Abstract:
A module including a bias circuit that generates gate bias voltages by resistance dividers creates a problem in that the values of the resistances constituting the bias circuit must be finely adjusted, and accordingly extra trimming tasks are required. The present invention provides current generators that generate currents varying with desired characteristics responsive to a control voltage, independent of variations in transistor threshold voltages, connects output resistors to parallel transistors in respective stages to form current mirror circuits, and supplies currents from the current generators thereto to drive them, instead of supplying dividing voltages.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a multi-stage high-frequency power amplifier circuit with a plurality of cascaded semiconductor amplifier devices and technology that is useful when applied to wireless communication devices such as cellular phones incorporating a high-frequency power amplifier circuit, and more particularly to a high-frequency power amplifier circuit capable of obtaining output with desired characteristics, independent of variations in semiconductor amplifier device characteristics.  
         [0002]     The transmission output stage of car phones, cellular phones, and other wireless communication devices (mobile communication devices), as shown in  FIG. 1 , includes a multi-stage high-frequency power amplifier circuit with cascaded semiconductor amplifier devices Q 1 , Q 2 , and Q 3  made of MOSFETs (Metal Oxide Semiconductor Field-Effect Transistors), GaAs-MESFETs (Metal Semiconductor Field-Effect Transistors), or other applicable kinds of transistors. The high-frequency power amplifier circuit shown in  FIG. 1  generally includes a discrete last-stage semiconductor amplifier device Q 3  (such as an output power MOSFET), and preceding-stage semiconductor amplifier devices Q 1  and Q 2  and a bias circuit BIAS that are integrated onto a single semiconductor chip as a semiconductor integrated circuit. The combination of this discrete semiconductor amplifier device part and a semiconductor integrated circuit including a bias circuit, together with capacitive elements and other circuit elements will be referred to as a high-frequency power amplifier module or just as a module hereinafter.  
         [0003]     In general, a cellular phone system is configured to change its output (transmission power) in different communication environments according to power-level command signals from a base station, so as not to interfere with other cellular phones. For example, a high-frequency power amplifier module in the transmission output stage of cellular phones adopting the U.S. 900-MHz band standard system or the European GSM (Global System for Mobile Communications) system is configured so that the gate bias voltages of the output power MOSFETs Q 1  to Q 3  are controlled by the output voltage Vapc of an Automatic Power Control (APC) to produce the output power required for communication.  
         [0004]     Conventionally, the gate bias voltages of the output power MOSFETs are generated by using a bias circuit BIAS consisting of resistance dividers as shown in  FIG. 1 , in which the output voltage Vapc of the APC circuit is divided by the ratios of paired resistances R 11  and R 12 , R 21  and R 22 , and R 31  and R 32  to generate gate bias voltages Vg 1 , Vg 2 , and Vg 3  (see, for example, Unexamined Japanese Patent Publication No. Hei 11(1999)-150483).  
         [0005]     Some conventional systems, as shown in  FIG. 2 , use a bias circuit that is configured with a plurality of resistances R 1  to R 4  connected in series with a MOSFET Qd that functions as a diode, forming a resistive voltage in which the ratio of the resistance values is adjusted so that the maximum output power can be obtained when Vapc is in the high neighborhood of 2 V, generating the gate bias voltages Vg 1 , Vg 2 , and Vg 3  of the output power MOSFETs in each stage (see, for example, Unexamined Japanese Patent Publication No. 2001-102881).  
         [0006]     As described above, all of the conventional gate bias circuits above apply bias voltages generated by dividing the output voltage Vapc of the APC circuit to the gates of the output power MOSFETs.  
       SUMMARY OF THE INVENTION  
       [0007]     Output power MOSFETs show variations in threshold voltages due to manufacturing process variations and temperature changes. In addition, the last-stage MOSFET Q 3  among the output power MOSFETs, in particular, is often a discrete part. Therefore, the last-stage MOSFET Q 3  and preceding-stage MOSFETs Q 1  and Q 2  differ in regard to the variations in the threshold voltage. More specifically, the gate voltage-drain current characteristics of the output power MOSFETs are different from each other.  
