Linear compensating circuit

Abstract: A linear compensating circuit is provided which comprises a first level detection circuit (3) for detecting the level of an input signal to an amplifier, a second level detection circuit (7) for detecting the level of an output signal from the amplifier, an amplitude compensating circuit (2) for controlling the amplitude of the input signal in accordance with the difference between the signal level detected by the first detection circuit and the signal level detected by the second level detection circuit for improving the linearity of an input-output characteristic of the amplifier, and a phase compensating circuit (4) for controlling the phase of the input signal in accordance with the signal level detected by the second detection circuit. This linear compensating circuit performs the compensations of amplitude and phase and as a result, the degradation of phase characteristic induced due to the compensation of amplitude can be prevented, resulting in a reduction in the distortion of an output signal of the amplifier even when it is being operated in the vicinity of the saturation range. This linear compensating circuit is effective when applied to a power amplifier, particularly to a high frequency power amplifier. It has a characteristic set preferably to cancel the phase change of the output signal induced caused by the compensation of amplitude by the amplitude compensating circuit.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a linear compensating circuit and more particularly, to a linear compensating circuit for compensating the linearity of an input-output characteristic of an amplifier and to an amplifier provided with such a circuit. 2. Description of the Related Art Generally, in a linear compensating circuit of a high frequency power amplifier, the output power of the amplifier is made equal in level to the input power thereof and compared therewith and then, controlled so that the difference in variation between the input and output powers of the amplifier can be made zero thereby keeping its input-output characteristic linear. Such a circuit is disclosed in US-A-3900823. An application example of such a linear compensating circuit will be explained below. In the front stage of a high frequency power amplifier, an amplitude compensating circuit for compensating an amplitude of an input signal to be inputted to the amplifier is provided. In addition there are provided with a first detection circuit for detecting a level of the power of the input signal, a second detection circuit for detecting a level of the power of an output signal of the amplifier and a differential amplifier for detecting a difference in power between the input and output signals of the amplifier and outputting a signal in accordance with the difference thus detected to the amplitude compensating circuit. The output signal is made equal in level to that of the input signal (that is, the level before amplification) and then, sent to the second detection circuit. The amplitude compensating circuit controls the amplitude of the input signal in advance in accordance with the signal from the differential amplifier so that the difference in power variation between the input and output signals is made substantially zero. As explained above, with the conventional linear compensating circuit, an input signal to be inputted to the amplifier is subjected to amplitude compensation and then, sent thereto, thus keeping linear the input-output power characteristic of the entire circuit (ie. both the linear compensating circuit and amplifier). According to the conventional linear compensating circuit as described above, the linearity of an input-output power characteristic of a high frequency power amplifier or other amplifier can be improved so that the distortion of an output signal can be reduced or eliminated when the amplifier is to be operated in the non-linear range because the level of an input signal is high. With this linear compensating circuit, however, when the amplifier is being operated in the saturation range because the input signal level is further increased, a satisfactory input-output characteristic cannot be obtained, and there thus remains the problem that the desired reduction in signal distortion cannot be attained. US-A-3900823 discloses that feedback can be used to reduce phase distortion and teaches controlling a variable phase shift network on the basis of a phase difference between the input and output signals. Thus, an object of this invention according to Claim 1 is to provide a linear compensating circuit capable of amplifying a signal at an extremely low level of signal distortion even when an amplifier is being operated in the saturation range. Another object of this invention is to provide an amplifier capable of amplifying a signal at an extremely low level of signal distortion in a wide range of an input signal. SUMMARY OF THE INVENTION In a first aspect of this invention, a linear compensating circuit is provided, which comprises first level detection means for detecting a level of an input signal to be inputted to amplifying means, second level detection means for detecting a level of an output signal to be outputted from the amplifying means, amplitude compensating means for controlling the amplitude of the input signal in accordance with a difference between a signal level detected by the first level detection means and a signal level detected by the second level detection means thereby improving the linearity of an input - output characteristic of the amplifying means, and phase compensating means for controlling the phase of the input signal, characterised in that said phase is controlled in accordance with a level of an output signal detected by the second level detection means. In general, when an input signal to an amplifier is subjected to amplitude compensation by an amplitude compensating circuit, an output signal of the amplifier has the phase characteristic degraded compared with the case when being not subjected to the amplitude compensation. Particularly, when it is applied to a power amplifier, if