JPS6147443B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an output control circuit for preventing output fluctuations due to variations in load impedance of a high frequency power amplifier circuit and overloading of the power amplifier circuit.

Two examples of conventional automatic output control circuits are shown in FIGS. 1 and 2. Figure 1 shows a C-class power amplifier circuit using a π-type matching circuit, in which the variable capacitor in the matching circuit is mechanically rotated by a control motor to follow changes in load impedance and always maintain the optimum matching state. It is. In FIG. 1, 11 is an excitation input terminal, 12 is a beam tube, 13 is a control grid bias power terminal, 14 is a power grid power terminal, 1
5 is an anode power supply terminal, 16 and 28 are coupling capacitors, 17 is a high frequency choke coil, 18 is a high frequency coil with a coupling coil, 19, 21 and 27 are bypass capacitors, 20 is a resistor, 22 is a current-voltage converter, 23 , 24 are variable capacitors, 25 and 26 are control motors, and 27 is a control circuit. In this circuit, load fluctuations within a certain range are always controlled to the correct matching state, but the disadvantages are that the response speed including mechanical control is slow, reliability is low, and maintenance is labor-intensive. Ta.

FIG. 2 is an example of output control using a conventional electronic circuit. In Fig. 2, 31 is a power supply terminal, 32 is a carrier wave input terminal, 33 is an audio input terminal, 34 is a switching class D amplifier circuit, 35 is a low pass filter, 36 is a pulse width modulation switching amplifier circuit, and 37 is an output matching circuit. , 38 is a coupling circuit, 3
9 is an output terminal, 40 is a detection circuit, and 41 is a smoothing circuit that averages the audio signal down to the lowest frequency and passes only the DC component. This circuit operates as a pulse width modulation AM modulation circuit. That is, a voltage that continuously changes from 0 to the power supply voltage is applied to the PA in accordance with the 36 switching duty ratios, and the pulse width is modulated in proportion to the audio signal, resulting in AM modulation. The LPF 35 blocks the pulse repetition frequency and passes the DC component and audio component.

Normally, a voltage switching class D amplifier has a small internal resistance and operates almost as a voltage source, so if the impedance seen from the input side of the output matching circuit decreases due to changes in load impedance, the output increases and the PA The input current will also increase, and there is a risk that the rating will be exceeded. Therefore, in FIG. 2, the output is controlled by controlling the pulse width of pulse width modulation and changing the PA voltage so that the output does not become excessive. In this case, PA depending on the output control
Since the voltage changes, the degree of modulation also changes, and the modulation impedance (load of the low-pass filter) also changes, which has the disadvantage of affecting the modulation characteristics.

Therefore, the present invention aims to improve the above-mentioned drawbacks of the prior art, and its purpose is to provide an automatic output control circuit that performs electronic control and performs output control without loss or change in modulation characteristics. , a rectifier circuit whose input terminal is connected in series and inserted between the output of the high frequency power amplifier circuit and the matching circuit of the load;
a first switching element that switches the output voltage of the rectifier circuit via a low-pass filter including an inductance element; and a second switching element that returns current from the inductance element to a power source when the first switching element is turned off. , a pulse width modulation circuit that controls the switching duty cycle of the first switching element; and a voltage corresponding to the DC input current or high frequency output current of the high frequency power amplifier circuit is detected as an input to the pulse width modulation circuit. The automatic output control circuit has a detection circuit that provides a detection circuit, and prevents output fluctuation and overload of the high frequency power amplifier circuit by controlling the output voltage of the rectifier circuit. This will be explained in detail below with reference to the drawings.

FIG. 3 shows a first embodiment of the present invention, with 51
is a power input terminal, 52 is an oscillation circuit, 53 is a class D amplifier circuit, 54 is a high frequency transformer with a high degree of coupling,
55 is an output matching circuit having bandpass filter characteristics.