         [0008]     In such a high-frequency power amplifier module configured with output power MOSFETs having different variations in their threshold voltages, if a gate bias voltage that is generated by dividing the output voltage Vapc of the APC circuit according to the ratio of resistances is applied to the gate terminals of the output power MOSFETs, the output characteristic of the high-frequency power amplifier circuit may deviate greatly from a desired characteristic. As a result, a module with a bias circuit that generates gate bias voltage by dividing resistances requires fine tuning of the resistance values making up the bias circuit; this obviously creates a problem in that extra trimming tasks or trimming resistors are required.  
         [0009]     Accordingly, an object of the present invention is to provide a high-frequency power amplifier circuit capable of obtaining desired characteristics without trimming the values of resistors making up the bias circuit.  
         [0010]     Another object of the present invention is to provide a high-frequency power amplifier circuit with better output controllability.  
         [0011]     Another object of the present invention is to provide a high-frequency power amplifier circuit capable of efficiently obtaining higher output with lower power consumption.  
         [0012]     The aforementioned and other objects and new features of the present invention will become clear from the description in this specification when read with reference to the attached drawings.  
         [0013]     The outline of a typical mode of practicing the invention disclosed herein will be described below.  
         [0014]     In a multi-stage high-frequency power amplifier circuit with a plurality of cascaded output semiconductor amplifier devices Q 1 , Q 2 , and Q 3 , the invention typically provides semiconductor amplifier devices Q 11 , Q 12 , and Q 13  connected to the plurality of output semiconductor amplifier devices to form current mirror circuits respectively, causing electric currents I 11 , I 12 , and I 13  changing with given characteristics according to control voltage to flow into the semiconductor amplifier devices and driving the plurality of output semiconductor amplifier devices with the currents.  
         [0015]     The method described above drives the output semiconductor amplifier devices with currents having given characteristics, thereby making it possible to obtain a high-frequency power amplifier circuit with output characteristics not sensitive to possible variations in the threshold voltages and other characteristics of the output semiconductor amplifier devices.  
         [0016]     The semiconductor amplifier devices are preferably field effect transistors, and the given characteristics are their gate voltage-drain current characteristics. Since the drain current of a field effect transistor is proportional to the square of the gate voltage, the control voltage can reduce the rate of change of the output in the vicinity of the threshold voltage of the field effect transistor and increase the rate of change of the output by increasing itself, thereby making it possible to achieve higher output controllability and larger output power.  
         [0017]     According to another aspect of the invention disclosed herein, in a high-frequency power amplifier circuit having a multi-stage output circuit with a plurality of cascaded semiconductor amplifier devices Q 1 , Q 2 , and Q 3  and a bias circuit that drives the semiconductor amplifier devices responsive to a control voltage, the invention provides semiconductor amplifier devices Q 11 , Q 12 , and Q 13  that are connected to the plurality of output semiconductor amplifier devices so as to form current mirror circuits; the bias circuit has a voltage-to-current converter  10 , a first resistance R 1  that converts currents I 1  and I 3  supplied from the voltage-to-current converter, a first constant-current source  31 , and a first semiconductor amplifier device Q 32  connected in series thereto; also included is a control voltage generator  30  that generates a voltage equal to the threshold voltage of the first semiconductor amplifier device; a second semiconductor amplifier device Q 21  (Q 31 ) generates current according to a combination of the voltage generated by the control voltage generator and the voltage converted by the first resistance; and currents I 11 , I 12 , and I 13  with the same characteristic as that of current I 21  flowing through the second semiconductor amplifier device are passed through the semiconductor amplifier devices connected to the plurality of output semiconductor amplifier devices in pair respectively so as to form the current mirror circuits to drive the plurality of output semiconductor amplifier devices.  
         [0018]     Preferably, the control voltage generator has a voltage follower  33  including a first differential circuit that receives a voltage equal to the threshold voltage of the first semiconductor amplifier device; the first resistance R 1  is connected to the output terminal of the voltage follower; and currents I 1  and I 3  fed from the voltage-to-current converter via the first resistance are caused to flow through the voltage follower. This enables the voltage follower, which has small output impedance, to sink sufficient currents I 1  and I 3  supplied from the voltage-to-current converter and generate a voltage proportional to the currents I 1  and I 3  supplied from the voltage-to-current converter through the first resistance to be applied to the control terminal of the second semiconductor amplifier device to produce current.  
