# Patent

## Title
Method and apparatus for reducing wake up time of a powered down device

## Abstract

A dummy circuit is provided to which a bias circuit of an low noise amplifier (LNA) can be coupled to during power down of the LNA. The dummy circuit maintains the bias circuit at an approximately normal operating state to reduce wake up time of the LNA.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation application of U.S. patent application Ser. No. 10/753,101, filed Jan. 6, 2004, now U.S. Pat. No. 7,184,799, which claims the benefit of U.S. Provisional Application 60/470,687, filed May 14, 2003. The entire disclosure of each of the prior application is hereby incorporated by reference.


BACKGROUND
A key building block of a wireless radio frequency (RF) transceiver is a low noise amplifier (LNA). A LNA amplifies an RF signal. A bias circuit typically supplies bias signals to the LNA.
FIG. 1 shows a conventional bias circuit 100 coupled to a LNA 102. LNA 102 includes a power source 104, resistors 106-108, an inductor 110, capacitors 112-114, and a bipolar transistor 116. Bias circuit 100 includes a reference current source 118, resistor 120, and a comparative amplifier 122. Bias circuit 100 provides a bias voltage 124 to LNA 102. Comparative amplifier 122 of bias circuit 100 generates bias voltage (Vbias) 124 by comparing a voltage at node 126 in LNA 102 to a reference voltage (Vref) generated by bias circuit 100 at node 128.
FIG. 2 shows an example comparative amplifier 122 in greater detail. Comparative amplifier 122 includes PMOS transistors 200-202 and bipolar transistors 204-208. Comparative amplifier 122 also typically includes a capacitor 210 coupled to node 212. Capacitor 210 generally has a large capacitance to filter high frequency supply noise and prevents the high frequency supply noise from being passed into LNA 102 (FIG. 1), where the high frequency supply noise could adversely affect performance of LNA 102 (FIG. 1).
Wireless RF transceivers are typically powered down (e.g., into a sleep mode) to conserve energy during periods of inactivity. A critical factor in performance of wireless devices is the wake up time of the device. The wake up time of a device is the total amount of time required for a device to return to a normal operating (e.g., functioning) state after having been powered down.
Referring to FIGS. 1 and 2, LNA 102 that is controlled by conventional bias circuit 100 will have a long wake up time due to the large capacitance of capacitor 210. More specifically, when a wireless transceiver is powered down, current flowing through bipolar transistor 116 is shut off (e.g., by pulling the base of bipolar transistor 116, at node 130, low). Once the current flowing through bipolar transistor 116 is shut off, the voltage at node 126 also goes high, and as a result, the voltage at node 212 shifts from a normal operating voltage to VDD (or ground depending upon the design of the comparative amplifier). During wake up, due to the large capacitance at node 212, the charging of capacitor 210 will delay for a significant amount of time the return of node 212 to a normal operating voltage.
SUMMARY
In general, in one aspect, this specification describes a biasing system for biasing a main circuit configured to operate in a first mode and a power down mode. The biasing system includes a circuit configured to maintain a bias circuit in an operating state during a time a main circuit is operating in a power down mode, and a switch circuit configured to switch a bias output of the bias circuit from the main circuit to the circuit when the main circuit is powered down from the first mode to the power down mode.
Particular implementations can include one or more of the following features. The bias circuit can be configured to provide the bias output to the main circuit during a time the main circuit is operating in the first mode. The bias output can be a bias current or a bias voltage. The main circuit can consume less power when operating in the power down mode than when the main circuit is operating in the first mode. The circuit can be configured to maintain a lead to the bias circuit at a predetermined operating voltage level during a time the main circuit is operating in the power down mode. The main circuit can be a low noise amplifier (LNA).
In general, in another aspect, this specification describes a method for biasing a circuit to be powered down. The method includes switching a bias output from a bias circuit coupled to a first circuit to a second circuit, and maintaining the bias circuit at an operating state while the first circuit is powered down.
Particular implementations can include one or more of the following features. The method can further include powering down the first circuit including reducing power consumption of the first circuit, or shutting off current flowing through the first circuit. The first circuit can be an LNA. The second circuit can have a lower current draw than the first circuit. The second circuit can be operational only when the first circuit is powered down. Switching a bias output can include switching a bias current, or a bias voltage, from the first circuit to the second circuit. Maintaining the bias circuit at the operating state can include maintaining a lead to the bias circuit at a predetermined voltage level.
In general, in another aspect, this specification describes a wireless transceiver. The wireless transceiver includes an amplifier configured to receive an input RF signal. The amplifier includes a biasing system comprising a low noise amplifier (LNA) to amplify the input RF signal, a bias circuit configured to provide a bias output to the LNA during a time the LNA is operating in a first mode, a circuit configured to maintain the bias circuit in an operating state during a time the LNA is operating in a power down mode, and a switch circuit configured to switch the bias output from the LNA to the circuit when the LNA is powered down from the first mode to the power down mode.
Particular implementations can include one or more of the following features. The wireless transceiver can be compliant with one or more of the following IEEE standards—802.11, 802.11a, 802.11b, 802.11e, 802.11g, 802.11h, and 802.11i, and 802.14.
A dummy circuit is provided to which a bias circuit of an LNA can be coupled to during power down of the LNA. The dummy circuit maintains the bias circuit at an approximately normal operating state to advantageously reduce wake up time of a LNA that has been powered down. The effect of a large capacitance (in the bias circuit) on the wake up time is reduced.
The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims.



DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic diagram illustrating a conventional bias circuit and LNA.
FIG. 2 is a schematic diagram illustrating a comparative amplifier of the bias circuit of FIG. 1.
FIG. 3 illustrates a biasing system.
FIG. 4 shows a method for controlling a bias circuit during power down of a main circuit.
FIG. 5 illustrates a biasing scheme for a bias circuit of an LNA during a power down.
FIG. 6 illustrates a dummy circuit.
FIG. 7 is a flowchart of a process for LNA bias control during power down.
FIG. 8 is a block diagram of a wireless transceiver.



Like reference symbols in the various drawings indicate like elements.
DETAILED DESCRIPTION
FIG. 3 shows a biasing system 300. Biasing system 300 includes a main circuit 302, a bias circuit 304, a dummy circuit 306, and a switch circuit 308, including a switch control 310. Main circuit 302 is a circuit in a device that operates in at least two modes: a normal mode (i.e., powered on), and a power down mode (e.g., a sleep mode). Bias circuit 304 includes a reference voltage source or a reference current source (not shown), and provides a bias current Ibias and/or a bias voltage Vbias to main circuit 302. Bias circuit 304 further includes a capacitor (not shown) that must be quickly charged (or discharged) to a normal operating state (e.g., to a normal operating voltage). Dummy circuit 306 includes circuitry for maintaining bias circuit 304 at a normal operating state when main circuit 302 is powered down. Switch circuit 308 can be any of a number of conventional switching devices. Switching circuit 308 includes a switch control 310 to switch an output bias of the bias circuit between main circuit 302 and dummy circuit 306.
FIG. 4 shows a method 400 for controlling a bias circuit (e.g., bias circuit 304 of FIG. 3) during power down of a main circuit (e.g., main circuit 302 of FIG. 3). As shown, the main circuit is powered down (step 402). The main circuit can be powered down so that the main circuit consumes less power. Any conventional technique can be used to power down the main circuit (e.g., a shut off switch). An output bias (Ibias, Vbias) of the bias circuit is switched to a dummy circuit (e.g., dummy circuit 306 of FIG. 3) (step 404). Accordingly, at power down of the main circuit, the bias circuit is maintained in a normal operating state (step 406).
FIG. 5 illustrates a biasing system for a bias circuit (e.g., bias circuit 304) to reduce a wake up time of an LNA. The biasing system includes a dummy circuit 500 to which bias circuit 304 can be coupled (or switched by switch circuit 308) during power down of the LNA (e.g., LNA 102 of FIG. 1). Dummy circuit 500 can include any combination and types of CMOS transistors, bipolar transistors, resistors, capacitors, inductors, and the like, and operate to maintain bias circuit 304 at or near a normal operating state during power down of the LNA. For example, dummy circuit 500 can maintain a positive lead 502 of comparative amplifier 122 at a voltage level near a normal operating level. By maintaining positive lead 502 of comparative amplifier 122 at a voltage level near the normal operating level, the voltage at node 212 (FIG. 2) shifts only a small amount, if any, away from a normal operating voltage. In one implementation, dummy circuit 500 is in use (i.e., operational) only when the LNA is powered down.
FIG. 6 illustrates an implementation of dummy circuit 500, formed by a resistor 600 and a bipolar transistor 602. To set a voltage at node 604 for positive lead 502, the magnitude of current flowing through resistor 600 can be controlled as well as controlling physical characteristics of resistor 600 or bipolar transistor 602 (e.g., size). Dummy circuit 500 can draw a small amount of current, and yet provide a large voltage at node 604 by including a relatively large resistance for resistor 600. Thus, dummy circuit 500 can have a low current draw and low power consumption (i.e., a lower current draw and a lower power consumption than LNA 102). Bias voltage 124 is provided as input to a base of bipolar transistor 602, and can be used as well to control the voltage of positive lead 502.
FIG. 7 shows a method 400 for controlling a bias circuit of a LNA. The LNA is powered-down (step 702). The LNA can be powered down so that the LNA consumes less power. An output bias of the bias circuit that is used to bias the LNA is switched to a dummy circuit (step 704). The dummy circuit maintains the bias circuit at a normal operating state. At power up, the output bias of the bias circuit is switched to the LNA (step 708). The LNA is powered up (step 710). The bias circuit is, therefore, operating at approximately near a normal operating state when the LNA is powered up, resulting in a reduced wake up time for the LNA.
FIG. 8 shows a wireless transceiver 800 for communicating information. The receive path of wireless transceiver 800 includes an amplifier 802 for amplifying an input signal 804. Amplifier 802 includes bias circuit 304, LNA 102, dummy circuit 500, and switch circuit 308. A mixer 806 combines an amplified version of input signal 804 with a RF LO signal 808. A filter 810 and adjustable amplifier 812 filter and amplify the combined signal, and mix the generated signal with an Intermediate Frequency (IF) LO signal. An analog-to-digital converter (ADC) 814 converts the mixed signal to a digital signal for further processing. Wireless transceiver 800 can be IEEE 802 compliant with the following standards 802.11, 802.11a, 802.11b, 802.11e, 802.11g, 802.11h, 802.11i, and 802.14.
A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. For example, steps of the methods discussed above may be performed in any order. Accordingly, other implementations are within the scope of the following claims.

