JP2008252267A - High-frequency power amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-frequency power amplifier in which an output power detector having a simple configuration and easily mounted controls an impedance variable element of an output matching circuit. <P>SOLUTION: Power consumption Ip of an FET 16 is detected as a monitor voltage Vp based on voltage fall by a resistor 20, as information source for controlling a variable capacity diode in the matching circuit 30 configuring a load impedance, and capacitance as impedance of the variable capacity diode provided in the matching circuit 30 is changed by a control voltage Vc corresponding to the detected monitor voltage Vp so that power efficiency may become large. Thus, the load impedance can be changed so that the power efficiency can increase in response to the output high-frequency power. The resistor 20 can be easily mounted because it has a simple configuration and is inserted into a power supply side. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a high-efficiency high-frequency power amplifier used in a transmission output stage of a wireless device or the like.

  The maximum output power and efficiency of the high-frequency power amplifier (efficiency = output high-frequency power / power consumption of the high-frequency power amplifier) vary depending on the impedance of the output matching circuit. It is impossible to obtain maximum output power and maximum efficiency at the same time, and these are in a trade-off relationship.

  Portable wireless communication devices such as mobile phones and wireless LANs always control transmission power according to the distance from the base station from the viewpoint of radio wave utilization efficiency and power consumption. In addition, when a digital modulated wave having an amplitude fluctuation component (digital modulated wave such as QPSK, QAM, OFDM, etc.) is amplified, the transmission power fluctuates even in an extremely short time.

  Usually, a high-frequency power amplifier for transmission is designed so as to obtain high efficiency and necessary low distortion characteristics when the transmission power is maximum, so that efficiency decreases when transmission power is reduced.

  Generally, a high output is obtained when the load impedance viewed from the output transistor of the high-frequency power amplifier is low, while a high output is obtained when the load impedance is high.

  As a method for improving the efficiency at the time of low output using this relationship, a method of varying the load impedance has been proposed (Patent Documents 1, 2, and 3).

  Patent Document 1 describes a high-frequency power amplifier that changes the load impedance of an output matching circuit based on information on transmission power supplied to an output setting terminal.

  Patent Document 2 describes a power amplifier that changes the load impedance of an amplifying transistor according to input power.

  Patent Document 3 describes a power amplifier that detects the amplified output power of a transistor and changes the load impedance of the output matching circuit by changing the voltage applied to the variable capacitance diode whose anode is grounded by the detection output. .

Japanese Patent Laid-Open No. 11-220338 (FIG. 2) Japanese Unexamined Patent Publication No. 2000-174559 (FIG. 1) Japanese Patent Laying-Open No. 2005-79967 (FIG. 4)

  However, the technique according to Patent Document 1 has a problem that an external output setting signal is required as an information source for controlling load impedance.

  Further, in the technique according to Patent Document 2, the input power is extremely small compared to the output power, so that the level cannot be detected accurately, and the input power is not exactly proportional to the output power. There is a problem that matching cannot be optimized.

  Furthermore, the technique according to Patent Document 3 requires a high-frequency detection circuit in order to obtain a detection output, and the high-frequency detection circuit has a problem that mounting design on a signal line is difficult and costly. In the technique according to Patent Document 3, since the impedance is changed by changing the reverse voltage of the variable capacitance diode whose anode is grounded, the variable capacitance diode becomes forward biased when the output power is large. There is a problem that distortion is increased in a signal having a large amplitude fluctuation component.

  The present invention has been made in consideration of such problems, and a high-frequency power amplifier capable of changing a load impedance so as to increase power efficiency in accordance with output power with a simple and easy configuration. The purpose is to provide.

  A high frequency power amplifier according to the present invention includes a transistor that amplifies an input high frequency signal and outputs the amplified high frequency signal, and a matching circuit that is provided on the output side of the transistor and matches an output impedance of the transistor and a load. The power amplifier includes a consumption current detection circuit that detects a consumption current supplied from a power source to the transistor, and an impedance variable element provided in the matching circuit, and is in accordance with the consumption current detected by the consumption current detection circuit. Then, the impedance of the variable impedance element is changed.

