Bias circuit with threshold voltage change compensation function and temperature change compensation function

A bias circuit which applies a bias voltage to a control terminal of a first active element for an RF signal amplification, includes a threshold voltage change compensation circuit and a first temperature compensation circuit. The threshold voltage change compensation circuit contains a second active element and compensates the bias voltage based on a change in threshold voltage of the first active element by using the second active element. The first temperature compensation circuit is connected between the control terminal and the voltage change compensation circuit and configured to compensate a change in the bias voltage based on a temperature change.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a bias circuit that compensation for change in threshold voltage of a transistor and temperature compensation can be accomplished.

2. Description of the Related Art

When an applied voltage to a transistor is fixed in an integrated circuit (IC), there are two problems. That is, one is the change in characteristics of the integrated circuit and decrease in a production yield due to deviation in threshold voltage (Vth) of the transistor, and the other is change in characteristics of the integrated circuit due to use temperature change. Therefore, it is required to compensate for both of the threshold voltage change of the transistor and the temperature change.

FIG. 1is a circuit diagram showing a conventional bias circuit with a temperature compensation function (IEEE TRANS. MTT, VOL.49, No12, December 2001). The bias circuit shown inFIG. 1is connected with a gate bias point106of an RF amplifying transistor104in an amplifier151, and has high impedance enough for a high frequency signal due to the resistance108. The bias circuit includes a diode202and a resistance204. A voltage201is applied to the anode of the diode202, and a voltage205is applied to the cathode of the diode202through the resistance204. A node203between the cathode and the resistance204is connected to the gate bias point106of the RF amplifying transistor104through the resistance108.

The amplifier151includes a capacitor107, the RF amplifying transistor104, and the resistance108. A radio frequency signal is inputted to one end of the capacitor107. The gate bias point106of the RF amplifying transistor104is connected with the other end of the capacitor107and one end of the resistance108. The drain thereof is connected to one end of a resistance102. A voltage101is applied to the drain of the RF amplifying transistor104through the resistance102, and a voltage105to the source thereof.

The bias circuit shown inFIG. 1utilizes a characteristic of the diode shown inFIG. 2, in which a forward voltage Vfincreases when temperature decreases in a state that a forward current is kept constant. Generally, a current-voltage characteristic of a Scotty barrier diode is expressed by
If=Js(exp(qVf/kT)−1)
where Js=AT2exp(−qΦB/kT), Ifis a forward current, q is unit electric charge, Vfis a forward voltage, k is the Boltzmann's constant, T is temperature, A is the effective Richardoson's constant, φB is a Scotty barrier height of the diode. That is, the voltage drop in the diode202becomes large when the temperature decreases, and therefore the voltage at the node203on the side of the cathode of the diode202lowers. As a result, the voltage V106of the gate bias point106lowers, so that the gain of the transistor104is restricted. On the other hand, the voltage drop at the diode202becomes small when the temperature increases. Therefore the gate bias voltage V106rises so that the gain of the transistor104becomes large.

FIG. 3shows a dependency on the temperature change, of drain current103in the transistor104(Id: unit mA) using the bias circuit shown inFIG. 1. As shown inFIG. 3, the drain current103is restricted with the temperature decrease. In the other words, the bias circuit shown inFIG. 1has the temperature compensation effect.

Next,FIG. 4is a circuit diagram showing a conventional bias circuit with a compensation function of a change in threshold voltage (2003 IEEE MTT-S Digest TU5B-3, pp. 121–124). The bias circuit shown inFIG. 4is connected with the gate bias point106of the RF amplifying transistor104in the amplifier151and has high impedance enough for a high frequency signal by the resistance108. The bias circuit includes a transistor305having a same DC characteristic as the RF amplifying transistor104, a resistance303, and a resistance306. A voltage301is applied to the drain of a transistor305through the resistance303. A voltage308is applied to the source of the transistor305. A voltage307is applied to the gate of the transistor305through the resistance306. It should be noted that the amplifier151inFIG. 4is the same as shown inFIG. 1.