         [0019]     More preferably, second constant-current sources  21   a  and  21   c  that are connected to the control terminals of the second semiconductor amplifier devices and sink current supplied from the voltage-to-current converter are provided. This can produce an initial control voltage at which the current passing through the second semiconductor amplifier devices starts to change, thereby making it easier to obtain a desired current characteristic.  
         [0020]     The first constant-current source  31  is configured to include a second differential circuit  312  receiving a constant voltage as an input from a band gap reference circuit  311  that generates a constant voltage and a third semiconductor amplifier device Q 30  that carries constant current from the output of the second differential circuit. This can make the current characteristic of the first output semiconductor amplifier device constant regardless of variations in the power source voltage.  
         [0021]     In addition, the second differential circuit  312  has its output fed back to the other input through a circuit including an amplifier device Q 44  and a second resistance R 4 . This feedback causes the second differential circuit to output a voltage Vc 1  proportional to the constant voltage Vref from the output terminal. The third semiconductor amplifier device Q 30  forms a current mirror with the amplifier device Q 44  to pass a current responsive to the value of the second resistance R 4  as a constant current I 4 . The second resistance R 4  is constituted by an external device. Since an external device can be provided with higher accuracy than a device integrated on a chip, using it can improve operation accuracy of the circuit.  
         [0022]     In addition, the second constant-current source  21   a  ( 21   c ) is configured to include a third differential circuit  321  that receives a constant voltage (Vref) as an input from the band gap reference circuit  311 , which generates a constant voltage with little dependecy on the power source voltage, and a fourth semiconductor amplifier device Q 31  that carries constant current responsive to the output Vc 2  of the third differential circuit. This makes it possible to provide the first output semiconductor amplifier device with a constant current characteristic independent of variations in the power source voltage.  
         [0023]     The output of the third differential circuit  321  is fed back to the other input via a circuit including an amplifier device Q 45  and a third resistance R 5 . This feedback causes the third differential circuit to output a voltage Vc 2  proportional to the constant voltage Vref from the output terminal. The fourth semiconductor amplifier device Q 31  constitutes a current mirror with the amplifier device Q 45  and is configured to pass current responsive to the value of the third resistance R 5  as a constant current I 5   a  (I 5   c ); the third resistance R 5  is formed integrally with the first to fourth semiconductor amplifier devices in the same semiconductor chip. When the output currents I 1  and I 3  vary due to manufacturing process variations in the value of the resistance R 2  provided in the voltage-to-current converter  10 , this configuration causes the same variation in the value of the third resistance R 5  as in the resistance R 2  in the voltage-to-current converter, which causes a current I 5   a  (I 5   c ) to change, thereby enabling the changes in the output currents I 1  and I 3  to be canceled.  
         [0024]     Furthermore, the first resistance, the second semiconductor amplifier device, and the second current source are provided for each of the plurality of the output semiconductor amplifier devices; the resistance value of the first-resistance and the current value of the second current source are set to mutually differing values. This enables separate control with desired characteristics of the plurality of output semiconductor amplifier devices, thereby making it possible to achieve a high-frequency power amplifier circuit with better output controllability and capable of efficiently obtaining higher output with lower power consumption. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]      FIG. 1  is a circuit diagram showing an example of a conventional multi-stage high-frequency power amplifier circuit;  
         [0026]      FIG. 2  is a circuit diagram showing another example of a bias circuit provided in a conventional high-frequency power amplifier circuit;  
         [0027]      FIG. 3  is a circuit diagram showing an embodiment of a high-frequency power amplifier circuit according to the present invention;  
         [0028]      FIG. 4  is a graph showing relations between a control voltage Vapc in a bias circuit and a gate voltage of a bias-current generating MOSFET in the high-frequency power amplifier circuit according to the present invention;  
         [0029]      FIG. 5  is a graph showing relations between a control voltage Vapc in the bias circuit and a drain current of the bias-current generating MOSFET in the high-frequency power amplifier circuit according to the present invention;  
         [0030]      FIG. 6  is a circuit diagram showing a specific example of a voltage-to-current converter that converts a control voltage Vapc to a current supplied to the bias circuit;  
         [0031]      FIG. 7  is a circuit diagram showing a specific example of a constant voltage circuit that generates a constant voltage independent of a power source voltage;  
         [0032]      FIG. 8  is a circuit diagram showing a specific example of a constant voltage circuit that generates a constant voltage that cancels variations in current output from the voltage-to-current converter;  
         [0033]      FIG. 9  is a circuit block diagram showing a part of a cellular phone including the high-frequency power amplifier circuit shown in  FIG. 3 ; and  
         [0034]      FIG. 10  is a block diagram showing the entire structure of a cellular phone using the high-frequency power amplifier circuit according to the embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     Preferable embodiments of the present invention will be described with reference to the attached drawings below. Although the following description will use a field effect transistor (FET) as an example of a semiconductor amplifier device, the semiconductor amplifier device is not limited to a field effect transistor; it may be a bipolar transistor, hetero-junction bipolar transistor (HBT), high-electron-mobility transistor (HEMT), or another applicable kind of transistor; a semiconductor substrate forming a transistor is not limited to a silicon substrate; it may also be a silicon-germanium or gallium-arsenic substrate, or one made of another applicable material.  