  According to the present invention, the current consumption of the transistor is detected as an information source for controlling the load impedance. A field effect transistor or a bipolar transistor is selected as the high-frequency transistor. In an amplifier biased to class AB, class B, or class C, the current that flows through the collector (or drain) of the transistor, that is, the consumption current, changes according to the output high-frequency power. Therefore, the current consumption is detected by the current consumption detection circuit, and the impedance of the impedance variable element provided in the matching circuit is changed according to the detected current consumption so as to increase the power efficiency. The load impedance can be varied to increase power efficiency. Since the current consumption is only required to detect the current flowing out of the power supply, the current consumption detection circuit can be configured with a simple and easy configuration.

  For example, the consumption current is supplied from the power supply to the transistor through a choke inductor, and the consumption current detection circuit detects the consumption current flowing from the power supply through the choke inductor between the power supply and the choke inductor. By doing so, the mounting is easy and the configuration is simple.

  Specifically, a small resistor for current-voltage conversion can be inserted between the power supply and the choke inductor, and the voltage appearing at both ends of the small resistor can be detected.

  Of course, a transformer (for example, a transformer configured to detect a current flowing in the wiring between the power source and the choke inductor with a configuration similar to that of a current probe using a Hall element capable of measuring current from a direct current, or 1 The secondary coil is a wiring between the power supply and the choke inductor, and the secondary coil is connected in parallel with the coil and a resistor. Further, the voltage at both ends of the resistor is integrated by an integrator to obtain a DC fluctuation value. It is also possible to detect the current consumption flowing out of the power supply by means of a current detector using the principle of the transformer etc. of the configuration.

  Even in this case, current consumption can be detected with an easy configuration and a simple configuration.

  The variable impedance element may include a variable capacitance diode, and the variable capacitance diode may be connected in parallel with the load or connected in series with the load.

  If comprised in this way, the variable impedance diode contained in an output matching circuit can be driven with the detection voltage according to consumption current, and load impedance can be changed.

  In the configuration in which the variable capacitance diode is connected in series with the load, the high-frequency signal component passes through the variable capacitance diode, so that no high voltage is generated at both ends of the variable capacitance diode, so that a large signal can be amplified.

  According to the present invention, the load impedance is changed so as to increase the power efficiency in accordance with the consumption current corresponding to the output power. Therefore, the load impedance can be changed with a simple and easy configuration. .

  First, it will be described that the power efficiency is increased by changing the load impedance in accordance with the output power of the transistor.

  FIG. 4 shows the load line 2 and current voltage waveforms Id and Vd for an ideal transistor in which the drain current Id changes in proportion to the change in the gate voltage Vg. The load line 2 is a load line when a field effect transistor (FET) is operated in a class B with a power supply voltage of 20 [V] and a maximum drain current of 2.0 [A]. The output impedance is 10 [Ω] (= 20 [V] /2.0 [A]).

  When this FET is driven at maximum with a sine wave, the half-wave operating voltage waveform Vd and the half-wave operating current waveform Id of the FET are Vd = 20−20 sin θ and Id = 2 sin θ, and the average power consumption Pt in one period (2π). Pt = ∫ (Vd · Id) dθ / 2π = 2.73 [W] (the integration range is 0 to π).

  The output power Po at this time is Po = (20 / √2 [V] × 2 / √2 [A]) / 2 = 10.0 [W].

  Therefore, the efficiency is η = Po / (Pt + Po) = 79 [%]. This is the maximum theoretical efficiency of a class B amplifier.

  FIG. 5 shows the load line 4 and the current-voltage waveform when the input power is reduced to 2.5 [W], which is 1/4, with the same bias and the same output impedance.

  In this case, the half-wave operating voltage waveform Vd = 20−20 sin θ and the half-wave operating current waveform Id = 1 sin θ, and the average power consumption Pt in one cycle is Pt = ∫ (Vd · Id) dθ / 2π = 3.87 [ W] (the integration range is 0 to π).

