DIGITAL POWER AMPLIFIER
1. A digital power amplifier comprising two or more individually activatable amplifiers, wherein the outputs of the amplifiers are connected causing an activated amplifier of the two or more amplifiers to load modulate another activated amplifier of the two or more amplifiers.
A digital power amplifier comprising two or more individually activatable amplifiers. The outputs of the amplifiers are connected causing an activated amplifier of the two or more amplifiers to load modulate another activated amplifier of the two or more amplifiers.
- 1. A digital power amplifier comprising two or more individually activatable amplifiers, wherein the outputs of the amplifiers are connected causing an activated amplifier of the two or more amplifiers to load modulate another activated amplifier of the two or more amplifiers.
Embodiments described herein relate generally to digital power amplifiers and in particular to digital power amplifiers using load modulation.
Contemporary communications, broadcast and wireless standards like LTE, DVB/ISDB and 802.11ax, are all based on orthogonal frequency division multiplexing (OFDM) modulation. Although OFDM is very spectrally efficient, it does so at the expense of a high peak-to-average power ratio (PAPR), which for the RF power amplifier (PA) in a transmitter equates to low average efficiency. It is likely that future, fifth generation (5G) standards will have an even greater PAPR, leading to even lower average operating efficiency.
Conventional single device RF PAs (i.e. class A, B, C, E, F and J) are capable of high efficiency only in their saturation region, i.e. peak output power (P). As POUT is reduced, efficiency degrades. In know device the input power may be reduced to the amplifiers to allow accommodating the peaks without clipping them and causing distortion. This is referred to as “backing-off”. Contemporary communications, broadcast and wireless connectivity standards require that on average, a PA operates at significant back-off to accommodate the large signal peaks. These peaks can often be 10 dB larger than the average POUT.
Arrangements of the present invention will be understood and appreciated more fully from the following detailed description, made by way of example only and taken in conjunction with drawings in which:
According to an embodiment there is provided a digital power amplifier comprising two or more individually activatable amplifiers. The outputs of the amplifiers are connected to cause an activated amplifier of the two or more amplifiers to load modulate another activated amplifier of the two or more amplifiers.
The connection that connects the amplifier outputs may in particular be designed so that when more than one amplifier is operating, the operating amplifiers experience load-pull to increase their output power.
The amplifiers can have the same or different supply voltages.
Each amplifier has its own transistor device. The same model of devices can be used for the amplifiers. Alternatively different models may be used.
The outputs of two of the two or more amplifiers are connected by two signal paths.
The outputs of each pairing of two amplifiers of the two or more amplifiers may be connected with two signal paths.
The outputs of two or more amplifiers may be connected by a transmission line network.
The transmission line network can be based on a rat-race combiner. The impedances of the transmission lines can be altered from those of a standard ring combiner to enable said load modulation.
The impedances of transmission lines in the transmission line network may be selected so that the impedance presented to a first activated amplifier by the combination of the transmission line network and any other activated amplifiers sets the output power of the first activated amplifier so that the first activated amplifier operates in saturation.
The transmission line networks can include a phase inverter.
The two or more amplifiers may operate in current saturation at different saturation powers.
The digital power amplifier may further comprise a digital pre-distorter configured to predistort the input signal based on the properties of the combination of the two or more amplifiers and the connection of the amplifier outputs. Signal distortion introduced by the combination of the two or more amplifiers and the connection of the amplifier outputs reduces or removes the pre-distortion introduced by the pre-distorter.
The digital power amplifier may be configured to receive digital signals for individually activating and deactivating the two or more amplifiers, said signals individually applied as gate bias to transistors of the amplifiers.
The digital power amplifier may further comprise a signal splitter configured to split the input signal, wherein individual outputs of the splitter are individually connected to inputs of the amplifiers.
The splitter may split the signal into unequal output signals to operate the amplifiers in saturation.
The two or more amplifiers may be discrete devices. Discrete devices can handle larger output power levels, say around 100 W or more, than CMOS implementations (which are limited to below 100 W).
In an embodiment the two or more amplifiers and their connections are configured to provide output power levels in saturation optimised for a predetermined signal modulation scheme.
According to another embodiment there is provided a device comprising the above described digital power amplifier. The device may be a mobile phone basestation or a DVB transmitter.
In the DPA of embodiments the amplitude of the output RF modulated signal are controlled by digital means. The DPA itself forms the digital-to-analogue converter (DAC) of the transmitter and may also be referred to as an RFDAC.
The present embodiments are capable of achieving three separate POUT levels with around 1 W peak transmit power. Combined with an off-state makes four separate POUT levels are generated, producing a 2-bit DPA.
In embodiments comprising two amplifiers the output network enables the two amplifiers to load-pull each other to achieve the required output powers (POUT) at their respective states, thereby providing full 2-bit operation.
The transmission line network, in particular the connection of the amplifier outputs via two paths that comprise transmission lines, allows the number of states to be increased.
An RF input is applied to the inputs of both amplifiers. Amplifier 1 consists of an active device Q1. In this example the active device is a Field Effect Transistor (FET) device; however, a bipolar transistor or other form of active device may be used instead. The active device Q1 has a gate that is driven by the RF signal through an input matching network (Input MN) and a biasing network. The input matching network is for impedance matching of the input signal.
The biasing networks are connected to respective input nodes through which they receive input respective input signals D0 and D1. D0 and D1 are derived from the input data as shown in
The input signals D0 and D1 are switching signals that turn Amplifiers 1 and 2 on respectively when the signal is high and that turn Amplifiers 1 and 2 off respectively when the signal is low. The biasing networks are configured to convert the respective input digital signals (D0 and D1) into a signal of appropriate voltage for switching the active device on and off.