## Claims

1. A method for biasing a circuit to be powered down, the method comprising:
switching a bias output from a bias circuit coupled to a first circuit to a dummy circuit, the first circuit including a low noise amplifier that operates in either a normal mode or a sleep mode; and
maintaining the bias circuit at an operating state using the dummy circuit while the first circuit including the low noise amplifier is powered down in the sleep mode including maintaining a positive lead of an amplifier of the bias circuit at a predetermined voltage level, wherein maintaining the positive lead includes maintaining the positive lead coupled between the dummy circuit and the amplifier of the bias circuit at the predetermined voltage level, the amplifier including a comparative amplifier.

2. The method of claim 1, further comprising powering down the first circuit including reducing power consumption of the first circuit.

3. The method of claim 1, further comprising powering down the first circuit including shutting off current flowing through the first circuit.

4. The method of claim 1, further comprising:
powering the first circuit including the low noise amplifier while deactivating the dummy circuit in the normal mode; and
switching the bias output from the dummy circuit back to the first circuit.

5. The method of claim 1, wherein the dummy circuit has a lower current draw than the first circuit.

6. The method of claim 1, wherein the dummy circuit is operational only when the first circuit is powered down.

7. The method of claim 1, wherein switching a bias output includes switching a bias current from the first circuit to the dummy circuit.

8. The method of claim 1, wherein switching a bias output includes switching a bias voltage from the first circuit to the dummy circuit.

9. A method comprising:
providing a bias output to a first circuit by a bias circuit during a time the first circuit is operating in a first mode, the first circuit including a low noise amplifier;
maintaining the bias circuit in an operating state using a dummy circuit during the time the first circuit is operating in a power down mode including maintaining a positive lead coupled between the dummy circuit and a comparative amplifier of the bias circuit at a predetermined voltage level; and
switching the bias output from the first circuit to the dummy circuit when the first circuit is powered down from the first mode to the power down mode.

10. The method of claim 9, further comprising:
powering the dummy circuit only when the first circuit is operating in the power down mode.

11. The method of claim 9, further comprising:
activating the first circuit from the power down mode to the first mode;
switching the bias output from the dummy circuit back to the first circuit; and
deactivating the dummy circuit.

12. A system comprising:
means for switching a bias output from a bias circuit coupled to a first circuit to a means for maintaining the bias circuit at an operating state while powering down the first circuit and activating the means for maintaining, the first circuit including a low noise amplifier that operates in either a normal mode or a sleep mode; and
the means for maintaining including means for maintaining a positive lead of an amplifier of the bias circuit at a predetermined voltage level, wherein the means for maintaining the positive lead includes means for maintaining the positive lead coupled between the dummy circuit and the amplifier of the bias circuit at the predetermined voltage level, the amplifier including a comparative amplifier.

13. The system of claim 12, further comprising:
means for powering down the first circuit and reducing power consumption of the first circuit.

14. The system of claim 12, further comprising:
means for powering down the first circuit including shutting off current flowing through the first circuit.

15. A system comprising:
means for providing a bias output to a first circuit by a bias circuit during a time the first circuit is operating in a first mode, the first circuit including a low noise amplifier;
means for maintaining the bias circuit in an operating state during the time the first circuit is operating in a power down mode including means for maintaining a positive lead coupled between the dummy circuit and a comparative amplifier of the bias circuit at a predetermined voltage level; and
means for switching the bias output from the first circuit to the means for maintaining when the first circuit is powered down from the first mode to the power down mode.

16. The system of claim 15, further comprising:
means for activating the first circuit from the power down mode to the first mode;
means for switching the bias output from the dummy circuit back to the first circuit; and
means for deactivating the dummy circuit.

17. The system of claim 15, wherein the dummy circuit has a lower current draw than the first circuit.