the input power is increased so that the amplifier is operated near its saturation range, the phase thereof is largely changed, resulting in a serious degradation. As a result, the output signal is largely distorted due to the degradation of phase characteristic in the vicinity of the saturation range. With the linear compensating circuit according to this invention, an input signal to an amplifier is subjected to amplitude and phase compensations with each other, so that the degradation of a phase characteristic induced due to amplitude compensation can be reduced or prevented. Accordingly, even when it is being operated if the vicinity of the saturation range, the distortion of an output signal from the amplifier can be satisfactorily reduced. The linear compensating circuit of this invention is effective when applied to a power amplifier, particularly to a high frequency power amplifier. As the first and second level detection means, for example, any detector that is known can be employed. The input or output signal of each of the first and second level detection means is preferably supplied through a branching circuit. In addition, it is preferable that the output signal has its level controlled to the same level as the input signal before being inputted to the second level detection means. As the amplitude compensating means, any circuit can be employed which can control the amplitude of the input signal in accordance with the level difference detected by the first and second level detection means. As the phase compensating means, any circuit can be employed which can control the phase of the input signal in accordance with the level of the output signal. In addition, it is preferable that the phase compensating means has a phase characteristic set so as to reduce or cancel the phase change of the output signal induced due to the amplitude compensation by the amplitude compensating means. For example, when it is applied to a power amplifier, if the input power becomes large, the phase of the output power is advanced in general. Therefore the phase compensating means has a phase compensating characteristic set so as to delay the phase of the input power signal, thus resulting in almost no change in phase after amplification. In a second aspect of this invention, an amplifier is provided which is extremely small in output distortion over a wide range of input signal. This amplifier has the linear compensation circuit according to the first aspect of this invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a circuit including a high frequency power amplifier with a linear compensating circuit according to an embodiment of this invention. Fig. 2 is graphs showing input-output characteristics of the circuit shown in Fig. 1. DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of this invention will be described below while referring to the drawings attached. Fig. 1 shows a high frequency power amplifier in which a linear compensating circuit of an embodiment of this invention is applied. In Fig 1, a high frequency power amplifier 5 has in the front stage thereof a first branching circuit 1 for branching an input signal to the amplifier 5, an amplitude compensating circuit 2 for compensating an amplitude of the input signal, and a phase compensating circuit 4 for compensating a phase of the input signal connected in series in that order . An input signal supplied from an input terminal 9 is passed through the first branching circuit 1, amplitude compensating circuit 2 and phase compensating circuit 4 and sent to the amplifier 5. The input signal branched by the first branching circuit 1 is sent to a first detector 3 composed of a diode for detecting the level thereof. The level signal thus detected is inputted to one input terminal of a differential amplifier 8. The first branching circuit 1 can control the level of a signal to be sent out. The amplifier 5 has a second branching circuit 6 for branching its output signal in the back stage thereof. The output signal thus amplified by the amplifier 5 is taken out from an output terminal 10. The output signal branched by the second branching circuit 6 is sent to a second detector 7 composed of a diode thereby to detect the level thereof. The level signal thus detected is delivered to the other input terminal of the differential amplifier 8 and simultaneously to the phase compensating circuit 4. The second branching circuit 6 also can control the level of a signal to be sent out. The output signal branched by the second branching circuit 6 is made equal in level to the input signal (that is, the level before amplification and amplitude compensation) in the second branching circuit 6 itself, and then, sent to the second detector 7. The level control of this output signal may be performed through a suitable circuit which is provided outside the second branching circuit 6. The differential amplifier 8 amplifies the difference between the signal level detected by the first detector 3 and that detected by the second detector 7 and sends it to the amplitude compensating circuit 2. The amplitude compensating circuit 2 controls, in other words, the increase or decrease the amplitude of the input signal in accordance with an output signal from the differential amplifier 8. Thus, the difference in variation between the input signal power from the input terminal 9 and the output signal power from the output terminal 10 is made substantially zero, thus keeping the input-output characteristic of the circuit shown in Fig. 1 linear. The phase compensating circuit 4 controls the phase of the input signal which is subjected to the compensation of amplitude in accordance with the level of an output signal from the second detector 7 so that the phase change of the input signal induced due by the compensation of amplitude can be canceled. With the circuit structured as above, the input signal supplied from the input terminal 9 is compensated in amplitude through the amplitude compensation circuit 2 and successively compensated in phase through the phase compensating circuit 4 so as to cancel the phase change induced due to the