JPH05129846A - Linear power amplifier - Google Patents

Abstract

PURPOSE:To suppress an output power change caused by a temperature change at a minimum by inserting a temperature compensating circuit, which compensates the temperature characteristic of an output detector, to the output side of an input detector. CONSTITUTION:A temperature compensating circuit 9 for temperature compensation and a loop band limiter 10 are added to the envelope feedback type linear power amplifier. Namely, since the output power is changed by the temperature characteristic of an output detector 2 although input power is fixed, the temperature compensating circuit 9 is inserted to the output side of an input detector 6 and the gain is changed so that the detected voltage change caused by the temperature change of the output detector 2 can be canceled and the change of the output power can be prevented. Further, the loop band limiter 10 decides whether the linear compensation of an amplitude is executed or not. Namely, in the case of large output power requiring the linear compensation, a loop band is widened and in the case of low output power unnecessitating the linear compensation, the loop band is narrowed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a linearized power amplifier for a wireless device used for mobile communication or the like.

[0002]

2. Description of the Related Art A power amplifier used for digital cellular and the like adopts a linear modulation system (π / 4 shift DQPSK system), and therefore needs to be a linear power amplifier. Usually, since power efficiency and linearity are contradictory, a power amplifier having a saturation characteristic as shown in FIG. 2 is often used after being linearized and compensated.

As a typical example of linearization compensation, there is an envelope feedback type linearization power amplifier (hereinafter referred to as linear PA). FIG. 3 is a circuit diagram showing an example of this linear PA. In FIG. 3, the output RF is output from the directional couplers 12 and 11.
The signal (Pout) and a part of the input RF signal (Pin) are respectively taken out, and the envelope components thereof are obtained by the detectors 2 and 6. The above envelope components are gain variable units 3 and 7, respectively.
After setting the appropriate gain with, it is compared with the comparator 4,
The error between the input and output amplitudes is applied to the power control terminal 5 as a feedback signal. Saturated power amplifier (PA)
The gain of 1 is the voltage (Vco
nt) can be varied as shown in FIG. Therefore, if the loop band of the loop shown in FIG. 3 has a band that can sufficiently follow the amplitude fluctuation, the amplitude of the output power changes following the amplitude of the input power, and linearization is performed.

Next, output power control will be described. In a system such as a car phone, the output power level of a terminal is changed stepwise according to the distance from a base station for the purpose of reducing interference with adjacent channels and reducing power consumption. For example, in a North American digital cellular car telephone system, the average output power level is controlled in 9 steps every 4 dB as shown in FIG. The level fluctuation (20 dB) at each stage indicates the fluctuation of the instantaneous amplitude due to the modulation.

In the linear PA shown in FIG. 3, the above-mentioned control of the output power level is realized by switching the gains of the gain changers 3 and 7 with the power control signal generated by the power signal generator 8.

[0006]

However, the linear PA having the above structure has a problem that the output power changes due to the temperature change of the detection characteristic of the output detector, and the amplitude distortion occurs at the low output power.

Here, let us consider the temperature change of the above-mentioned detection characteristics. For example, the detection characteristics when using a GaAs Schottky barrier diode changes as shown in FIG. 6 depending on the temperature. That is, when the temperature is high, the detection sensitivity (change of the detection voltage Vdet with respect to the output power Po) is A
When the temperature is low, on the contrary, the detection sensitivity becomes large as shown in B. Further, the fluctuation of the detection voltage due to the temperature change is large when the detection voltage is low. Furthermore, when the detection voltage becomes low, the linearity of the detection characteristic deteriorates.

On the other hand, since the output power is controlled over 50 dB as a whole as shown in FIG. 5, the output detector correspondingly obtains a linear detection output over a wide range over 50 dB. There is a need. In this case, even if the maximum detection voltage is 10V, the minimum detection voltage is reduced to 30mV.

Therefore, the temperature change of the detection characteristic at a particularly low voltage level poses a serious problem. That is, there is a problem that a change in the detection voltage at a low power level causes a change in the output power level, and a change in the linearity of the detection characteristic causes distortion during linearization compensation. In order to eliminate the above-mentioned problem of fluctuations in output power level due to temperature changes, the present invention provides a temperature compensating means on the input detector side of an envelope feedback type linearized power amplifier, and outputs the output in response to temperature changes. An object of the present invention is to provide an excellent linearized power amplifier that stabilizes the power level and stops the linear compensation at a low output power level to prevent the occurrence of amplitude distortion.