  The output power Po at this time is Po = (10 / √2 [V] × 1 / √2 [A]) / 2 = 2.5 [W].

  Therefore, the efficiency is η = Po / (Pt + Po) = 39 [%], and is reduced to about ½.

  FIG. 6 shows the load line 6 and the current voltage when the output impedance is changed to 40 (= 20 [V] /0.5 [A]) [Ω] when the output power is reduced to ¼. Waveforms Vd and Id are shown.

  In this case, Vd = 20−20 sin θ, Id = 0.5 sin θ, and the average power consumption Pt in one cycle is Pt = ∫ (Vd · Id) dθ / 2π = 0.683 [W] (the integration range is 0 to π).

  The output power Po at this time is Po = (20 / √2 [V] × 0.5 / √2 [A]) / 2 = 2.5 [W].

  Therefore, the efficiency is η = Po / (Pt + Po) = 79 [%], and the maximum efficiency 79 [%] can be maintained. Thus, high efficiency can be maintained by changing the output impedance (load impedance) according to the output power. This method is called a load modulation method. In the present invention and the embodiments described below, this load modulation method is applied.

  That is, for example, assuming a signal in which the output power change of the class B amplifier (amplitude change of the output envelope) changes linearly from 2.5 [W] to 10 [W], the load impedance is If it is not changed, an average efficiency of only 59 [%] (= (39 + 79) / 2) can be obtained. However, by changing the load impedance so that the power efficiency becomes ideal according to the change in the output power, the efficiency can be ideally increased by 20 [%]. In practice, the range of increase in efficiency is limited by inaccuracy of variable load impedance, mismatch of output matching, change in transistor dynamic characteristics, and the like.

  In the embodiment described below, the load impedance at the high frequency employs a method of changing the capacitance value as in Patent Documents 1 and 3.

  FIG. 1 is a circuit diagram showing a configuration of a high-frequency amplifier circuit 10 according to an embodiment of the present invention.

  A circuit configuration using a field effect transistor (FET) 16 as a power amplifying element is shown.

  The FET 16 biased to class AB, class B or class C has a gate terminal connected to the input terminal 12 via the input matching circuit 14, and the input terminal 12 is connected to the signal source 8 having a characteristic impedance of 50 [Ω]. Has been. The drain terminal is connected to a power supply terminal 22 to which a DC power supply + Vcc is supplied and output through a choke inductor 18 for power supply and a resistor 20 that converts the consumption current Ip into a monitor voltage (consumption current monitor voltage) Vp. The output terminal 32 is connected to the output terminal 32 via the matching circuit 30. The output terminal 32 is connected to a load 34 having a characteristic impedance of 50 [Ω]. The source terminal is grounded.

  The monitor voltage (detection voltage corresponding to the power consumption current) Vp is supplied to the output matching circuit 30 as the control voltage Vc via the voltage converter 28.

  FIG. 2 is a circuit diagram showing a configuration of the matching circuit 30a using the variable capacitance diode 54 as the impedance variable element.

  In the matching circuit 30a, the transmission line 50 is inserted between the drain terminal of the FET 16 and the output terminal 32, and the variable capacitor diode 54 having the capacitor 52 and the anode terminal grounded is connected between the transmission line 50 and the ground. Further, the control voltage Vc from the voltage conversion unit 28 is applied to the cathode terminal of the variable capacitance diode 54 via the resistor 56.

  The variable capacitance diode 54 that operates as a variable capacitor according to the change in the control voltage Vc has a characteristic that when the positive control voltage Vc is high, the capacitance value (capacitance) becomes smaller than when the control voltage Vc is low. .

  In the high-frequency amplifier circuit 10 configured as shown in FIGS. 1 and 2, when the FET 16 is biased to class AB, class B, or class C, the current flowing through the drain of the FET 16 changes according to the output high-frequency power. To do. The current flowing through the drain appears as a potential difference across the resistor 20 inserted between the power supply terminal 22 and the other end of the choke inductor 18 having one end connected to the drain terminal of the FET 16. This potential difference is supplied to the input terminal of the voltage converter 28 as the monitor voltage Vp.