         [0036]      FIG. 3  shows an embodiment of a high-frequency power amplifier circuit according to the present invention. In  FIG. 3 , Q 1 , Q 2 , and Q 3  indicate output power MOSFETs used as output transistors; these output power MOSFETs Q 1 , Q 2 , and Q 3  are interconnected so that the drain terminal of the preceding-stage FET is connected to the gate terminal of the next-stage FET via capacitive elements C 1  and C 2 . The gate terminal of the first-stage output power MOSFET Q 1  receives a radio-frequency signal RFin via a capacitive element C 0 ; the drain terminal of the last-stage output power MOSFET Q 3  is connected to an output terminal OUT via a capacitive element C 3 , eliminates the direct-current components of the radio-frequency signal Rfin, amplifies only the alternate-current components, and outputs the resultant signal. The output power at this time is controlled by the bias circuit described below.  
         [0037]     This embodiment provides MOSFETs Q 11 , Q 12 , and Q 13  paired in parallel with the output power MOSFETs Q 1 , Q 2 , and Q 3 , their drains being connected to the gates of the output power MOSFETs Q 1 , Q 2 , and Q 3  respectively to provide a diode function; the gates of MOSFETs Q 11 , Q 12 , and Q 13  are connected with the gates of the output power MOSFETs Q 1 , Q 2 , and Q 3 , thereby forming current mirror circuits.  
         [0038]     In the high-frequency power amplifier circuit shown in  FIG. 3 , the last-stage output power MOSFET Q 3  and MOSFET Q 13  forming a current mirror circuit with Q 3  are formed onto a single semiconductor chip as a discrete part; the preceding-stage output power MOSFETs Q 1  and Q 2  and MOSFETs Q 11  and Q 12  paired with them respectively to form current mirror circuits, and a bias circuit that feeds bias current to MOSFETs Q 11  to Q 13  are formed on another single semiconductor chip as a semiconductor integrated circuit. The capacitive elements C 0 , C 1 , C 2 , and C 3  are discrete parts, connected to the semiconductor integrated circuit including the bias circuit as external devices.  
         [0039]     The bias circuit includes a voltage-to-current converter  10  that outputs currents I 1  and I 3  proportional to a control voltage Vapc; current generators  20 A,  20 B, and  20 C that generate currents with desired characteristics little depencency on thresholds, based on the currents I 1  and I 3 , and feed the currents to the MOSFETs Q 11 , Q 12 , and Q 13  for forming the current mirror circuits; and a gate voltage control circuit  30  that generates a bias voltage for these current generators  20 A,  20 B, and  20 C and causes the current generators  20 A,  20 B, and  20 C to output currents with given characteristics.  
         [0040]     The current generators  20 A and  20 C have the same circuit configuration. Although not limited to this, this embodiment has a structure in which the current generator  20 B that generates current I 12  fed to the MOSFET Q 12  connected to the second-stage output power MOSFET Q 2  to form a current mirror circuit includes a MOSFET Q 20  connected to an output MOSFET Q 22  of the current generator  20 A to form a current mirror circuit, and the MOSFET Q 20  generates current I 12  having the same characteristic as that of the current I 11  fed from the current generator  20 A to the MOSFET Q 11 . The ratio of currents I 11  and I 12  can be adjusted by changing the size ratio of MOSFETs Q 22  and Q 20 .  
