Bypass power amplifier for improving efficiency at low power

Embodiments of a two-stage bypass power amplifier are provided. In general, the two-stage bypass power amplifier is configured to receive a RF signal that is to be transmitted to a remote device and provide gain to the RF signal prior to the RF signal being transmitted to the remote device. The two-stage bypass power amplifier is configured to operate efficiently (in terms of power) at two different gain or output power levels and can be extended to operate efficiently at additional gain or output power levels.

FIELD OF THE INVENTION

This application relates generally to power amplifiers and more particularly to a two-stage bypass power amplifier.

BACKGROUND

Mobile devices, such as cell phones, are generally designed to transmit a radio frequency (RF) signal at a high output power level when located far away from a receiving base station (or when the transmission channel is noisy) to ensure adequate reception of the signal. Conversely, mobile devices are generally designed to transmit a RF signal at a comparatively lower output power level when located close to the receiving base station (or when the transmission channel is less noisy). Mobile devices can determine the power level at which to transmit the RF signal by, for example, using information received from the base station.

A power amplifier is used in a mobile device to amplify the RF signal prior to transmitting it to the base station. Therefore, depending on the distance of the mobile device from the base station (or the conditions of the channel between the mobile device and base station) the gain of the power amplifier can be adjusted to provide more or less amplification of the RF signal prior to transmission. For example, if the distance of the mobile device from the base station is relatively small, the gain of the power amplifier can be reduced to provide less amplification of the RF signal than if the distance of the mobile device from the base station was comparatively larger. Adjusting the gain of the power amplifier depending on the distance of the mobile device from the base station (or conditions of the channel) allows the mobile device to conserve power. In other words, the power amplifier does not need to continually operate under the assumption of worst case operating conditions (e.g., under the assumption that the mobile device is located at a far distance from the base station) and the gain of the power amplifier can be reduced, when operating conditions permit, to conserve power.

However, the power efficiency of a typical power amplifier does not scale linearly with its gain. In general, the typical power amplifier, and the impedance matching network at the output of the power amplifier, are designed to provide maximum power efficiency at a single operating gain. This single operating gain is typically set at the maximum gain that the power amplifier is expected to operate at (or at least at a very high gain). Assuming the power amplifier is designed to operate most efficiently at this high gain setting, any reduction in the gain of the power amplifier will lead to a reduction in the power amplifier's efficiency. Therefore, although reducing the gain of the power amplifier can reduce its power consumption, often the reduction in power consumption is not as significant as desired because of the further reduction in the power amplifier's efficiency.

For example, a power amplifier designed to operate at a high gain level may amplify the power of a RF signal from 1 mW to 500 mW with 40% power efficiency. At 40% power efficiency, the power amplifier will consume around 1.25 W of power. If the gain of the same power amplifier is reduced, however, such that the power of the RF signal is amplified from 1 mW to only 50 mW (a ten times reduction in the gain), the power efficiency of the power amplifier may fall to 5%. At 5% power efficiency, the power amplifier will consume around 1 W. Thus, although the gain of the power amplifier was reduced by a factor of ten, the power consumed by the power amplifier was only reduced by 20%.

Therefore, what is needed is a system and a method for improving the efficiency of power amplifiers used when operating at lower gains or lower output power levels.

DETAILED DESCRIPTION

FIG. 1illustrates a two-stage bypass power amplifier (PA)100, according to embodiments of the present invention. Two-stage bypass PA100can be implemented in a number of different wired or wireless communication devices including, for example, mobile phones, personal digital assistants (PDAs), laptop computers, and desktop computers. In general, two-stage bypass PA100is configured to receive a RF signal that is to be transmitted to a remote device and provide gain to the RF signal prior to the RF signal being transmitted to the remote device. Two-stage bypass PA100is configured to operate efficiently (in terms of power) at two different gain or output power levels and can be extended to operate efficiently at additional gain or output power levels as will be appreciated by one of ordinary skill in the art based on the teachings herein.

As illustrated inFIG. 1, two-stage bypass PA100specifically includes a first PA105, a first switch (S1)110, a gain path115, and a gain path120. By utilizing two different gain paths, each path can be designed to operate efficiently at a different gain. Gain path115includes an input matching network125, a second PA130, and an output matching network135. Gain path120includes an output matching network140, a second switch (S2)145, and a quarter wavelength transmission line150, where the quarter wavelength transmission line150is determined at the frequency of interest (e.g. center frequency) of the input RF signal. Gain path115is configured to efficiently (in terms of power) provide the RF signal at a first power level to a transducer (not shown), such as an antenna or a cable, and second gain path115is configured to efficiently (in terms of power) provide the RF signal at a second power level to the transducer. The first power level is greater than the second power level.

In specific operation of two-stage bypass PA100, an input RF signal to be transmitted is initially received by first PA105. First PA105is configured to amplify the RF signal to provide a first amplified RF signal. Switch110is then configured to couple the first amplified RF signal to one of the two gain paths (i.e., either gain path115or120) depending on the desired output power level for the RF signal.

Assuming a high output power level is desired, switch110is controlled to bypass gain path120by coupling the first amplified RF signal to gain path115. Switch145is further controlled to couple a first end of quarter wavelength transmission line150to ground. By coupling the first end of quarter wavelength transmission line150to ground, the second end of quarter wavelength transmission line150appears as a high impedance (and, in theory, an infinite impedance) to the RF signal provided at the output of gain path115. Thus, quarter wavelength transmission line150substantially prevents the load of gain path120, which includes output matching network140, from interfering with the RF signal provided at the output of gain path115.