  The resistor 20 is preferably as small as possible within the range in which the circuit operates normally.

  As the amplitude fluctuation component is larger, in other words, as the peak power is larger, it is necessary to increase the back-off of the FET 16, and it becomes difficult to ensure high efficiency. The high-frequency amplifier circuit 10 of FIG. 1 can achieve high efficiency even for a signal having a large amplitude fluctuation component. Generation of distortion can be reduced by a variable load impedance mechanism.

  The choke inductor 18 is formed by, for example, a stub, and the impedance of the choke inductor 18 is sufficiently large for a high frequency to be amplified (for example, 2 [GHz]), and the amplitude component of the amplitude component is an envelope frequency component. For this, a value that is sufficiently small is usually used.

  Here, the frequency component of the envelope is a frequency component of several kHz to several MHz in the case of a modulated wave having an amplitude variation component (QPSK, QAM, OFDM, etc.), for example.

  By determining the choke inductor 18 in this way, the monitor voltage Vp appearing at both ends of the resistor 20 becomes a voltage corresponding to the consumption current Ip which is an envelope current.

  Here, the resistor 20 and the voltage converter 28 can easily realize a high-speed operation that immediately responds to the frequency component of the envelope described above in real time.

  For this reason, the load impedance comprised from the capacitor | condenser 52 and the variable capacitance diode 54 can be adjusted with the output control voltage Vc of the voltage conversion part 28 according to envelope power.

  In practice, since the load impedance is configured to include the transmission line 50 and the variable capacitance diode 54, the load impedance increases when the monitor voltage Vp decreases, that is, when the envelope power decreases. The output control voltage Vc may be controlled so that

  Note that a method other than controlling the variable capacitance diode can be used to vary the load impedance.

  The voltage conversion unit 28 is not only an analog circuit configuration using a differential amplifier by an operational amplifier, but also a DSP (digital signal processor) including an A / D converter, a digital voltage conversion unit, and a D / A conversion unit. A digital circuit can be used.

  In the output matching circuit 30 a shown in FIG. 2, a large-amplitude high-frequency voltage that is a signal voltage is applied to the cathode terminal of the variable capacitance diode 54 connected in parallel with the load 34. In this case, since the anode terminal of the variable capacitance diode 54 is grounded, the large-amplitude high-frequency voltage is applied between both terminals of the variable capacitance diode 54. For this reason, the capacitance value of the variable capacitance diode 54 slightly changes, causing distortion. Further, when a high-frequency voltage having a larger amplitude is applied, the capacitance value of the variable capacitance diode 54 may fluctuate greatly, or in the worst case, the variable capacitance diode 54 may become conductive and may not function as a capacitor.

  FIG. 3 is a circuit diagram showing a configuration of an output matching circuit 30b having a configuration suitable for amplifying a large amplitude high frequency signal.

  In the matching circuit 30 a, a variable capacitance diode 54 is inserted in series with the transmission line 50 inserted between the drain terminal of the FET 16 and the output terminal 32, that is, in series with the load 34, and is further connected to the anode terminal of the variable capacitance diode 54. The control voltage Vc is applied via the resistor 56.

  At the high frequency, 2 [GHz] described above, the variable capacitance is compared with the impedance when the transmission line 50 is viewed from the cathode terminal of the variable capacitance diode 54 and the impedance when the output terminal 32 side is viewed from the anode terminal of the variable capacitance diode 54. When the impedance of the diode 54 is low, the variable capacitance diode 54 can be regarded as being short-circuited at the high frequency, and the high-frequency voltage amplitudes appearing at the cathode terminal and the anode terminal are substantially equal. That is, the high-frequency voltage difference between both ends of the variable capacitance diode 54 becomes substantially zero, and the cause of distortion is eliminated. For the same reason, it is suitable for large amplitude signal amplification.