[0010]

In order to achieve the above object, the present invention provides an error obtained by comparing an envelope of an input signal detected by an input detector with an envelope of an output signal detected by an output detector. In a linearized power amplifier that performs linearization by controlling output power with a voltage, a temperature compensation circuit that compensates for the temperature characteristic of the output detector is inserted on the output side of the input detector.

In addition, the present invention adds a loop band controller for switching the frequency band of the error voltage in accordance with the switching of the average power level of the output signal to the above-mentioned configuration.

[0012]

[Operation] The output of the output detector fluctuates due to temperature changes.
On the other hand, the output from the input detector changes according to the fluctuation due to the temperature compensation circuit. Therefore, the above-mentioned fluctuation does not appear in the output of the comparator that compares the outputs of the output detector and the input detector, and the output power does not change.

When operating at low output power, the loop band limiter narrows the loop band, and the linear compensation by the loop is stopped. Therefore, generation of amplitude distortion due to linearity deterioration of the output detector is avoided.

[0014]

FIG. 1 is a circuit diagram showing an embodiment of the present invention. In the conventional envelope feedback type linearized power amplifier shown in FIG. 3, a temperature compensating circuit 9 for temperature compensation and a loop band limiter are provided. 10 and 10 are added. Therefore, since the basic operation for linearization is similar to that of the linearization power amplifier of FIG. 3, the description thereof is omitted and only the temperature compensation will be described below.

In FIG. 1, when the detection characteristic of the output detector 2 changes with temperature as shown in FIG. 6, when the output power P _O is low, the detection voltage (Vd of the output detector 2 is
When et) is small, the detected voltage and the output power controlled by the loop operation change as follows due to the temperature change.

When the temperature rises: The detection voltage of the output detector 2 increases. Therefore, the comparator 4 determines that the output power has increased, and the loop operation causes the output power to decrease as compared with the case of normal temperature.

When the temperature decreases: The detection voltage of the output detector 2 decreases. Therefore, the comparator 4 determines that the output power has decreased, and the loop operation causes the output power to increase as compared with the case of normal temperature.

As described above, due to the temperature characteristics of the output detector 2, the output power varies even though the input power is constant. Therefore, in the present embodiment, the temperature compensating circuit 9 is inserted on the output side of the input detector 6 and the gain thereof is changed to cancel the fluctuation of the detection voltage due to the temperature change of the output detector 2 to reduce the output power. It prevents fluctuation. Therefore, the temperature compensating circuit 9 may be configured so that the gain increases when the temperature rises and the gain decreases when the temperature falls.

FIG. 7 shows an example of the temperature compensating circuit 9, which is composed of a temperature-sensitive resistor (having a positive temperature coefficient) R2 whose resistance value increases as the temperature rises and an ordinary resistor R1. In FIG. 7, as the temperature rises, the resistance value of the temperature sensitive resistor R2 increases, so the output increases. When the temperature compensating circuit 9 is inserted on the output side of the input detector 6 as shown in FIG. 1, the temperature compensating circuit 9 increases the detection voltage of the input detector 6 when the temperature rises. This produces the same effect as when the input power is increased, and cancels the decrease in the output power due to the change in the detection characteristic of the output detector 6 when the temperature rises as described above. Even when the temperature drops, the output power is prevented from increasing similarly.

Since the input detector 6 has a constant input power, it can be set so as to operate in a region where the detection voltage is always large, and changes in the detection characteristics due to temperature changes are almost ignored. be able to.

Next, the loop band limiter 10 will be described. FIG. 8 shows an example of the configuration of the loop band limiter 10. By switching the switch SW, an LPF can be inserted between the input and the output to limit the pass band. With this loop band limiter 10,
Determines whether linear compensation of amplitude is performed or not. That is,
When the SW of the loop band limiter 10 is switched to the 1 side to make the loop band wide enough, the loop follows the amplitude change of the modulated wave and performs the amplitude compensation operation. On the other hand, when the above SW is switched to the 2 side and the loop band is narrowed, the loop does not follow the amplitude change of the modulated wave, and the voltage of the average level of the amplitude fluctuation is applied to the control terminal 5 of the saturation power amplifier 1. Return to. Therefore, the loop band is widened at the time of large output power for which linearization compensation is required, and narrowed at the time of low output power for which linearization compensation is unnecessary. The switching of the loop band is performed by switching the switch SW of the loop band limiter 10 with the power control signal generated by the power control signal generator (CPU) 8.