The secondary side of the transformer 54 is double-wave rectified by a prism-connected diode 57 having a short reverse recovery time. The rectified DC voltage has the amplitude of the AC voltage of the secondary winding, and if the value of the DC voltage is forcibly set, the voltage of the secondary winding is also limited to approximately that magnitude. That is, a constant voltage drop determined in accordance with the rectified voltage can be generated between the output of the PA and the output matching circuit. In other words, if the magnitude can be controlled, the output can be controlled, so in order to set the voltage to an arbitrary value without loss, the rectified output is passed through a low-pass filter 58 and then connected to a switching power FET.
Switching is performed by 59. By controlling the switching pulse width, that is, the ON duty, the rectified voltage can be forcibly controlled. Reference numeral 60 is a pulse width modulator (PWM) using a voltage comparator, which compares the sine wave output voltage of the oscillator 52 with the input voltage, and determines the polarity of the output pulse such that the higher the input voltage, the longer the low level period of the output pulse. The oscillator 52 is configured to perform pulse width modulation in which the repetition frequency is the frequency of the oscillator 52. Since the voltage drop (proportional to the power supply current of PA53) of the low resistance 61 inserted between the negative power supply terminal of PA53 and the ground is applied as the input of PWM60,
The PWM 60 operates in such a direction that the input voltage of the PWM 60 increases and the low level period of the output of the PWM 60 increases as the PA current increases. That is, P.A.
As the current increases, the ON duty of FET 59 decreases. When the FET 59 performs a switching operation in accordance with the pulse width modulation signal of the PWM 60, the rectified current by the diode bridge 57 flows to the FET 59 or the diode 62 via the low-pass filter 58. During the ON period of FET59,
The voltage flows from the drain of the FET 59 to the source of the FET 59, and the voltage between the drain and source of the FET 59, that is, the voltage on the coil side of the low-pass filter 58 becomes almost zero. During the period when FET59 is OFF,
The energy stored in the coil of the low pass filter (LPF) 58 becomes a current to the diode 62,
Since the continuity of the current in the coil is maintained, the anode side of the diode 62, that is, the low-pass filter 5
The voltage on the coil side of No. 8 is approximately the power supply voltage (voltage at the voltage input terminal 51). Therefore, it is understood that a pulse width modulation waveform whose amplitude is the power supply voltage is generated on the coil side of the low-pass filter 58, but the voltage on the capacitor side of the low-pass filter 58 is the average voltage, that is, the FET 59. The voltage is proportional to the OFF duty. This can be considered to mean that the high-frequency voltage amplitude of the secondary winding of the transformer 54 is limited to the voltage on the capacitor side of the low-pass filter 58 by the diode bridge 57, and the voltage of the secondary winding of the transformer is It is proportional to the voltage of the primary winding, and the voltage drop due to the primary winding of the transformer 54 inserted in series with the load of the PA 53 can be controlled in proportion to the voltage on the capacitor side of the low-pass filter 58. thus,
The voltage of the primary winding of the transformer 54 can be controlled by the duty ratio of the switching operation of the FET 59. If the PA current attempts to increase due to changes in load impedance, etc.
Since the voltage drop across the low resistance 61 increases and the input voltage to the PWM 60 increases, the OFF duty of the FET 59 increases and the voltage on the capacitor side of the LPF 58 increases. As a result, the voltage drop across the primary winding of the transformer 54 increases, the load current of the PA 53 decreases, and the power supply current of the PA 53 becomes constant. In this way, even if load fluctuations occur,
The voltage drop due to the transformer 54 is controlled so that the power supply current of the PA 53 is constant.
Automatic output control is performed, and a substantially constant output is obtained at the output terminal 56. In this case, the unnecessary power due to the voltage drop in the transformer 54 is transferred to the FET 59.
Since it is stored in the coil of the low-pass filter 58 during the ON period and returned to the power source by the diode 62 during the OFF period, no loss occurs.

As explained above, in the first embodiment, all output control is performed by electronic circuits, so there is an advantage that the response speed can be extremely fast. In addition, output control can be performed indirectly without changing the PA excitation or power supply.
Since it can be controlled as if by adjusting the load, it has the advantage of reducing fluctuations in the operating characteristics of the PA. Furthermore, since the rectifier output voltage is controlled by switching and all of the power is returned to the power source, there is an advantage that no power loss occurs.

The first embodiment is an example of output control of a carrier wave output power amplifier circuit, but as shown in FIG. 4, control of AM transmitter output is also possible.

