Analog switch for RF front end
Techniques for improving the linearity of radio-frequency (RF) front-end switches. In an aspect, open-loop techniques are disclosed for superimposing the output voltage of one or more negative rectifiers on a negative substrate bias voltage to reduce the non-linearities associated with voltage-dependent substrate leakage current. In another aspect, closed-loop techniques are further disclosed for maintaining the substrate bias voltage close to a reference voltage. Exemplary embodiments of the circuit blocks are further described.
1. An apparatus comprising:
a transistor having a substrate; and
a negative rectifier configured to rectify a negative portion of a voltage related to a drain-to-source voltage of the transistor,
the substrate coupled via a first resistor to a constant bias voltage and coupled via a second resistor to an output of the negative rectifier.
2. The apparatus of claim 1 , further comprising a gain element configured to generate the voltage related to the drain-to-source voltage of the transistor as a gain voltage proportional to the drain-to-source voltage of the transistor.
3. The apparatus of claim 2 , the gain element comprising a series resistive divider, wherein at least one resistor of the series resistive divider has an adjustable resistance.
4. The apparatus of claim 2 , further comprising:
closed-loop circuitry for adjusting a gain of the gain element such that a substrate bias voltage of the transistor is approximately equal to a reference voltage.
5. The apparatus of claim 4 , the closed-loop circuitry comprising:
a low-pass filter coupled to the substrate bias voltage; and
operational amplifier circuitry having a first input coupled to the low-pass filter and having a second input coupled to the reference voltage, wherein the operational amplifier circuitry is coupled to a resistor in the gain element that has an adjustable resistance.
6. The apparatus of claim 2 , the gain element comprising a capacitive divider.
7. The apparatus of claim 1 , the negative rectifier comprising a negative diode rectifier.
8. The apparatus of claim 7 , the negative diode rectifier comprising a negative voltage doubler.
9. The apparatus of claim 1 , further comprising a plurality of transistors coupled in series with the transistor, wherein each of the plurality of transistors is coupled to the negative rectifier.
10. The apparatus of claim 9 , wherein gates of the plurality of transistors are coupled to a negative bias voltage.
11. An apparatus comprising:
means for rectifying a negative portion of a voltage related to a drain-to-source voltage of a transistor to generate a rectified voltage;
means for coupling the rectified voltage to a substrate of the transistor;
means for generating the voltage related to the drain-to-source voltage of the transistor as a voltage proportional to the drain-to-source voltage of the transistor; and
means for adjusting the voltage proportional to the drain-to-source voltage to maintain a constant rectified voltage.
12. The apparatus of claim 11 , the means for rectifying the negative portion comprising means for coupling the voltage proportional to the drain-to-source voltage to a negative voltage doubler.
13. The apparatus of claim 12 , the means for rectifying the negative portion further comprising a negative diode rectifier.
14. A method comprising:
rectifying a negative portion of a voltage proportional to a drain-to-source voltage of a transistor to generate a rectified voltage; and
coupling the rectified voltage via a first resistor to a substrate of the transistor, wherein the substrate of the transistor is coupled via a second resistor to a constant bias voltage.
15. The method of claim 14 , wherein rectifying the negative portion comprises coupling the voltage proportional to the drain-to-source voltage to a negative voltage doubler.
16. The apparatus of claim 7 , the negative diode rectifier comprising:
an input terminal;
an output terminal;
a capacitor coupled to ground and coupled to the output terminal;
a resistor coupled to ground and coupled to the output terminal; and
a diode coupled to the input terminal and coupled to the output terminal.
17. The apparatus of claim 1 , wherein the transistor is included in a shunt switch configured to ground voltages associated with an inactive signal path of a transceiver.
18. The apparatus of claim 17 , wherein the inactive signal path is tuned to a first frequency band of a plurality of frequency bands.
19. The apparatus of claim 11 , wherein the means for adjusting the voltage proportional to the drain-to-source voltage of the transistor maintains a substantially constant rectified voltage.