         [0041]     The gate voltage control circuit  30  includes a constant-current source  31 ; a MOSFET Q 32  connected to the constant-current source  31  in series; and a differential amplifier  33  that receives a drain voltage of the MOSFET Q 32  to behave as a voltage follower. The MOSFET Q 32  with its gate and drain connected to function as a diode outputs a voltage equal to the threshold voltage Vgs 1  from the drain terminal to the differential amplifier  33 ; the differential amplifier  33  outputs a voltage at the same level as that of the input voltage, or at the same level as that of the threshold voltage Vgs 1  of the MOSFET Q 32 .  
         [0042]     The current generator  20 A includes a source-follower MOSFET Q 21 ; a resistance R 1  inserted between the gate terminal of the MOSFET Q 21  and the output terminal of the differential amplifier  33  in the gate voltage control circuit  30 ; a constant-current source  21   a  that is connected between the gate terminal of the MOSFET Q 21  and ground and passes constant current I 5   a ; a MOSFET Q 22  inserted in series between the drain terminal of the MOSFET Q 21  and a power source voltage terminal Vdd; and a MOSFET Q 23  with the gate terminal connected to the gate of the MOSFET Q 22  to form a current mirror circuit.  
         [0043]     The termination of the wiring for supplying current I 1  responsive to the control voltage Vapc output from the voltage-to-current converter  10  is linked to a connection node n 1  of the resistance R 1  and the gate terminal of the MOSFET Q 21 . This causes the current I 1  from the voltage-to-current converter  10  to be distributed to the constant-current source  21   a  and the output terminal of the differential amplifier  33  if I 1  is larger than I 5   a  (I 1 &gt;I 5   a ), and to be fed all to the constant-current source  21   a  if I 1  is smaller than I 5   a  (I 1 &lt;I 5   a ). If I 1  is smaller than I 5   a  (I 1 &lt;I 5   a ), current with a value equal to I 5   a -I 1  flows from the output terminal of the differential amplifier  33  toward the constant-current source  21   a  via the resistance R 1 .  
         [0044]     The operations of the current generator  20 A will be described below.  
         [0045]     First, suppose the current generator  20   a  without the constant-current source  21   a . Current I 1  from the voltage-to-current converter  10  flows through the resistance R 1  and the output terminal of the differential amplifier  33  into the differential amplifier. The output voltage of the differential amplifier  33  is at the same level of that of the threshold voltage Vgs 1  of the MOSFET Q 32  as described above. Therefore, the gate voltage VG 2  is higher than the output voltage Vgs 1  of the differential amplifier  33  by R 1 ·I 1 , or Vgs 1 +R 1 ·I 1 . The current I 1  fed from the voltage-to-current converter is proportional to the control voltage Vapc as described above.  
         [0046]     Therefore, the gate voltage VG 2  of the MOSFET Q 21  varies almost linearly with the control voltage Vapc, as indicated by line A shown in  FIG. 4 . Then, the current I 21  flowing through the MOSFET Q 21  varies according to the drain current characteristic of the MOSFET responsive to the control voltage Vapc, as indicated by curve a shown in  FIG. 5 . Since the MOSFETs Q 32  and Q 21  are formed on a single semiconductor chip, the amounts of variation in their threshold voltages are the same. Specifically, as the threshold voltage of Q 32  increases, the threshold voltage of Q 21  increases by the same amount; as the threshold voltage of Q 32  decreases, the threshold voltage of Q 21  also decreases by the same amount.  
         [0047]     As a result, the current I 21  flowing into the MOSFET Q 21  varies with a given characteristic regardless of the variations in the threshold voltage. Then the current I 21  is passed through two current mirror circuits formed by the pairs of MOSFETs Q 22  and Q 23  and MOSFETs Q 1   l  and Q 1  to the output power MOSFET Q 1 . Therefore, the output power MOSFET Q 1  carries a current with the same characteristic as the drain current characteristic of the MOSFET Q 21 . More specifically, even if the threshold voltage of the MOSFET Q 1  deviates from a desired value due to manufacturing process variations and temperature changes, an output characteristic independent thereof can be obtained.  
