JP5921482B2 - Doherty amplifier - Google Patents

Description

  Embodiments described herein relate generally to an improvement of a Doherty amplifier used for microwave communication or the like.

  In an amplifier used for a cellular phone or the like, an amplifier is used to efficiently amplify a high frequency signal such as CDMA (Code Division Multiple Access) or OFDM (Orthogonal Frequency Division Multiplex) having a large difference between average power and peak power. Therefore, it is necessary to make the amplifier saturation level higher than the average output power. However, if the back-off is used from the saturation level, the efficiency of the amplifier decreases. In order to prevent this efficiency reduction, a Doherty amplifier is applied.

  The Doherty amplifier includes a main amplifier for class AB or class B operation and a peak amplifier for class C operation. Only the main amplifier operates in the region where the output power is low, and the main amplifier and peak amplifier operate when the output level approaches the saturation region. This has the advantage that high efficiency can be obtained over a wide range of output levels. .

WO2008 / 35396

A Dual-Band Parallel Doherty Power Amplifier for Wireless Applications (IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL.60, No.10, OCTOBER 2012) A Methodology for Realizing High Efficiency Class-J in a Linear and Broadband PA (IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL.57, No.12, DECEMBER 2009)

  By the way, in the Doherty amplifier, high-efficiency frequency characteristics cannot be obtained in a wide band, and means for improving it are being studied.

  An object of the present invention is to provide a Doherty amplifier capable of obtaining a highly efficient frequency characteristic in a wide band.

  According to the embodiment, the Doherty amplifier always operates when an input signal is supplied, operates when the input signal is larger than a predetermined level, and a main amplifier that amplifies the input signal. Is connected between the main amplifier and the load unit, and corresponds to the quarter wavelength of the input signal, the peak amplifier for amplifying the signal, the load unit for combining and outputting the output of the main amplifier and the output of the peak amplifier A Doherty-type amplifier including a quarter-wave transmission line is provided, and a half-wave transmission line corresponding to a half wavelength of an input signal, which is connected between a peak amplifier and a load unit, is provided.

1 is a schematic configuration diagram of a Doherty amplifier according to a first embodiment. FIG. The figure which shows the amplitude characteristic of the signal input into main amplifier and peak amplifier in 1st Embodiment. FIG. 3 is a circuit diagram showing an equivalent circuit of a general Doherty amplifier as a first comparative example. In the 1st comparative example, the frequency characteristic figure from the main amplifier to the output load in the operating point which took 6 dB or more backoff. The circuit diagram which shows the equivalent circuit of the Doherty amplifier used as the 2nd comparative example. In the 2nd comparative example, the frequency characteristic figure from the main amplifier to the output load in the operating point which took 6 dB or more backoff. FIG. 3 is a circuit diagram showing an equivalent circuit of the Doherty amplifier according to the first embodiment. FIG. 6 is a frequency characteristic diagram from the main amplifier to the output load at the time of 6 dB back-off in the first embodiment. The frequency characteristic figure which shows the impedance of the load part seen from the main amplifier in the said 2nd comparative example. The frequency characteristic figure which shows the impedance of the load part seen from the main amplifier in the 1st embodiment. The circuit diagram which shows the equivalent circuit of the Doherty type amplifier concerning 2nd Embodiment. The frequency characteristic figure which shows the impedance of the load part seen from the main amplifier in the 2nd embodiment. The circuit diagram which shows the equivalent circuit of the Doherty type amplifier concerning 3rd Embodiment. In the 3rd Embodiment, the frequency characteristic figure which shows the impedance of the load part seen from the main amplifier.

  Hereinafter, embodiments will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a schematic configuration diagram of a Doherty amplifier (hereinafter referred to as a Doherty amplifier) according to the first embodiment.

  In FIG. 1, an input signal is distributed to two systems by a hybrid circuit 1, one is supplied to a main amplifier 2, and the other is supplied to a peak amplifier 3. The main amplifier 2 amplifies the power of the input signal to a predetermined signal level. Similarly to the main amplifier 2, the peak amplifier 3 amplifies the power of the input signal to a predetermined signal level.

  The output of the main amplifier 2 is taken out via the quarter wavelength line 4 and the load unit 5 corresponding to the quarter wavelength of the input signal. The output of the peak amplifier 3 is combined with the output of the main amplifier 2 and taken out via the load unit 5.