A first DC signal VDC1 is supplied to a first inductor L1, which in turn is connected to the drain of the active device Q1. The first inductor L1 is a high value inductor, or RF choke, which has high impedance at the RF operating frequency. The source of the active device is connected to ground.
In the present case, an N-channel FET is used. Accordingly, when a low voltage is applied to the gate, the conductivity of the FET drops thereby increasing the voltage at the drain. Conversely, when a high voltage is applied to the gate, the conductivity of the FET increases, lowering the drain voltage towards ground. The active device therefore amplifies the input signal by controlling the proportion of the supply voltage that is output at the drain.
Amplifier 2 is arranged in an identical manner, with a respective input signal D1, biasing network, input matching network, gate Q2 and first inductor L2. Having said this, the two amplifiers are configured to have different peak output powers, as shall be discussed below. To provide improved efficiency across a wide range of input powers, Amplifier 1 and Amplifier 2 are tuned for different power ratings.
In the embodiment the six transmission line sections labelled in
Optimum operating impedances for the transmission lines TL1 to TL6 are determined from the load pull data to achieve the maximum efficiency at the required POUT. Example load-pull data is shown in
During the design process, simple optimisation algorithms are used to determine the dimensions of all the transmission line sections used in the output network, based on the required POUT for the three “on” states (first state: Amplifier 1 ON, Amplifier 2 OFF; second state: Amplifier 1 OFF, Amplifier 2 ON; third state: Amplifier 1 ON, Amplifier 2 ON) and an efficiency target. A number of different combinations exist which met the requirements, representing different output network structures.
Standard rat-race combiners have moderate operating bandwidths. In an embodiment the operating bandwidth is increased by replacing two of the transmission line length (with a combined length of λ/2, equivalent to 180°) which a phase inverter, as shown in
The embodiment of
Embodiments are not limited to the use of only two amplifiers and in other embodiments more than two amplifiers may be connected to each other in a manner that allows the amplifiers involved to load modulate each other. An example of this is shown in
The impedances presented to the transistors of the amplifiers are tuned for a set of particular POUTs. They could for example follow the 6 dB steps size of a conventional DAC or be mapped onto a different set of amplitudes, for example the 16-QAM or 32-APSK constellation diagrams shown in
So far, the transmission line network has been used exclusively to determine the POUT steps. In an embodiment this is expanded by selecting different physically sized devices, or different supply voltages (VDC) for each amplifier.
As far as is known, the embodiments present the only DPAs that exploits load modulation. Other, know DPAs do not use load modulation and do not scale to high powers. Embodiments moreover differ from known analogue load modulating amplifiers in that the amplifiers of the DPAs of the embodiment experience load modulation whenever more than one amplifier is in operation. In Doherty amplifiers in contrast the peaking amplifier only load modulates the carrier amplifier in the region of the amplitude peaks. In Outphasing amplifiers both amplifiers experience an equal amount of load modulation at back-off.
The load modulation operation of this DPA is explained in
In state 1 (a), Amplifier 1 is enabled and Amplifier 2 is disabled. Amplifier 2 appears as just a capacitor CDS. In this 2-bit system, Amplifier 1 is the least significant bit (LSB) and Amplifier 2 the most significant bit (MSB). Amplifier 1 is operating in saturation, the voltage at its output has a maximum swing of VMax Swing. There is an accompanying current flowing into the RF Combining Network I1 Swing. The RF Combining Network provides the correct transformation so the appropriate impedance is presented to the output of Amplifier 1.
In state 2 (b) where only Amplifier 2 is active, Amplifier 1 appears as capacitor CDS. Its operation is similar to state 1, except that this is the MSB, so produces a larger voltage across RL, hence VL2. In this example Amplifier 1 and Amplifier 2 have the same supply voltage (VDC1 and VDC2). It will be noted that I2 Swing is larger than I1 Swing while VMax Swing remains the same.
In state 3 (c), both Amplifier 1 and Amplifier 2 are enabled. The voltage across RL is equal to the sum of the two voltages from first and second states (VL1+VL2). This results in a larger current flowing into RL than the previous two states. Both Amplifier 1 and Amplifier 2 must provide a greater output current as shown by the larger I1 and I2. This is the load modulation effect. To achieve state 3, the impedance presented to the two amplifiers must be lower than that presented in states 1 and 2. This is effectively moving along the POUT contour lines shown in the Smith Chart in
The two or more amplifiers are all driven by the same RF input signal. This will generally have a fixed amplitude, but contain the phase modulation. A power splitter will be used to pass the RF input signal to each amplifier. This can be a splitter with equal ratio splitting or asymmetric with an unequal ratio as required to ensure that all amplifiers operate in saturation. Each amplifier is enabled and disabled by controlling its gate bias. Applying a voltage just above the devices threshold will enable it for amplifying an RF signal applied to its input. Applying a strong negative bias will turn the transistor hard-off.
Embodiments are specifically designed for a high POUT using discrete devices (as opposed to low power CMOS integration), for applications like TV broadcast transmitters. The transmission line architecture used also lends itself well to high power implementations since losses are low and heat can be easily dissipated.
While certain arrangements have been described, the arrangements have been presented by way of example only, and are not intended to limit the scope of protection. The inventive concepts described herein may be implemented in a variety of other forms. In addition, various omissions, substitutions and changes to the specific implementations described herein may be made without departing from the scope of protection defined in the following claims.