         [0048]     On the other hand, considering a case in which the constant-current source  21   a  is provided and current I 5   a  flows therein, since the current I 5   a  reduces a current flowing from the voltage-to-current converter  10  into the differential amplifier  33  via the resistance R 1 , the gate voltage VG 2  of MOSFET Q 21  is Vgs 1 +R 1  I 1 ·R 1 ·I 5   a . Specifically, when the constant-current source  21   a  is provided, the gate voltage VG 2  of the MOSFET Q 21  can be plotted by line B in  FIG. 4 , which is line A shifted downward by R 1 ·I 5   a.    
         [0049]     The current I 5   a  flowing into the constant-current source  21   a  is kept constant regardless of the control voltage Vapc. The current I 1  fed from the voltage-to-current converter  10  is proportional to the control voltage Vapc. Therefore, the slope of line B is the same as that of line A. This causes the current I 21  flowing through the MOSFET Q 21  varies responsive to the control voltage Vapc along curve b in  FIG. 5 . The slopes of lines A and B shown in  FIG. 4  can be adjusted by the value of the resistance R 1 , and can be reduced by reducing the value of the resistance R 1 , thus the controllability of the output power MOSFETs Q 1  to Q 3  by a control voltage Vapc can be improved.  
         [0050]     The current generator  20 C, as is the case with the current generator  20 A, includes a source-follower MOSFET Q 31 ; a resistance R 3  inserted between the gate terminal of the MOSFET Q 31  and the output terminal of the differential amplifier  33  in the gate voltage control circuit  30 ; a constant-current source  21   c  that is connected between the gate terminal of the MOSFET Q 31  and ground and passes a constant current I 5   c ; a MOSFET Q 32  inserted between the drain terminal of the MOSFET Q 31  and the power source voltage terminal Vdd in series; and a MOSFET Q 33  with the gate terminal connected to the gate terminal of the MOSFET Q 32  to form a current mirror circuit. The values of a current I 5   c  flowing through the constant-current source  21   c  and the resistance R 3  are set to values different from the values of a current I 5   a  flowing through the constant-current source  21   a  in the current generator  20 A and the resistance R 1 .  
         [0051]     This causes the gate voltage VG 3  of the MOSFET Q 31  varies according to line C in  FIG. 4 , for example. As a result, the drain current, or the bias current I 13  flowing in the current mirror circuit formed by the pair of MOSFETs Q 13  and Q 3  has a characteristic indicated by curve c in  FIG. 5 . Optimal setting of the values of the current I 5   c  flowing through the constant-current source  21   c  in the current generator  20 B and the resistance R 3 , and the values of the current I 5   a  flowing through the constant-current source  21   a  in the current generator  20 A and the resistance R 1  makes it possible to obtain larger output with a smaller operating current.  
         [0052]     The bias circuit according to the embodiment shown in  FIG. 1  configures the current generator  20 B that generates a bias current for the second-stage output MOSFET Q 2  only with the MOSFET Q 20  connected to the MOSFET Q 22  in the current generator  20 A to form a current mirror, but it is also possible to configure the current generator  20 B in the same way as with the current generator  20 A and appropriately set the values of a current flowing through a current source equivalent to the constant-current source  21   a  and a resistance equivalent to the resistance R 1  according to the output MOSFETs Q 1  to Q 3  used therein, thereby obtaining a desired characteristic for the entire module.  
         [0053]      FIG. 6  shows a specific example of the voltage-to-current converter  10  that generates currents I 1  and I 3  fed to the current generators  20 A and  20 C based on a control voltage Vapc.  
         [0054]     The voltage-to-current converter  10  of this embodiment includes a differential amplifier  11  configured with a CMOS differential amplifier circuit that receives a control voltage Vapc as an input; MOSFETs Q 41 , Q 42 , and Q 43  with the gate terminals to which a potential at the inverting output node of the differential amplifier  11  is applied; and a resistance R 2  that is connected to the MOSFET Q 41  in series: in which a potential at the connection node of the MOSFET Q 41  and the resistance R 2  is fed back to the other input terminal of the differential amplifier  11 , whereby a drain current proportional to the input voltage Vapc is fed to each of the MOSFETs Q 42  and Q 43 . The value of the drain current to the MOSFETs Q 42  and Q 43  can be set arbitrarily according to the size ratio (gate width W ratio) of the MOSFETs Q 41  and Q 42  and the value of the resistance R 2  and the size ratio of the MOSFETs Q 41  and Q 43  and the value of the resistance R 2 , which is output as currents I 1  and I 3  fed to the current generators  20 A and  20 C respectively.  