  In FIG. 1, the operating point of the main amplifier 2 is set to class AB or class B by adjusting the bias of each amplifier, and the operating point of the peak amplifier 3 is set to class C. Since the peak amplifier 3 operates in class C, the drain level of the peak amplifier 3 does not flow if the output level of the main amplifier 2 is small and linearly operates. However, when the main amplifier 2 starts to saturate, the peak amplifier 3 The amplified signal begins to be output.

  FIG. 2 shows an example of a signal having a ratio of average power to peak power of 6 dB. In this case, the bias voltage of the peak amplifier 3 is set to a gate bias that turns on at a value 6 dB lower than the peak power level as shown in FIG. As a result, the peak amplifier 3 is turned off in a region where the output power is low (for example, below the average power in FIG. 2), and when the output power approaches the saturation region, the drain current of the peak amplifier 3 reaches the same magnitude as the main amplifier 2. stand up.

Next, the operation in the above configuration will be described.
First, before describing the operation of the first embodiment, the frequency characteristics of a general Doherty amplifier will be described.

(Description of the first comparative example)
FIG. 3 is an equivalent circuit diagram of a general Doherty amplifier as a first comparative example.

  Here, the main amplifier 2 and the peak amplifier 3 are current sources, and a 1/4 wavelength line 4 that is 1/4 wavelength of the input signal at the center frequency is mounted between the output of the main amplifier 2 and the load unit 5. ing. In FIG. 3, as a general circuit, the main amplifier 2 and the peak amplifier 3 can supply the maximum output power when the load is 50Ω.

  Since the load resistance (RL) of the load section 5 of the Doherty amplifier is 25Ω, at the center frequency at the 6 dB back-off point, 50Ω × 50Ω ÷ 25Ω = 100Ω by the 50Ω 1/4 wavelength line 4, and the main amplifier 2 is saturated in voltage and can operate with high efficiency.

  However, since the 1/4 wavelength line 4 has frequency characteristics, the characteristics deteriorate when the operating frequency deviates from the design center frequency. FIG. 4 shows S21 from the main amplifier 2 to the output load at the operating point where the back-off is 6 dB or more. Here, the output of the main amplifier 2 is 100Ω and the load unit 5 is 25Ω. The center frequency of the 1/4 wavelength line 4 is 1000 MHz. As the frequency deviates from the center frequency, S21 decreases.

  Note that the general Doherty amplifier of FIG. 3 does not have frequency characteristics when the peak amplifier 3 is in operation if the main amplifier 2 and the peak amplifier 3 are ideal current sources. The reason for this is that the impedance (ZT in FIG. 3) viewed from the output of the quarter wavelength line 4 becomes load resistance (RL) × 2 = 50Ω due to the current flowing from the peak amplifier 3, and the impedance is below 6 dB back-off point. This is because the 50Ω 1/4 wavelength line 4 operating as the conversion line can be matched with the load resistance.

(Description of the second comparative example)
Next, as an example of a broadband Doherty amplifier, the circuit described in `` A Dual-Band Parallel Doherty Power Amplifier for Wireless Applications IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 60, NO. 10, OCTOBER 2012 '' is explained. To do.

  FIG. 5 is a circuit diagram showing an equivalent circuit of a Doherty amplifier as a second comparative example.

  The Doherty amplifier in the first comparative example has a narrow frequency characteristic because the 1/4 wavelength line 4 of the output of the main amplifier 2 operates as an impedance converter at the time of 6 dB backoff, but at the time of peak operation, Since the quarter wavelength line 4 can be matched with the load resistance RL, it has a wide band characteristic. The circuit of FIG. 5 is 1/4 even when the Doherty amplifier operates only at the main amplifier 2 or less, and when the Doherty amplifier is saturated and both the main amplifier 2 and the peak amplifier 3 operate. Although the wavelength line 4 operates as an impedance converter, the impedance conversion ratio is lower than that of the circuit of the first comparative example. In the circuit of the first comparative example, the load resistance 25Ω is converted to 100Ω at the time of 6 dB backoff, whereas in the circuit of the second comparative example, the load resistance RL of 50Ω is converted to 100Ω. For this reason, the band characteristic at the peak operation is inferior to that of the first comparative example, but the characteristic at 6 dB back-off is improved from the circuit of the first comparative example, and the operation of the total Doherty amplifier is higher than that of the first comparative example. Improved.