compensation of amplitude, and thereafter, sent to the amplifier 5. As a result, the linearity of an input-output characteristic of the circuit shown in Fig. 1 can be improved and the phase change can be almost perfectly canceled at the same time, thus making possible to output a signal from the output terminal 10 that is extremely small in distortion over a wide range of input signal power. Fig. 2 shows a change in an output signal (that is, signal power outputted from the output terminal 10) with a change in an input signal (that is, signal power inputted from the input terminal 9) and a change in the phase of an output signal with a change in an input signal of the circuit shown in Fig. 1. In the case of not using the amplitude compensating circuit 2, as the input power becomes large, the output power changes non-linearly as shown at (a) of Fig. 2, however, in the circuit shown in Fig. 1, the input signal is subjected to the compensation of amplitude through the amplitude compensating circuit 2 before being sent to the amplifier 5, so that the output power varies linearly with respect to the change of the input power as shown at (b) of Fig. 2. In addition, in the case of not using the amplitude compensating circuit 2, as the input power increases, the phase of the output power changes as shown at (c) of Fig. 2, however, by using the amplitude compensating circuit 2, the phase change is sharply increased, as shown at (d) of Fig . 2. That is, the use of the amplitude compensating circuit 2 makes possible to improve the linearity of the input-output characteristic, but adversely to increase the phase change. In the circuit shown in Fig.1, however, the phase characteristic of the phase compensating circuit 4 is set so as to oppose the characteristic shown at (d) of Fig. 2 in accordance with the signal level detected by the second detector 7, so that the signal level outputted from the second detector 7 becomes large as the output power of the amplifier 5 is increased, and the phase compensating circuit 4 operates so as to cancel the phase change of the output power of the amplifier 5. As a result, the phase characteristic as shown at (c) of Fig. 2 which is achievable when the amplitude compensating circuit 2 is not used still can be obtained. As explained above, according to the linear compensating circuit of this invention, the phase compensating circuit 4 is provided, which serves to operate oppositely to the phase change of the output power of the amplifier 5 in accordance with the output level of the same, and so that , not only the compensation of amplitude but also the compensation of phase of an input signal of the amplifier 5 becomes possible, resulting in obtaining an output signal with an extremely small distortion therefrom. In this embodiment, the amplitude compensating circuit 2 is disposed before the phase compensating circuit 4, but not limited thereto, thus, they may be inversely disposed, that is, the phase compensating circuit 4 may be disposed in the preceding stage of the the amplitude compensating circuit 2 because the both circuits 4 and 2 are disposed independently of each other. This means that the amplitude and phase of an input signal may be compensated before sending to the amplifier 5. In this embodiment, no specific circuits for the amplitude compensating circuit 2 and phase compensating circuit 4 are shown. However, any circuits may be used for this purpose with no limitation, provided they can operate to compensate amplitude or phase as required. As the phase compensating circuit 4, for example, a PIN diode may be employed and its bias voltage is changed in accordance with the output level of the amplifier 5. This fact can be applied to the first and second branching circuits 1 and 6 and as a result, any circuit may be employed for this purpose with no limitation, if it can branch and extract the input or output signal. As the first and second detection circuits 3 and 7, a diode is employed in this embodiment, but they are not limited thereto, any circuit may be employed for this purpose, if it can detect the level of a signal.

Claims: A linear compensating circuit for compensating non-linearity in an input-output characteristic of amplifying means comprising: first level detection means far detecting a level of an input signal to be inputted to said amplifying means; second level detection means for detecting a level of an output signal to be outputted from said amplifying means; amplitude compensating means for controlling an amplitude of said input signal in accordance with a difference between a signal level detected by said first detection means and a signal level detected by said second detection means thereby improving the linearity of an input-output characteristic of said amplifying means; and phase compensating means for controlling a phase of said input signal, characterized in that said phase is controlled in accordance with a level of an output signal detected by said second level detection means. The linear compensating circuit as claimed in claim 1, wherein said phase compensating means has a characteristic that at least partially cancels a phase change of said output signal induced due to amplitude compensation by said amplitude compensating means. The linear compensating circuit as claimed in Claim 1 or Claim 2, wherein at least one of said first and second level detection means is a detector. The linear compensating circuit as claimed in any preceding claim, wherein at least one of said input and output signals is extracted through a branching circuit. The linear compensating circuit as claimed in claim 4, wherein said branching circuit has means for controlling the level of a signal to be branched. An amplifier comprising a linear compensating circuit as claimed in any preceding claim. The amplifier as claimed in Claim 6, wherein said amplifying means is a power amplifier.

Inventors: FUJITA NORIYUKI C O NEC CORPOR, TANAKA MASAHIKO C O NEC CORPOR, FUJITA, NORIYUKI, C/O NEC CORPORATION, TANAKA, MASAHIKO, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1992
Main classification: H03G3
IPC codes: H03G3, H03F1