         [0055]      FIG. 7  shows a specific example of a constant-current circuit as the constant-current source  31  that constitutes the gate voltage control circuit  30 . The constant-current source  31  of this embodiment includes a band gap reference circuit  311  that generates a reference voltage Vref with little dependency on the power source; a differential circuit  312  receives a reference voltage Vref from the band gap reference circuit  311  as an input at one of its input terminals; a MOSFET Q 44  that receives the output of the differential circuit  312  at the gate terminal; a resistance R 4  inserted between the MOSFET Q 44  and ground; and MOSFET Q 30  that receives the output voltage Vc 1  of the differential circuit  312  at the gate terminal: in which feedback is carried out from the point of interconnection of the MOSFET Q 44  and the resistance R 4  to the other input terminal of the differential circuit  312 .  
         [0056]     In the constant-current source  31  shown in  FIG. 7 , the output of the differential circuit  312  is fed back to the other input terminal thereof through a circuit including the MOSFET Q 4  and the resistance R 4 . This makes it possible to obtain a constant voltage Vc 1  responsive to the value of the resistance  4  from the output terminal of the differential circuit  312 . Since the constant voltage Vc 1  is also supplied to the gate of the MOSFET Q 30 , the MOSFET Q 30  forms a current mirror in combination with the MOSFET Q 44 . Therefore, the MOSFET Q 30  passes a current responsive to the value of the resistance R 2  as a current I 4 . This embodiment uses an external resistive element as the resistance R 2 . This provides a configuration capable of outputting a current I 4  with higher accuracy than obtainable with a resistance formed on the chip.  
         [0057]      FIG. 8  shows a specific example of a constant-current circuit as the constant-current source  21   a  ( 21   c ) that constitutes the current generator  20 A ( 20 C). The constant-current source  21   a  ( 21   c ) of this embodiment includes a differential circuit  321  that receives a reference voltage Vref output from a band gap reference circuit  311  shared with the constant-current source  31  as an input at one of the input terminals; a MOSFET Q 45  that receives the output of the differential circuit  321 ; a resistance R 5  inserted between the MOSFET Q 45  and ground; a MOSFET Q 31  that receives the output voltage Vc 2  of the differential circuit  321  at the gate terminal; a MOSFET Q 32  that is connected to the MOSFET Q 31  in series; and a MOSFET Q 33  that is connected to the Q 32  to form a current mirror: in which feedback is carried out from the point of interconnection of the MOSFET Q 45  and the resistance R 5  to the other input terminal of the differential circuit  312 .  
         [0058]     In the constant-current circuit  21   a  ( 21   c ) shown in  FIG. 8 , the output of the differential circuit  321  is fed back to the input terminal via a circuit including the MOSFET Q 45  and the resistance R 5 . This makes it possible to obtain a constant voltage Vc 2  responsive to the value of the resistance R 5  from the output terminal of the differential circuit  321 . Since the constant voltage Vc 2  is also supplied to the gate terminal of the MOSFET Q 31  that constitutes a current mirror in combination with the MOSFET Q 45 , the current responsive to the value of the resistance R 5  flows through MOSFET Q 31 . The circuit is configured so that the drain current flowing in the MOSFET Q 31  is copied by MOSFETs Q 32  and Q 33  that form a current mirror circuit in pair and a constant current I 5   a  (I 5   c ) flows through the MOSFET Q 33 .  
         [0059]     Unlike the constant-current source  31  that passes the constant current I 4  shown in  FIG. 7 , the constant-current source of this embodiment employs a resistance formed on the chip as the resistance R 5 , rather than an external device. When the output currents I 1  and I 3  change due to manufacturing process variations of the resistance R 2  provided in the voltage-to-current converter  10  shown in  FIG. 6 , this configuration causes the same variation in the value of the resistance R 5  that constitutes the constant-current source  21   a  ( 21   c ) shown in  FIG. 8  as in the resistance R 2 , which causes an output current I 5   a  to change, thereby enabling the changes in the output current I 1  and I 3  fed from the voltage-to-current converter  10  to be cancelled.  
         [0060]     The differential circuits  312  and  321 , although not limited to this configuration, include a pair of differential transistors Qp 1  and Qp 2 , current mirror transistors Qc 1  and Qc 2  that constitute the load circuits thereof, and a constant current device I.  