  In the second comparative example, quarter wavelength lines 8-1 and 8-2 having different impedances are connected between the peak amplifier 3 and the load unit 5. The impedance of the quarter wavelength line 8-1 is set to “70.7Ω” in order to convert the load resistance RL to 100Ω. The impedance of the ¼ wavelength line 8-2 is necessary to open the impedance on the peak amplifier 3 side from the output synthesis point at the time of backoff, and the impedance is “50Ω” like the load impedance of the peak amplifier 3. Set to

  FIG. 6 illustrates a frequency characteristic S21 from the main amplifier 2 to the load unit 5 at the time of 6 dB back-off. Here, the output of the main amplifier 2 is 100Ω and the load unit 5 is 50Ω. The center frequency of the 1/4 wavelength line 4 is 1000 MHz. It can be seen from the first comparative example that the band has a wide band characteristic.

(Description of operation of the first embodiment)
Therefore, in the first embodiment, the broadband characteristics are further improved as compared with the second comparative example.

  FIG. 7 is a diagram illustrating an equivalent circuit of the Doherty amplifier according to the first embodiment. In FIG. 7, the same parts as those in FIG.

  That is, the half-wave line 6 is mounted between the peak amplifier 3 and the load resistance of the load unit 5. The 1/2 wavelength line 6 has a function of correcting the frequency characteristic of the 1/4 wavelength line 4 of the output of the main amplifier 2 when the Doherty amplifier performs a 6 dB back-off operation.

  FIG. 8 illustrates a frequency characteristic S21 from the main amplifier 2 to the output load at the time of 6 dB backoff. Here, as in the previous case, the output of the main amplifier 2 is a 100Ω system, but the load resistance RL of the load unit 5 is a 25Ω system. The center frequency of the 1/4 wavelength line 4 of the output of the main amplifier 2 and the 1/2 wavelength line 6 of the output of the peak amplifier 3 is 1000 MHz.

  The characteristics of FIG. 8 are wider than the frequency characteristics of the first comparative example (shown in FIG. 4), but are substantially equivalent to the frequency characteristics of the second comparative example (shown in FIG. 6).

  By the way, comparing the configuration of the first embodiment with the configuration of the second comparative example has the following advantages.

  (1) FIG. 9 shows the impedance of the load section 5 viewed from the main amplifier 2 in the second comparative example. FIG. 10 shows the impedance of the load unit 5 as viewed from the main amplifier 2 in the first embodiment. Compared with the circuit of the second comparative example, the circuit of the first embodiment is the real part of the impedance of the load section 5 viewed from the main amplifier 2 between 651.2 MHz and 1349 MHz (FIGS. 9 and 10). It can be seen that there is little change in). In the second comparative example, there is little change in the real part of the impedance of the load unit 5 (indicated by □ in FIGS. 9 and 10) viewed from the main amplifier 2 in the vicinity of 1000 MHz, but a band lower than 900 MHz and 1100 MHz. When the band becomes high, the change in the real part of the impedance of the load unit 5 viewed from the main amplifier 2 becomes large.

  For example, as described in the literature "A Methodology for Realizing High Efficiency Class-J in a Linear and Broadband PA IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 57, NO. 12, DECEMBER 2009" If the linear operation is performed without clipping the waveform by adjusting the impedance of the wave, the efficiency and output power can be maximized by making the real part of the impedance constant. For this reason, it is considered that the first embodiment may be able to broaden the efficiency characteristic at the time of 6 dB back-off compared to the second comparative example.

  (2) The circuit of the second comparative example has frequency characteristics as described above during peak operation. However, the first embodiment does not have frequency characteristics during peak operation if the main amplifier 2 and the peak amplifier 3 are ideal current sources. During peak operation, the impedance ZT of the output of the main amplifier 2 seen from the quarter wavelength line 4 becomes load resistance (RL) × 2 = 50Ω due to the current flowing from the peak amplifier 3 side. Can be matched with the load resistance RL of the load section 5. Similarly, the impedance ZT ′ viewed from the half-wave line 6 of the output of the peak amplifier 3 becomes load resistance (RL) × 2 = 50Ω due to the current flowing from the main amplifier 2 side, and the half-wave line 6 is the load section 5. This is because matching with the load resistance RL is possible.

