The present invention relates to a power amplifier, module designed for a portable terminal unit used in a mobile communication system. More specifically, the present invention is related to a power amplifier module designed for a cellular telephone system requiring a high efficiency and high linearity.
As conventional techniques related to power amplifier modules, there are JP-A-7-154169 and U.S. Pat. No. 5,629,648.
Very recently, while mobile communication markets typically known as cellular telephone systems are considerably expanded, new communication systems such as the broadband CDMA system and the EDGE system have been conducted. In such systems, high efficiencies and high linearity of portable terminal units are required. In particular, as to power amplifier modules which constitute one of major electronic components of terminal units, there is such a serious problem that these contradicting performance requirements are realized at the same time. Conventionally, many attempts have been made so as to solve this problem. As a typical example, in general, one control system has been proposed. In this system, an input power level of a power amplifier module is detected, and then, an operation condition of a post-stage amplifier which constitutes the power amplifier module is controlled by this signal. For instance, in a prior-investigated technical example shown in FIG. 3, a gate voltage of a transistor 111 which constitutes a post-stage amplifier 101 is controlled based upon a DC voltage obtained by envelope-detecting and smoothing an output signal of a pre-stage amplifier 102 in a DC voltage generating circuit 103 which is arranged by a directional coupling device 106, a detecting diode 107, and a low-pass filter 108. The DC voltage is increased/decreased in response to a power level inputted into a terminal 104, namely, a gate voltage of the post-stage amplifier 101 is controlled in response to the input power level. Also, in a prior-investigated technical example shown in FIG. 4, a power supply current of a first amplifier 201 is detected by a power supply voltage control circuit 203, and a power supply voltage corresponding to this current value is generated so as to control a power supply voltage of a second amplifier 202. Since the power supply current of the first amplifier 201 is changed in correspondence with a level of power inputted into a terminal 204, the power supply voltage of the second amplifier 202 is controlled in response to the input power level.
In the above-explained prior-investigated technical example, the DC voltage generating circuit 103 is required in order to envelope-detect and smooth the output power level of the pre-stage amplifier 102. However, since this DC voltage generating circuit 103 must be separately provided with reference to the amplifiers 101 and 102, there is such a problem that the stable characteristic of this DC voltage generating circuit 103 can be continuously and hardly guaranteed without any adjustment with respect to variations of environment conditions such as manufacturing deviation and ambient temperatures, and also power supply voltages. Furthermore, since the DC voltage generating circuit 103 is constituted by such components having different characteristics as the directional coupling device 106, the detecting diode 107, and the low-pass filter 108, there is another problem that the DC voltage generating circuit 103, and the amplifiers 101, 102 can be hardly formed in an integrated circuit.
In the prior-investigated technical example of FIG. 4, the DC-to-DC converter is required to be employed as the power supply voltage control circuit 203 for controlling the second amplifier 202. However, there are such problems that the use of such a DC-to-DC converter may constitute a factor of impeding the integrating manufacture of electronic components, and furthermore, the module is made bulky as well as the manufacturing cost is increased.
Also, the circuit system disclosed in the above-described U.S. Pat. No. 5,629,648 corresponding to the prior art may be considered. In this circuit system, since the output of the reference transistor, namely, the voltage at the bias point corresponding to the base input of the main transistor is not substantially changed in connection with the change in the input amplitudes, there is a problem that the output of the reference transistor can hardly give effectively influences to up/down of the operating point of the main transistor.
An object of the present invention is to solve the problems of the above-described prior art and also the problems of the above-explained prior-investigated technical examples, and is to provide a low-cost power amplifier module which owns a high efficiency and a high linear characteristic, and further, can be easily manufactured in an integrated circuit form.
To achieve the above-described object, a power amplifier module, according to the present invention, is featured by that a reference amplifier is newly employed which simulates operations of respective stages of amplifiers for constituting the power amplifier module, a current flowing through an input terminal of this reference amplifier in response to an input power level is detected/amplified, and then, the detected/amplified current can be supplied as an input current of the above-described amplifiers.
In the present invention, an input signal is supplied via individual capacitors to the reference amplifier and the respective stage amplifiers. At this time, although the current responding to the input power level flows through the input terminal of the reference amplifier, no current flow through the input terminals of the respective stage amplifiers. Therefore, if a DC component of the input current of the reference amplifier is detected and amplified, and the detected/amplified DC component current is supplied to the input terminals of the respective stage amplifiers, then the respective stage amplifiers commence high frequency operations. When the input power level is increased, the input current of the reference amplifier is increased, so that the input currents supplied to the respective stage amplifiers are also increased. Conversely, when the input power level is lowered, the input current of the reference amplifier is decreased, so that the input currents supplied to the respective stage amplifiers are also decreased. In other words, since the operating point corresponding to the input power can be set, even when the small input power is supplied, a relatively high efficiency can be obtained.