         [0061]      FIG. 9  is a block diagram showing a portion of a cellular phone incorporating the high-frequency power amplifier circuit shown in  FIG. 3 .  
         [0062]     In this cellular phone, as shown in  FIG. 9 , an RF transmission signal generated by oscillating operations of a modulation oscillator (VCO)  70  is input to the input terminal (Pin) of a high-frequency power amplifier module  1 . The RF transmission signal is amplified in the high-frequency power amplifier module  1  and output from the output terminal (Pout), then transmitted through a power detector  71  and transmission filter  72  to an antenna  73 , and transmitted as a radio wave therefrom.  
         [0063]     On the other hand, an RF receive signal is subject to signal processing in a receiver circuit  80 . The receiver circuit  80  outputs a received signal strength indication signal SRI, which is converted to a digital signal in an A/D converter  81  and supplied to a control logic  82 . The control logic  82  outputs a power-level command signal SPL, which is supplied to a control logic circuit  84  in an output level controller  83 . The control logic circuit  84  processes the received power-level command signal SPL to generate a control code, and the generated control code is converted to an analog signal in a D/A converter  85 , which is supplied to the automatic power control (APC) circuit  74  as a power level command voltage VPL. The APC circuit  74  forms a power control signal Vapc responsive to the power level command voltage VPL and supplies it to the high-frequency power amplifier module  1 , which drives an output transistor in response to this signal. Reference numeral  90  indicates a battery that supplies a power source voltage Vdd to the high-frequency power amplifier module  1 .  
         [0064]      FIG. 10  is a block diagram showing the entire structure of a cellular phone using the high-frequency power amplifier circuit of the embodiment.  
         [0065]     The cellular phone of this embodiment includes a liquid crystal panel  200  as a display unit; an antenna  321  for transmitting and receiving signals; a voice output speaker  322 ; a voice input microphone  323 ; a liquid crystal control deriver  310  that drives the liquid crystal panel  200  to provide a display; an audio interface  330  that inputs and outputs signal for the speaker  322  and the voice input microphone  323 ; a high-frequency interface  340  that makes a GSM-system cellular phone communication via the antenna  321 ; a Digital Signal Processor (DSP)  351  that performs signal processing of voice signals and transmitting and receiving signals; an Application Specific Integrated Circuits (ASIC)  352  that provides custom functions (user logic); a system controller  353  including a micro processor or micro computer that controls the entire system including display control; a memory unit  360  for storing data and programs; an oscillator (OSC)  370 ; and other components. The DSP  351 , ASIC  352 , and micro computer  353  as a system controller constitute a so-called base band unit  350 . The high-frequency power amplifier circuit of the embodiment above is employed in the transmission output unit of the high-frequency interface  340 .  
         [0066]     Although the invention has been described above in its preferred embodiments, it is needless to say that the invention is not limited to the specific embodiments described above and various changes may be made without departing from the spirit or scope of the invention. For example, the embodiments use three-stage output transistors, but it allows use of two-stage or four- or-more-stage structure. In addition, although it has been described that the last-stage output transistor Q 3  and the transistor Q 3  connected thereto to form a current mirror circuit are formed in a discrete chip, they may be formed together with the other output transistors Q 1  and Q 2  in the same chip as the one with the bias circuit, or in contrast, the output transistor Q 1  and the current mirror transistor Q 11  or the output transistor Q 2  and the current mirror transistor Q 12  may also be formed in a discrete chip.  
         [0067]     Although the invention made by the inventors has been described above mainly in relation to a high-frequency power amplifier circuit applied in wireless communication devices, the invention is not limited to this field, but may be widely applied to multi-stage amplifier circuits with a plurality of cascaded semiconductor amplifier devices and systems including such circuits.  
         [0068]     The outline of a typical mode of practicing the invention disclosed herein will be described below.  
         [0069]     Specifically, since the output transistors are driven with currents having desired characteristics responsive to a control voltage, it is possible to obtain a high-frequency power amplifier circuit with output characteristics not sensitive to possible variations in the threshold voltages of the output transistors. In addition, driving an output transistor at each stage with a separate bias current makes it possible to achieve a high-frequency power amplifier circuit with better output controllability and capable of efficiently obtaining higher output with lower power consumption.