signal is at least ten times greater than a bandwidth of the RF input signal.
17. The sigma-delta modulation ADC of claim 10 , wherein the output voltage is exponentially related to the RF input signal.
18. An active envelope detector to detect a radio-frequency (RF) input signal comprising:
a first transistor configured to operate in a sub-threshold mode and to generate a reference voltage;
a variable current source configured to generate an output current;
a second transistor configured to operate in the sub-threshold mode and to generate an output voltage based on the RF input signal and the output current; and
a clocked comparator configured to compare the reference voltage to the output voltage in response to a clock signal.
19. The active envelope detector of claim 18 , wherein the variable current source comprises:
a third transistor configured to vary the output current based on a voltage received from a digital-to-analog converter (DAC).