  According to the above-described embodiment, as the information source for controlling the variable capacitance diode 54 constituting the load impedance, the consumption current Ip of the FET 16 is detected by the resistor 20 as the monitor voltage Vp due to the voltage drop, and the detected monitor voltage Vp is obtained. The capacitance that is the impedance of the variable capacitance diode 54 provided in the matching circuit 30 is changed by the corresponding control voltage Vc so as to increase the power efficiency, so that the load is increased so that the power efficiency increases according to the output high-frequency power. Impedance can be changed.

  In this case, since the consumption current Ip detects the current flowing out from the power supply + Vcc, the consumption current detection circuit can be configured with a simple configuration of one resistor 20. Since the resistor 20 may be mounted on the so-called low frequency side power supply + Vcc side, the load of the FET 16 is extremely light and the mounting configuration and arrangement are easy.

  Of course, instead of the resistor 20, a configuration using a transformer, for example, a configuration similar to a current probe using a Hall element capable of measuring a current from a direct current, between the power supply terminal 22 and the choke inductor 18 is used. The transformer configured to detect the consumption current Ip flowing in the wiring, or the primary side coil, the wiring between the power supply terminal 22 and the choke inductor 18, the coil and the resistor are connected in parallel on the secondary side, Furthermore, the consumption current Ip flowing out from the power source + Vcc can be detected by a configuration in which the voltage across the resistor is integrated by an integrator and converted into the consumption current Ip. Even in this case, the consumption current Ip can be detected with a simple configuration that is easy to mount.

  As described above, according to the above-described embodiment, the load impedance is changed so that the power efficiency increases in accordance with the consumption current Ip corresponding to the output power. Therefore, the load impedance is changed with a simple and easy configuration. Can be made. In particular, by adopting the configuration of the matching circuit 30b of FIG. 3, it is possible to reduce the occurrence of distortion due to the variable load impedance mechanism and achieve high efficiency even for a signal having a large amplitude fluctuation component.

  Note that the present invention is not limited to the above-described embodiment, and it is needless to say that various configurations can be adopted based on the contents described in this specification.

1 is a circuit diagram showing a configuration of a high-frequency amplifier circuit according to an embodiment of the present invention. It is a circuit diagram which shows the structure of the matching circuit using a variable capacitance diode as an impedance variable element. It is a circuit diagram which shows the structure of the output matching circuit of the structure suitable for amplification of a large amplitude high frequency. It is explanatory drawing of the load line and current voltage waveform with respect to the ideal transistor in which drain current changes proportionally with respect to gate voltage. FIG. 5 is an explanatory diagram of a load line and a current-voltage waveform when the input power is reduced and the output power is reduced to ¼ with the same bias and the same output impedance as in the example of FIG. 4. It is explanatory drawing of a load line and a current voltage waveform when output impedance is quadrupled when output electric power falls to 1/4.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... High frequency amplifier circuit 16 ... Field effect type transistor (FET)
18 ... choke inductor 20 ... resistor (current consumption detection circuit)
28 ... Voltage converter 30 (30a, 30b) ... Matching circuit (output matching circuit)
50 ... Transmission line 54 ... Variable capacitance diode

Claims (4)

In a high-frequency power amplifier comprising a transistor that amplifies an input high-frequency signal and outputs it as an amplified signal, and a matching circuit that is provided on the output side of the transistor and matches an output impedance of the transistor and a load,
A consumption current detection circuit for detecting a consumption current supplied to the transistor from a power supply;
An impedance variable element provided in the matching circuit,
A high-frequency power amplifier, wherein the impedance of the variable impedance element is changed according to the consumption current detected by the consumption current detection circuit. The high frequency power amplifier according to claim 1, wherein
Supplying the current consumption from the power source to the transistor through a choke inductor;
The high-frequency power amplifier, wherein the consumption current detection circuit detects the consumption current flowing from the power source through the choke inductor between the power source and the choke inductor. The high-frequency power amplifier according to claim 1 or 2,
The high-frequency power amplifier, wherein the variable impedance element includes a variable capacitance diode, and the variable capacitance diode is connected in parallel with the load. The high-frequency power amplifier according to claim 1 or 2,
The high-frequency power amplifier, wherein the variable impedance element includes a variable capacitance diode, and the variable capacitance diode is connected in series with the load.

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