When the output power is low, the detection characteristic linearity is deteriorated because the temperature change amount of the detection characteristic of the output detector 2 is large. Therefore, if linear compensation is performed in this state, distortion occurs. In order to avoid this, it is a good idea to stop the loop compensation as described above at low output power. However, in this case as well, the loop operates as a power level control.

The relationship between the distortion compensation, the loop band, the temperature fluctuation of the detection voltage of the output detector, and the output power described above can be summarized as shown in FIG. That is, by changing the loop band and the presence / absence of temperature compensation according to the level of the output power, the fluctuation of the output power due to the temperature can be minimized.

Further, as shown in FIG. 6, the temperature change of the detection characteristic of the output detector 2 becomes larger as the output power becomes larger and the fluctuation width of the detection voltage becomes smaller, and becomes larger as the output power becomes smaller. Therefore, by switching the temperature compensation sensitivity according to the output power level, optimum compensation can be performed for each output power level. For this purpose, a temperature compensating circuit whose sensitivity with respect to temperature can be switched is required. FIG. 10 shows an example of the configuration of such a temperature compensating circuit, in which temperature-sensitive resistors R1 and R having temperature coefficients of a, b and c (a>b>c> 0), respectively.
Sensitivity to temperature is switched by selecting 2, R3 with switches SW1, SW2, SW3.

FIG. 11 is a circuit diagram showing another embodiment of the present invention. In the present embodiment, an RF band gain variable device 13 having excellent linearity and capable of changing the gain is inserted in a front stage (or a rear stage) of the saturation type power amplifier (PA) 1, and the saturation type power amplifier (PA) is inserted. ) 1, the gain of the gain varying device 13 is controlled by the feedback voltage, and the same effect as that of the embodiment shown in FIG. 1 can be obtained. The gain varying device 13 may be a high frequency attenuator using a PIN diode or the like.

[0026]

As described above in detail, according to the present invention, since the temperature compensating circuit for compensating the temperature characteristic of the output detector is provided, the fluctuation of the output power due to the temperature change can be minimized. it can.

Further, when the average power level of the output signal is low, the loop band limiter narrows the loop band to stop the linear compensation, so that the occurrence of amplitude distortion due to deterioration of the detection characteristic linearity at a low level of the output detector is avoided. And good linearization can be maintained.

Furthermore, by switching the gain of the temperature compensation circuit according to the operating region of the output detector, optimum temperature compensation can be realized for each operating region.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a diagram showing input / output characteristics of a power amplifier.

FIG. 3 is a circuit diagram showing a conventional envelope feedback type linearized power amplifier.

FIG. 4 is a diagram showing output power control characteristics.

FIG. 5 is an explanatory diagram of a power control range of digital cellular.

FIG. 6 is a diagram showing a detection characteristic of an output detector.

FIG. 7 is a circuit diagram of a temperature compensation circuit.

FIG. 8 is a circuit diagram of a loop band limiter.

FIG. 9 is an explanatory diagram of a relationship between distortion compensation, a loop band and the like and output power.

FIG. 10 is a circuit diagram of a temperature compensation circuit.

FIG. 11 is a circuit diagram showing another embodiment of the present invention.

[Explanation of symbols]

 1 Saturation type power amplifier 2 Output detector 3, 7, 13 Gain variable device 4 Comparator 6 Input detector 8 Power control signal generator 9 Temperature compensation circuit 10 Loop band limiting circuit 11, 12 Directional coupler

Claims (3)

[Claims] 1. Linearization by controlling output power with an error voltage obtained by comparing the envelope of an input signal detected by an input detector with the envelope of an output signal detected by an output detector In the linearized power amplifier, a temperature compensating circuit for compensating the temperature characteristic of the output detector is inserted on the output side of the input detector. 2. The linearized power amplifier according to claim 1, further comprising a loop band limiter that switches a frequency band of the error voltage in response to switching of an average power level of the output signal. 3. The linearized power amplifier according to claim 1, wherein the temperature compensation circuit switches its gain in response to switching of the average power level of the output signal.