  As described above, according to the first embodiment, by connecting the ½ wavelength line 6 between the peak amplifier 3 and the load unit 5, the 1 on the main amplifier 2 side during the 6 dB back-off operation. Since the frequency characteristic of the / 4 wavelength line 4 is corrected, the frequency characteristic of the real part of the impedance of the load unit 5 viewed from the main amplifier 2 is flat over a wide band (650 MHz to 1349 MHz) at the time of 6 dB backoff. can do.

  In the first embodiment, since the impedance of the half-wavelength line 6 is set to be twice the impedance of the load resistance RL of the load unit 5, the peak amplifier 3 is operated during the operation of the peak amplifier 3. The impedance of the load section 5 seen from the half-wave line 6 on the output side becomes load resistance × 2 due to the current flowing from the main amplifier 2 side, and the half-wave line 6 is matched with the load section 5, As a result, the frequency characteristic of the real part of the impedance of the load unit 5 viewed from the main amplifier 2 can be flattened over a wide band at the time of 6 dB backoff without having the frequency characteristic at the peak operation. Further, by adjusting the impedance of the harmonics, if the linear operation is performed without clipping the waveform, the real part becomes constant, and the efficiency and output power can be maximized.

(Second Embodiment)
In the second embodiment, the first embodiment will be described in a more general manner.

  FIG. 11 is a connection diagram of the Doherty amplifier according to the second embodiment.

  Here, as in the previous case, the center frequency of the Doherty amplifier is 1000 MHz, the Doherty amplifier has a back-off operation, the load impedance RL of the load unit 5 viewed from the main amplifier 2 is 100Ω, and the main amplifier 2 has a peak operation when the Doherty amplifier is in peak operation. The impedance RL viewed from the peak amplifier 3 is set to 50Ω.

  The impedance Z0 of the quarter wavelength (λ0) line 23 connected to the output side of the peak amplifier 3 is 50Ω.

  Further, after the peak amplifier 3 starts its operation, Zm ^ 2 = Z0 × RL × 2 is set so that the voltage of the main amplifier 2 remains constant even if the current of the main amplifier 2 changes at the center frequency.

FIG. 12 shows the impedance of the load unit 5 viewed from the main amplifier 2 at the time of backoff (when only the main amplifier 2 is operating) using RL as a parameter. The relationship between the RL value and Zm and Z0 is shown in Table 1 below.

  In FIG. 12, RL = 50Ω corresponds to the second comparative example (FIG. 5). When RL = 25Ω, it corresponds to the first embodiment (FIG. 7). The impedance RL is not limited to 25Ω. For example, the impedance RL may be 20Ω. Even when the impedance RL is 20Ω, the real part of the impedance is flatter than RL = 50Ω as the second comparative example.

  As described above, according to the second embodiment, not only the first embodiment in which the load resistance RL of the load unit 5 is “25Ω”, but also RL is “20Ω” close to “25Ω”. The same effect can be obtained.

(Third embodiment)
In the above embodiments, the load resistance RL has been set to “25Ω” or “20Ω”. Since the interface of a normal circuit is “50Ω”, it is necessary to convert “25Ω” to “50Ω”. One method is to use, for example, a tapered broadband impedance converter, but an impedance converter using a quarter wavelength line used in a conventional Doherty amplifier may be used. This will be described below.

  FIG. 13 shows a circuit obtained by adding a quarter wavelength line 31 to the output circuit of the second embodiment (FIG. 11) and converting the output impedance to “50Ω” as the third embodiment.

  Here again, as before, the center frequency of the Doherty amplifier is 1000 MHz, the Doherty amplifier is back-off operation, the load impedance viewed from the main amplifier 2 is “100Ω”, the main amplifier 2 when the Doherty amplifier is in peak operation, and the peak amplifier 3 The impedance seen from the viewpoint is “50Ω”.

  The impedance Z0 of the quarter wavelength line 23 connected to the output side of the peak amplifier 3 is “50Ω”. Also, after the peak amplifier 3 starts operating, Zm ^ 2 = 2 × Z0 × Za ^ 2 / so that the voltage of the main amplifier 2 remains constant even if the current of the main amplifier 2 changes at the center frequency. RL.