After switches110and145are controlled as described above, the first amplified RF signal is received by input matching network125from first PA105. In general, input matching network125is configured to match the output impedance of first PA105to the input impedance of second PA130. By matching the output impedance of first PA105to the input impedance of second PA130reflections are minimized and power transfer is maximized. Input matching network125can include one or more capacitive, inductive, and/or resistive components.

After being processed by input matching network125, the first amplified RF signal is received by second PA130. Second PA130is configured to amplify the first amplified RF signal to provide a second amplified RF signal. The second amplified RF signal is then provided to the transducer, via output matching network135, for transmission. In general, output matching network135is configured to match the output impedance of second PA130to the input impedance of the transducer. By matching the output impedance of second PA130to the input impedance of the transducer reflections are minimized and power transfer is maximized. Output matching network135can include one or more capacitive, inductive, and/or resistive components.

Assuming now that a low output power level is desired, switch110is controlled to bypass gain path115by coupling the first amplified RF signal to gain path120. Switch145is further controlled to couple a first end of quarter wavelength transmission line150to the output of output matching network140. The first end of quarter wavelength transmission line150appears as a low impedance (and, in theory, a zero impedance path) to the RF signal provided at the output of matching network140and couples the RF signal to the transducer.

After switches110and145are controlled as described above, the first amplified RF signal is received by output matching network140from first PA105. In general, output matching network140is configured to match the output impedance of first PA105to the input impedance of the transducer. By matching the output impedance of first PA105to the input impedance of the transducer reflections are minimized and power transfer is maximized. Output matching network140can include one or more capacitive, inductive, and/or resistive components.

Importantly, it should be noted that second PA130can be turned off when gain path115is being bypassed and gain path120is being utilized to conserve power.

It should be further noted that quarter wavelength transmission line150provides an effective means for isolating the output of gain path115from the load presented by gain path120and its use has several associated benefits. For example, the use of quarter wavelength transmission line150as an isolation means allows two-stage bypass PA100to be more efficiently implemented in a conventional complementary metal oxide semiconductor (CMOS) process. More specifically, because the output RF signal from gain path115can have a significant voltage magnitude (e.g., 15-20 V) a typical MOS switch in a CMOS process cannot, by itself, be used to provide isolation because at such high voltages conventional MOS switches break down. Although several MOS switches can be stacked together in cascode form to absorb the potentially large voltages at the output of gain path115and provide isolation, such an implementation is inefficient in terms of power.

FIG. 2illustrates one exemplary implementation of quarter wavelength transmission line150that substantially overcomes the issue described above, according to embodiments of the present invention. As illustrated inFIG. 2, quarter wavelength transmission line150includes a capacitor C1, a first inductor L1, and a second inductor L2. Capacitor C1is coupled between the first and second ends of quarter wavelength transmission line150, inductor L1is coupled to the first end of quarter wavelength transmission line150and ground, and inductor L2is coupled to the second end of quarter wavelength transmission line150and ground. In one embodiment, capacitor C1can be fully integrated on a chip together with the rest of two-stage bypass amplifier100, illustrated inFIG. 1, and inductors L1and L2can be implemented using bond wires coupled to the chip and ground.

Referring now toFIG. 3, a flowchart300of a method for operating a two-stage bypass amplifier is illustrated, according to embodiments of the present invention. Flowchart300is described with continued reference to exemplary two-stage bypass amplifier100depicted inFIG. 1. However, flowchart300is not limited to that embodiment.

Flowchart300starts at step305and transitions to step310. In step310, an input RF signal to be transmitted is amplified using first PA105to provide a first amplified RF signal.

In step315, a determination is made as to whether a high output transmission power (or alternatively a low output transmission power) is desired. If a low output transmission power is desired, flowchart300transitions to step320. If, on the other hand, a high output transmission power is desired, flowchart300transitions to step325.

Assuming a low output transmission power is desired, flowchart300transitions from step315to step320. At step320, switch110is controlled to bypass gain path115by coupling the first amplified RF signal to gain path120, and switch145is controlled to couple a first end of quarter wavelength transmission line150to the output of output matching network140. The first end of quarter wavelength transmission line150appears as a low impedance to the RF signal provided at the output of matching network140and couples the RF signal to the transducer.

Assuming a high output transmission power is desired, flowchart300transitions from step315to step325. At step325, switch110is controlled to bypass gain path120by coupling the first amplified RF signal to gain path115, and switch145is controlled to couple a first end of quarter wavelength transmission line150to ground. By coupling the first end of quarter wavelength transmission line150to ground, the second end of quarter wavelength transmission line150appears as a high impedance (and, in theory, an infinite impedance) to the RF signal provided at the output of gain path115. Thus, quarter wavelength transmission line150substantially prevents the load of gain path120, which includes output matching network140, from interfering with the RF signal provided at the output of gain path115. After the switches are controlled as described above, the first amplified signal provided by first PA105is processed by input matching network125and second PA130. Second PA130is configured to amplify the first amplified RF signal to provide a second amplified RF signal. The second amplified RF signal is then provided to the transducer, via output matching network135, for transmission.

The present invention has been described above with the aid of functional building blocks illustrating the implementation to interpret the claims. The Abstract section may set forth one or more but not all exemplary embodiments of the present invention as contemplated by the inventor(s), and thus, is not intended to limit the present invention and the appended claims of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed.