Abstract:
A power amplifier includes: a semiconductor substrate; a preceding-stage amplifying device on the semiconductor substrate, amplifying an input signal; a following-stage amplifying device on the semiconductor substrate, amplifying an output signal of the preceding-stage amplifying device; and an inter-stage matching circuit connecting the preceding-stage amplifying device to the following-stage amplifying device. The preceding-stage amplifying device has a first field effect transistor; the following-stage amplifying device has a heterojunction bipolar transistor; and the inter-stage matching circuit has a capacitance galvanically separating the output terminal of the preceding-stage amplifying device from the input terminal of the following-stage amplifying device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power amplifier formed by a BiFET process and, more particularly, to a power amplifier capable of realizing equivalent power characteristics of a HBT (Heterojunction Bipolar Transistor) power amplifier while obtaining low noise characteristics. 
     2. Background Art 
     GaAs-FET (Field Effect Transistor) power amplifiers have a negative threshold voltage and, therefore, have a drawback of requiring a negative gate bias voltage. In contrast, GaAs-HBT (Heterojunction Bipolar Transistor) power amplifiers require no negative gate bias voltage, being capable of single power supply operation and having more uniform device characteristics in comparison with FET power amplifiers. For this reason, use of GaAs-HBT power amplifiers in CDMA (Code Division Multiple Access) portable telephones, wireless LAN (Local Area Network) devices, etc., has been markedly increased. 
     A BiFET process for making a FET together with a HBT on a single substrate has recently been applied to products. Ordinarily, in a GaAs BiFET process, an HBT and a depletion mode FET (that is normally on) are mounted on a single substrate. Further, a process in which an enhancement mode FET (that is normally off) is made in addition to an HBT and a depletion mode FET on a single substrate has recently been reported in a learned society (IEEE: Radio Frequency Integrated Circuits Symposium 2008). 
     SUMMARY OF THE INVENTION 
       FIG. 13  is a circuit diagram showing an HBT power amplifier. This power amplifier is a two-stage amplifier. A preceding-stage amplifying device Q 1  and a following-stage amplifying device Q 2  are HBTs. Bias 1  and Bias 2  denote bias circuits for respectively supplying bias currents to the bases of the Q 1  and Q 2 . IN denotes an RF signal input terminal. OUT denotes an RF signal output terminal. R 1  to R 4  denote resistors. C 1  to C 10  denote capacitors. L 1  and L 2  denote inductors. L 3  to L 8  denote lines having particular electrical lengths and functioning as inductors. Vc 1  and Vc 2  respectively denote power terminals of Q 1  and Q 2 . Vcb denotes a power terminal of Bias 1  and Bias 2 . Vref denotes a terminal to which a reference voltage for Bias 1  and Bias  2  is externally supplied. 
       FIG. 14  is a circuit diagram showing a following-stage amplifying device and a following-stage bias circuit. Qb 1  to Qb 5  denote GaAs-HBTs. Rb 1  to Rb 6  denote resistors. The following-stage bias circuit Bias 2  is an emitter follower circuit and supplies to a base of the Q 2  a voltage according to the reference voltage. An RF signal input from a terminal RFin is inputted to the base of Q 2  via C 4  of an input matching circuit. The amplified RF signals are outputted from the collector of the Q 2  to the terminal RFout. The Bias 2  operates so as to constantly maintain the idling current Ictq 2  through Q 2  (the bias current when no RF signal is inputted) under varying temperature. The circuit configurations of the preceding-stage amplifying device Q 1  and the preceding-stage bias circuit Bias 1  are also the same. 
       FIG. 15  is a diagram showing the input-output characteristics of the power amplifier shown in  FIG. 13 . IF the input power Pin increases, the idle current Ictq 2  is constant but the output power Pout and the corrector current Ic 2  increase. From the fact that the power gain Gp is substantially constant, it is known that the power amplifier can distortion-freely and linearly amplify signals such as W-CDMA wherein the amplitude of the modulating signals are instantly significantly varied. 
       FIG. 16  is a block diagram showing a W/N (wide band/narrow band)-CDMA terminal machine which uses HBT power amplifiers. A BPF (Band-Pass Filter) provided in a stage before the power amplifier removes noise signals other than the output signal band of the RF/IF-LSI. 
       FIG. 17  is a diagram showing a spectrum waveform at the terminal PAIN shown in  FIG. 16 .  FIG. 18  is a diagram showing a spectrum waveform at the terminal OUT shown in  FIG. 16 . Here, Band 1  (having a transmitting band frequency of 1920 to 1980 MHz and a receiving band frequency of 2100 to 2160 MHz) of W-CDMA is used as an example. The receiving channel for a transmitting signal of 1950 MHz is 2130 MHz. However, as shown in  FIG. 17 , the noise level of the receiving band frequency at the terminal PAIN is reduced to the noise level of the natural world (−174 dBm/Hz) by the BPF. If this signal is provided to the power amplifier, the signals in the transmitting band frequency and the receiving band frequency are substantially uniformly amplified, and a main signal fo is amplified from Pin to Pout. In this case, the frequency characteristics of the power gain of the power amplifier are almost uniform near fo, thereby noise in the receiving band frequency is also amplified so as to obtain the substantially same gain as the gain obtained near fo. The main signal fo is modulated, thereby the wave form has fat tail as shown in  FIG. 18 . 
     If the power amplifier performs amplifying operations, noise near DC is mixed with the second harmonic of the power amplifier, thereby being up-converted to the neighborhood of the main signal fo. This is added to the noise of the receiving band frequency. As a result, as shown in  FIG. 18 , the noise of the receiving band frequency increases. Several tens of dB of this noise in the receiving band frequency is suppressed by a duplexer (a filter which divides the transmitting band frequency and the receiving band frequency) provided in the subsequent stage of the power amplifier. However, the leaked noise is directly inputted to the low noise amplifier in the receiver so as to deteriorate the receive sensitivity. In the W-CDMA terminal machine, the receiving band frequency noise level of the output of the power amplifier is required to be suppressed to about −135 to −140 dBm/Hz. 
     In general, the following ways are effective in reducing noise in the receiving band frequency during the power amplifying operation: (i) reducing the gain of the power amplifier; (ii) reducing the noise figure (NF) of the power amplifier in the receiving band frequency; (iii) suppressing the upconversion amount of DC noise; and (iv) reducing noise flowed from the bias circuit into the amplification stage of the HBT amplifier. 
     However large shot noise is generated in HBTs when carriers transport across pn junctions. Therefore, the HBTs have larger noise figures in comparison with FETs such as MOSFETs (Metal Oxide Semiconductor FETs) and HEMTs (High Electron Mobility Transistors). As a result, the HBT power amplifiers generally have higher receiving band frequency noise in comparison with the FET power amplifiers having same gains and same output powers. However, in comparison with the FETs, the HBTs can be fabricated at high yield, and have high power densities. Therefore, the chip size of the HBTs can be reduced if their output levels are not influenced by the heat. As a result, the HBT power amplifiers are presently used in many portable terminals. 
     In view of the above-described problem, an object of the present invention is to provide a power amplifier capable of realizing equivalent power characteristics of a HBT power amplifier and obtaining low noise characteristics. 
     According to one aspect of the present invention, a power amplifier includes: a semiconductor substrate; a preceding-stage amplifying device on the semiconductor substrate, amplifying an input signal; a following-stage amplifying device on the semiconductor substrate, amplifying an output signal of the preceding-stage amplifying device; a preceding-stage bias circuit supplying bias currents to an input terminal of the preceding-stage amplifying device; a following-stage bias circuit supplying bias currents to an input terminal of the following-stage amplifying device; and an inter-stage matching circuit connected between an output terminal of the preceding-stage amplifying device and the input terminal of the following-stage amplifying device; wherein the preceding-stage amplifying device has a first field effect transistor of enhancement mode or a depletion mode; the following-stage amplifying device has a heterojunction bipolar transistor; and the inter-stage matching circuit has a capacitor which galvanically divides the output terminal of the preceding-stage amplifying device and the input terminal of the following-stage amplifying device. 
     The present invention makes it possible to realize equivalent power characteristics of a HBT power amplifier and have low noise characteristics. 
     Other and further objects, features and advantages of the invention will appear more fully from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram showing a power amplifier according to the first embodiment. 
         FIG. 2  is a circuit diagram showing a preceding-stage amplifying device and a preceding-stage bias circuit according to the first embodiment. 
         FIG. 3  is a sectional view showing a power amplifier according to the first embodiment. 
         FIG. 4  is a circuit diagram showing a preceding-stage amplifying device and a preceding-stage bias circuit according to a second embodiment. 
         FIG. 5  is a circuit diagram showing a preceding-stage amplifying device and a preceding-stage bias circuit according to a third embodiment. 
         FIG. 6  is a block diagram showing a negative voltage generating circuit according to the third embodiment. 
         FIG. 7  is a circuit diagram showing a level control circuit which is used in the negative voltage generating circuit according to a third embodiment. 
         FIG. 8  is a diagram showing a reference voltage generation circuit according to the third embodiment. 
         FIG. 9  is a graph showing the Ids-Vds characteristics of a depletion mode FET. 
         FIG. 10  is a graph showing the Ids-Vds characteristics of an enhancement mode FET. 
         FIG. 11  is a graph showing the Ids/gm-Vgs characteristics of a depletion mode FET and an enhancement mode FET. 
         FIG. 12  is a circuit diagram showing a preceding-stage amplifying device and a preceding-stage bias circuit according to the fourth embodiment. 
         FIG. 13  is a circuit diagram showing an HBT power amplifier. 
         FIG. 14  is a circuit diagram showing a following-stage amplifying device and a following-stage bias circuit. 
         FIG. 15  is a diagram showing the input-output characteristics of the power amplifier shown in  FIG. 13 . 
         FIG. 16  is a block diagram showing a W/N (wide band/narrow band)-CDMA terminal machine which uses HBT power amplifiers. 
         FIG. 17  is a diagram showing a spectrum waveform at the terminal PAIN shown in  FIG. 16 . 
         FIG. 18  is a diagram showing a spectrum waveform at the terminal OUT shown in  FIG. 16 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       FIG. 1  is a circuit diagram showing a power amplifier according to the first embodiment. This power amplifier is a two-stage amplifier formed by a BiFET process for making a FET together with HBTs on a single substrate. 
     The area within the dotted-line frame is a GaAs chip, and other circuit elements are composed of chip parts and wirings formed on a module substrate. A preceding-stage amplifying device Fe 1  which amplifies an input signal and a following-stage amplifying device Q 2  which amplifies an output signal of Fe 1  are formed on a single GaAs substrate. Fe 1  denotes an enhancement mode FET (HEMT). Q 2  denotes a HBT. 
     Bias 1  denotes a preceding-stage bias circuit which provides bias currents to the gate of Fe 1 . Bias 2  denotes a following-stage bias circuit which provides bias currents to the base of Q 2 . The circuit configuration of Bias 2  is same as the circuit configuration shown in  FIG. 14 . IN denotes a RF signal input terminal. OUT denotes a RF signal output terminal. R 2  to R 4  denote resistors. C 1  to C 10  denote capacitors. L 1  and L 2  denote inductors. L 3  to L 8  denote lines having particular electrical lengths and functioning as inductors. Vc 1  denotes a power terminal of Fe 1 . Vc 2  denotes a power terminal of Q 2 . Vcb denotes power terminals of Bias 1  and Bias 2 . Vref denotes a terminal to which a reference voltage for Bias 1  and Bias  2  is externally supplied. In many cases the reference voltage for HBT is about 2.8 to 2.9 V. 
     C 3 , C 4 , and L 2  form an inter-stage matching circuit connected between the drain of Fe 1  and the base of Q 2 . Nowadays, in many cases, C 1 , C 2 , and L 1  acting as an input matching circuit and C 3 , C 4 , and L 2  acting as the inter-stage matching circuit are also integrated on the GaAs chip. 
       FIG. 2  is a circuit diagram showing a preceding-stage amplifying device and a preceding-stage bias circuit according to the first embodiment. Fe 2  and Fe 3  denote enhancement mode FETs. Rbb 1  and Rbb 2  denote resistors. The gate of Fe 2  is connected to Terminal Vref via Rbb 1 . A reference voltage is supplied to the gate of Fe 2 . The drain of Fe 2  is connected to a power supply via the terminal Vcb. The source of Fe 3  is grounded. The gate and drain of Fe 3  is connected to the source of Fe 2  via Rbb 2  and to the gate of Fe 1  via R 3 . 
       FIG. 3  is a sectional view showing a power amplifier according to the first embodiment. The HBT acting as the preceding-stage amplifying device and the HEMT acting as a following-stage amplifying device are formed on the single GaAs substrate  10 . The HBT includes a sub-collector layer  12 , a collector layer  14 , a base layer  16 , an emitter layer  18 , and an emitter contact layer  20  that are sequentially formed on the GaAs substrate  10 ; a collector electrode  22  connected to the sub-collector layer  12 ; a base electrode connected to the base layer  16 ; and an emitter electrode  26  connected to the emitter contact layer  20 . The HEMT includes a gate electrode  28 ; a source layer  30  and a drain layer  32  arranged on both sides of the gate electrode  28 ; a source electrode  34  connected to the source layer  30 ; and a drain electrode  36  connected to the drain layer  32 . 
     As described above, in this embodiment, an enhancement mode FET (HEMT) which has preferable noise characteristics is used as a preceding-stage amplifying device, and a HBT which has a high power density is used as a following-stage amplifying device. Thereby, equivalent power characteristics (an output power, a power gain, efficiency, and distortions) of the HBT power amplifier shown in  FIG. 13 , which has HBTs as the preceding-stage and following-stage amplifying devices, can be realized and low noise characteristics which are features of the HEMT can be obtained. 
     For example, in the case of the preceding-stage amplifying device (HBT) and the preceding-stage bias circuit as shown in  FIG. 13 , the noise figure in the 2 GHz band is about 4 dB or more (Only the HBT has the noise figure of 2 dB or more). On the other hand, in the case of the preceding-stage amplifying device (HEMT) and the preceding-stage bias circuit according to this embodiment, the noise figure in the 2 GHz band can be reduced to about 2 dB or less. As a result, the power amplifier according to this embodiment can reduce 2 dB or more receiving band frequency noise compared with the HBT power amplifier as shown in  FIG. 13 . 
     However, considering the fluctuation of manufacturing conditions, the threshold voltage of the enhancement mode HEMT needs to be about +0.15V or more so as to sufficiently suppress the leak current during the shutdown of the amplifier (to suppress the total leak current to about 10 uA or less when 3.4 V is applied to the terminals Vcb, Vc 1 , Vc 2  and the voltage of Terminal Vref is set to 0V). In case of the GaAs HEMP, the junction of gate is a Schottky junction (occasionally a pn junction), thereby a maximum gate voltage is limited to about 0.7-0.8 V (about 1.1-1.2V in the case of the pn junction). Therefore, if the threshold voltage is too high for the suppression of excessive leak current, the effective gate voltage range (0.8-0.15 V) becomes narrower so that it is hard to obtain a large current amplitude, so that the output power of the single transistor is deficient. If the threshold value of the enhancement mode FET is set by considering this matter, the leak current can be reduced equally to the HBT power amplifier and equivalent power characteristics (output power, power gain, efficiency, and distortion) of the HBT power amplifier can be obtained. 
     The power amplifier according to this embodiment includes an inter-stage matching circuit comprising two capacitors, C 3  and C 4 , which galvanically divide the drain voltage of the preceding-stage amplifying device Fe 1  and the base bias voltage of the following-stage amplifying device Q 2 . This division is in contrast with the circuit wherein the electrode terminal of the HEMT is directly DC connected to the electrode terminal of the HBT (see, e.g., Japanese Patent Laid-Open No. 2006-278544, Japanese Patent Laid-Open No. 2007-194412, Japanese Patent Laid-Open No. 62-242419, and Japanese Patent Laid-Open No. 9-246877). Because this embodiment is aimed at a narrow band frequency amplifier for wireless communications, the input, inter-stage, and output matchings can be realized by using relatively small capacitors that are suitable for amplifying only a specific RF frequency band. 
     The preceding-stage bias circuit Bias 1  according to this embodiment is the current mirror circuit shown in  FIG. 2  and is simpler than the circuit shown in  FIG. 14 . Therefore, even if the HEMT which has a lower power density than that of the HBT is used as the preceding-stage amplifying device, equivalent power characteristics can be realized with the very little increase of the total chip size compared with the HBT power amplifier. 
     In this embodiment, as shown in  FIG. 3 , the sub-collector layer  12  of the HBT and the source layer  30  and the drain layer  32  of the HEMT are respectively formed in different epitaxial layers, and the HBT is formed above the HEMT. However, the source and drain layers of the HEMT and the sub-collector layer of the HBT may be formed in a common layer (see, e.g., U.S. Pat. No. 7,015,519B2). Therefore, the wafer manufacturing cost can be reduced. The HEMT may be formed on the HBT (see, e.g., Japanese Patent Laid-Open No. 2006-228784 and Japanese Patent Laid-Open No. 2009-16597). As a result, the gate process which needs high processing accuracy can be performed in the flat situation (the situation including few differences in level on the wafer surface). 
     In the bias circuit Bias 1  shown in  FIG. 2 , an adequate capacitor may be connected between the gate (drain) of Fe 2  and a ground point. Therefore, a more stable power amplifying operation can be realized. 
     Second Embodiment 
       FIG. 4  is a circuit diagram showing a preceding-stage amplifying device and a preceding-stage bias circuit according to a second embodiment. Rbb 3  denotes a resistance. Cc 1  denotes a capacitor. Fe 4  denotes an enhancement mode FET (HEMT). All other components are similar to those described in connection with the first embodiment. 
     The preceding-stage amplifying device has a cascode configuration which includes not only Fe 1  but also F 4 . The source of Fe 4  is connected to the drain of Fe 1 . As a result, the component of the preceding-stage bias circuit also differs slightly. The gate and drain of Fe 3  are connected to the source of Fe 2  via Rbb 2  and Rbb 3 , are connected the gate of Fe 4  via Rbb 2 , and are connected to the gate of Fe 1  via R 3 . Cc 1  is connected between the gate of Fe 4  and a ground point. 
     Since the preceding-stage amplifying device is formed in a cascode configuration, a higher-gain than the first embodiment can be obtained. However, an increase in the source-drain voltage Vds by stacking FETs increases the minimum operating voltage of the drain voltage applied to the terminal Vc 1 . Other effects similar to those of the first embodiment can be obtained. 
     Third Embodiment 
       FIG. 5  is a circuit diagram showing a preceding-stage amplifying device and a preceding-stage bias circuit according to a third embodiment. Fe 5  and Fe 6  denote enhancement mode FETs (HEMTs). Rs 1  and Rs 2  denote resistors. Ven denotes an enable terminal of the circuit.  38  denotes a negative voltage generating circuit.  40  denotes a level control circuit. Fd 1  denotes a depletion mode FET (HEMT). Vss denotes a generated negative voltage. All other components are similar to those described in connection with the first embodiment. 
     The depletion mode Fd 1  is used as the preceding-stage amplifying device, instead of Fe 1  according to the first embodiment. Fe 5  switches whether the drain of Fd 1  is connected to the terminal Vc 1  (power source) or not depending on the enable signal applied to the terminal Ven. 
     The preceding-stage bias circuit includes a negative voltage generating circuit  38 , a level control circuit  40 , a Fe 6 , and a Rs 2 . The negative voltage generating circuit  38  and the level control circuit  40  are formed by a BiFET process using enhancement mode or depletion mode FETs. 
     Because the negative voltage needs to be generated as the gate bias voltage of the depletion mode FET Fd 1 , the negative voltage generating circuit  38  generates the negative voltage Vss. The level control circuit  40  converts the negative voltage Vss into a desired level in response to the reference voltage and outputs the converted negative voltage to the gate of Fd 1  via R 3 . As a result, Fd 1  is biased by a suitable idle current. 
     Fe 6  switches whether the negative voltage generating circuit  38  and the level control circuit  40  is connected to the terminal Vcb (power source) or not depending on the enable signal applied to the terminal Ven. For example, the voltage applied to the terminal Ven is changed from 0V to Vdd, the negative voltage generating circuit  38  and the level control circuit  40  can start operating. 
       FIG. 6  is a block diagram showing a negative voltage generating circuit according to the third embodiment.  42  denotes an oscillator or an external input signal buffer.  44   a  and  44   b  denote drive circuits which amplify outputs of the oscillator or the external input signal buffer  42  to a desired voltage amplitude.  46   a  and  46   b  denote charging pump circuits. The output voltage Vout 1  and Vout 2  of the oscillator or the external input signal buffer  42  are in a complementarity relation. The charging pump circuits  46   a ,  46   b  operate complementarily. 
       FIG. 7  is a circuit diagram showing a level control circuit which is used in the negative voltage generating circuit according to a third embodiment. Fd 2  denotes a depletion mode FET (HEMT). Fe 7  to Fe 10  denote enhancement mode FETs (HEMTs). D 1  to D 8  denote diodes. R 5  to R 7  denote resistors. Vdd denote a power source potential. VTRIM 2  denotes a reference voltage terminal. Ig 3  denotes an output current (a gate current through Fd 1 ). Ib 1 , Ib 2 , and Is 2  denote branch currents of the circuit. The terminal Vg 3  is connected to the gate of Fd 1  via the resistor R 3  as shown in  FIG. 5 . The level of the negative voltage outputted from the terminal Vg 3  is set according to the reference voltage applied to the terminal VTRIM 2 . 
       FIG. 8  is a diagram showing a reference voltage generation circuit according to the third embodiment. Fd 3  to Fd 6  denote depletion mode FETs (HEMTs). Fell and Fe 12  denote enhancement mode FETs (HEMTs). Q 3  denotes a HBT. R 8  to R 14  denote resistors. This reference voltage generation circuit generates a stable reference voltage (e.g., 2.85 V) which is not related to the voltage of the terminal Vcb. 
       FIG. 9  is a graph showing the Ids-Vds characteristics of a depletion mode FET.  FIG. 10  is a graph showing the Ids-Vds characteristics of an enhancement mode FET.  FIG. 11  is a graph showing the Ids/gm-Vgs characteristics of a depletion mode FET and an enhancement mode FET. From these figures, it is known that the depletion mode FET can obtain the higher current density per unit gate width than that of the enhancement mode FET. In this embodiment, the depletion mode FET is used as the preceding-stage amplifying device, thereby the gate width of the preceding-stage amplifying device can be smaller than those of the circuits according to the first and second embodiments. However, the enhancement mode FET generally has the higher mutual conductance gm than that of the depletion mode FET. 
     By the enhancement mode FETs Fe 5  and Fe 6 , the leak current during Ven=0 V (the shutdown of the amplifier) can be suppressed to that of the HBT power amplifier. Unlike the cases of the first and second embodiments, the enhancement mode FET does not perform RF amplifying operations, thereby its gate length can be longer than that of Fd 1 , thus the stable positive threshold voltage Vth can be realized. Other effects, which are the same as those of the first embodiment, can also be obtained. 
     Fourth Embodiment 
       FIG. 12  is a circuit diagram showing a preceding-stage amplifying device and a preceding-stage bias circuit according to the fourth embodiment. Rbb 4  and Rbb 5  denote resistors. Cc 1  denotes a capacitor. Fd 7  denotes a depletion mode FET (HEMT). All other components are similar to those described in connection with the third embodiment. 
     The preceding-stage amplifying device has a cascode configuration including not only Fd 1  but also Fd 7 . The source of Fd 7  is connected to the drain of Fd 1 . Fe 5  switches whether the drain of Fd 7  is connected to the terminal Vc 1  (power source) or not depending on the enable signal applied to the terminal Ven. The level control circuit  40  converts the negative voltage Vss into a desired level in response to the reference voltage and outputs the converted negative voltage to the gate of Fd 1  via R 3  and to the gate of Fd 7  via Rbb 4 . 
     Since the preceding-stage amplifying device is formed in a cascode configuration, a higher-gain than the third embodiment can be obtained. However, an increase in the source-drain voltage Vds by stacking FETs increases the minimum operating voltage of the drain voltage applied to the terminal Vc 1 . Other effects similar to those of the third embodiment can be obtained. 
     Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. 
     The entire disclosure of a Japanese Patent Application No. 2009-126427, filed on May 26, 2009 including specification, claims, drawings and summary, on which the Convention priority of the present application is based, are incorporated herein by reference in its entirety.