FIG. 14 shows the impedance of the load unit 5 viewed from the main amplifier 2 during backoff (when only the main amplifier 2 is operating), using the impedance (Zb) viewed from the output synthesis point as the parameter. Table 2 below shows the relationship between the values of RL, Zb, and Z0 and Zm and Za.

  In FIG. 14, when Zb = 50Ω, it corresponds to the second comparative example (FIG. 5). When Zb = 25Ω, this corresponds to a circuit in which a quarter wavelength line 31 is added to the output of the circuit of the first embodiment (FIG. 7). Even if the quarter-wave line 31 is added to the output of the circuit of the first embodiment (FIG. 7) and the output impedance is 50Ω, the back-off time is still higher than that of the second comparative example (FIG. 5). The real part of the impedance of the load unit 5 viewed from the main amplifier 2 is a wide band.

  As described above, according to the third embodiment, between the quarter wavelength line 21 and the quarter wavelength line 22 and the load unit 5, the quarter corresponding to the quarter wavelength of the input signal. Even if the wavelength line 31 is connected, the frequency characteristic of the impedance of the load unit 5 viewed from the main amplifier 2 at the time of backoff can be flattened in a wide band.

(Other embodiments)
In order to confirm the effect of each of the above embodiments, the characteristic example has been calculated and described by substituting a numerical value for the impedance of the transmission line in the block diagram, but is not limited to the numerical value.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment.

  2 ... main amplifier, 3 ... peak amplifier, 4 ... 1/4 wavelength line, 5 ... load section, 6 ... 1/2 wavelength line, 21, 22, 23, 31 ... 1/4 wavelength line.

Claims (5)

A main amplifier that always operates when an input signal is supplied, amplifies the input signal, and a peak that operates at the time of backoff when the input signal reaches a saturation level larger than a predetermined level and amplifies the input signal An amplifier, a load unit for synthesizing and outputting the output of the main amplifier and the output of the peak amplifier, and connected between the main amplifier and the load unit, so that a quarter wavelength of the input signal is obtained. A Doherty amplifier comprising a corresponding quarter wavelength transmission line,
Corresponding to 1/2 wavelength of the input signal connected between the peak amplifier and the load unit, the frequency characteristic of the 1/4 wavelength transmission line at the output of the main amplifier is corrected at the back-off time. A Doherty amplifier comprising a 1/2 wavelength transmission line.   2. The Doherty amplifier according to claim 1, wherein the half-wave transmission line has an impedance twice that of the load unit.   Furthermore, it comprises a quarter wavelength transmission line corresponding to a quarter wavelength of the input signal connected between the quarter wavelength transmission line and the half wavelength transmission line and the load section. The Doherty amplifier according to claim 1. A main amplifier that always operates when an input signal is supplied, amplifies the input signal, and a peak that operates at the time of backoff when the input signal reaches a saturation level larger than a predetermined level and amplifies the input signal An amplifier, a load unit for synthesizing and outputting the output of the main amplifier and the output of the peak amplifier, and connected between the main amplifier and the load unit, so that a quarter wavelength of the input signal is obtained. A corresponding first quarter wavelength transmission line, a second quarter wavelength transmission line and a third quarter wavelength transmission line which are connected between the peak amplifier and the load section and have different impedances from each other; A Doherty amplifier comprising:
The Doherty amplifier characterized by changing the impedance of the load section so that the frequency characteristic of the impedance of the load section viewed from the main amplifier is flat in a wide band. A main amplifier that always operates when an input signal is supplied, amplifies the input signal, and a peak that operates at the time of backoff when the input signal reaches a saturation level larger than a predetermined level and amplifies the input signal An amplifier, a load unit for synthesizing and outputting the output of the main amplifier and the output of the peak amplifier, and connected between the main amplifier and the load unit, so that a quarter wavelength of the input signal is obtained. A corresponding first quarter wavelength transmission line, a second quarter wavelength transmission line and a third quarter wavelength transmission line which are connected between the peak amplifier and the load section and have different impedances from each other; A Doherty amplifier comprising:
A fourth 1/1 wavelength corresponding to a 1/4 wavelength of the input signal connected between the first quarter wavelength transmission line and the third quarter wavelength transmission line and the load unit. de Hattie amplifier further comprising a quarter-wave transmission line.

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