In FIG. 4, 71 is a power supply terminal, 72 is a modulation input terminal, 73 is a class D power amplifier circuit, and 74 is a carrier wave oscillator. Excitation at frequency _c is given to the class D amplifier circuit, and a modulation signal is superimposed and applied to the power supply by a modulation transformer 75, so that amplitude modulation is performed. Its output is coupled to an output matching circuit 77 by an output transformer 76. 78 is an output terminal. A high frequency diode 7 connected in series with a bridge is connected to one side of the secondary winding of the output transformer.
The input side of the rectifier circuit 9 is connected to ground, and the output of the rectifier is connected to a low-pass filter 81 after removing the in-phase modulated wave component by a transformer 80 . A switching transistor 82 is connected to the coil side of the low-pass filter, and performs a switching operation in response to a pulse width modulation signal. 83 is a pulse width modulation circuit;
Pulse width modulation is performed in accordance with the power supply current of the class D power amplifier using the frequency _s of the sample carrier wave oscillator No. 4 as the repetition frequency. 85 is a shunt for converting the power supply current into voltage, and 86 is a low-pass filter for blocking ripples in the carrier frequency. The pulse width modulator is configured so that the voltage of the shunt 85 is 0.
If the duty ratio of the ON pulse is 1 and the duty ratio is set to change from 1 to 0 as the voltage increases, the output control during modulation will not distort the modulated wave. can be done. Since the voltage of the shunt 85 changes according to the modulation, the voltage drop caused by the rectifier also changes as shown between A and B and between A' and B' in Figure 5. The waveform shown is also B-
The waveform will look like B′.

Further, the power corresponding to the voltage drop is returned to the power source by the diode 87 as in the first embodiment, so no power loss occurs.

In FIG. 4, amplitude modulation is performed using a modulation transformer, but the same operation can be performed using any modulation method. Further, although a bridge type rectifier circuit is used as the rectifier circuit, a double wave rectifier circuit using a transformer with a center tap or various voltage doubler rectifier circuits can also perform the same operation. Further, a carrier wave may be used as a sample carrier wave for pulse width modulation. Further, the configuration of the low-pass filter 81 is not limited to the secondary one of LC, but any configuration is possible. In addition, the DC input current of the power amplifier circuit was detected for output control.
A similar effect can be obtained by detecting the high frequency output current or output power of the power amplifier circuit.

The present invention uses an electronic circuit to quickly control the output so as not to overload the power amplifier due to the influence of load fluctuations, so it can be used for solid-state high-frequency amplifiers that require high-speed protection. It is particularly useful for transmitters in frequency bands (medium wave bands and long wave bands) where circulators cannot be used.

[Brief explanation of the drawing]

Fig. 1 is an example of conventional output control, Fig. 2 is an example of another conventional output control, Fig. 3 is a block diagram of the first embodiment of the automatic output control circuit according to the present invention, and Fig. 4 is FIG. 5 is a block diagram of a second embodiment of the automatic output control circuit according to the present invention.
FIG. 3 is an explanatory diagram of output control during modulation in the embodiment. 11...Excitation input terminal, 12...Beam tube, 1
3...Bias power supply terminal, 14...Shiyahei grid power supply, 15...Anode power supply terminal, 16, 28...
Coupling capacitor, 17... High frequency choke coil, 18... High frequency coil, 19, 21, 27...
...Bypass capacitor, 20...Low resistor, 22...
... Current-voltage converter, 23, 24 ... Variable capacitor, 25, 26 ... Control motor, 27 ... Control circuit, 31 ... Power terminal, 32 ... Carrier wave input terminal, 33 ... Audio input terminal, 34... Class D amplifier circuit, 35... Low pass filter, 36... Pulse width modulation switching amplifier, 37... Output matching circuit, 38... Coupling circuit, 39... Output terminal, 4
0...Detection circuit, 41...Smoothing circuit, 51...Power input terminal, 52...Oscillation circuit, 53...Class D amplifier circuit, 54...High frequency transformer, 55...Output matching circuit, 56...Output Terminal, 57...Diode bridge, 58...Low pass filter, 59...
...Power FET, 60...Pulse width modulator, 61...
... Resistor, 62 ... Diode, 71 ... Power supply terminal, 72 ... Modulation input terminal, 73 ... Class D power amplifier circuit, 74 ... Carrier wave oscillator, 75 ... Modulation transformer, 76 ... Output transformer, 77 ... Output matching circuit, 78 ... Output terminal, 79 ... Diode bridge, 80 ... Transformer, 81 ... Low pass filter, 82 ... Switching transistor,
83... Pulse width modulation circuit, 84... Sample carrier wave oscillator, 85... Current shunt, 86... Low pass filter.

Claims (1)

[Claims] 1. A rectifier circuit whose input terminal is connected in series between the output of the high-frequency power amplifier circuit and the matching circuit of the load, and a rectifier circuit which switches the output voltage of the rectifier circuit through a low-pass filter including an inductance element. a second switching element that returns the current from the inductance element to the power supply when the first switching element is off; and a pulse width modulation that controls the switching duty cycle of the first switching element. An automatic output control circuit comprising: a detection circuit that detects and provides a voltage corresponding to the DC input current or high frequency output current of the high frequency power amplifier circuit as an input to the pulse width modulation circuit.