In the bias circuit shown inFIG. 4, when the threshold voltage Vthof the transistor changes by ΔVth, the circuit is set to satisfy the relational of ΔId2*R303=ΔVthwhere ΔId2is a difference of the drain current302, and R303is the resistance value of the resistance303. When the drain current302becomes large by ΔId2with the decrease of the threshold voltage Vth, the voltage304lowers by ΔVth. As a result, the voltage at a gate bias point106of the transistor104becomes low by ΔVth. On the other hand, when the threshold voltage Vthincreases, the threshold voltage change ΔVthis compensated oppositely.

FIG. 7shows a dependency upon the threshold voltage change (ΔVth: unit V), of the drain current103of the transistor104(Id: unit mA) when the bias circuit shown inFIG. 4is used.

FIG. 5is a circuit diagram showing a conventional circuit (of a self-bias method) which has both of a threshold voltage change compensation function and a temperature change compensation function (by Yasuyuki Itou, et al., “Base and application of MMIC technology”, May 31, 1996 Realize company, P. 130). In the circuit shown inFIG. 5, a resistance406is connected in series between the source of a RF amplifying transistor404and a ground potential GND. A capacitor407is connected to the ground in parallel to the resistance406for a high frequency signal. Also, a voltage401is applied to the drain of the RF amplifying transistor404through the resistance402. A radio frequency signal is supplied to the one end of a capacitor408. The other end of the capacitor408is connected to the gate of the RF amplifying transistor404.

In the circuit shown inFIG. 5, the following functions are accomplished when the drain current403of the transistor404is changed due to a temperature change and a threshold voltage change. For instance, when the drain current403increases, the voltage405becomes high, so that a voltage difference Vgsbetween the gate and the source in the transistor404decreases. As a result, the drain current403decreases. On contrary, when the drain current403decreases, the voltage difference Vgsbecomes large so that the drain current403increases. That is to say, the compensation functions in the circuit shown inFIG. 5are accomplished to keep the drain current403of the transistor404constant.

FIG. 6is a circuit diagram showing another conventional circuit which has functions compensating both of threshold voltage change and temperature change (2002 IEEE MTT-S Digest TH1B-4, pp. 1427–1430). The circuit shown inFIG. 6is a bias circuit which is connected with a gate bias point106of an RF amplifying transistor104in an amplifier151. The bias circuit includes a first circuit which includes a transistor504having a same DC characteristic as the RF amplifying transistor104, a resistance502connected with the drain of the transistor504, a diode506, and a resistance509connected with the source of the transistor504, and a second circuit which includes a resistance511and a diode513, which are connected with the gate of the transistor504.

The gate bias point106of the transistor104is connected to a node503between the drain of the transistor504and the resistance502through the resistance108to have high impedance enough for a high frequency signal. The drain of the transistor504is grounded through the resistance502. The anode of the diode506in the first circuit is grounded and the cathode of the diode506is connected with a negative voltage514through the resistance509. The source of the transistor504is connected to the node507between the diode506and the resistance509in the first circuit. The cathode of the diode513is connected to the negative voltage514, and the anode of the diode513is grounded through the resistance511in the second circuit. A gate of the transistor504is connected to a node512between the resistance511and the diode513in the second circuit. The amplifier151inFIG. 6is the same as shown inFIGS. 1 and 4.

When the threshold voltage Vthof the transistor is changed by ΔVthin the bias circuit shown inFIG. 6, the threshold voltage change ΔVthis compensated for to satisfy the relational expression of ΔId5*R502=ΔVth, where ΔId5indicates a change of the drain current501, and R502indicates the resistance value of the resistance502. The temperature characteristic of a forward voltage Vfof the diode (shown inFIG. 2) is used when the temperature change has occurred. For instance, the voltage drop across the diode513becomes large when the temperature decreases. Therefore, the voltage at the node512corresponding to a gate voltage of the transistor504increases. The voltage drop across the diode506becomes large similarly. Therefore, the voltage at the node507corresponding to the source voltage of the transistor504decreases. As a result, the voltage between the gate and the source in the transistor504increases, so that the drain current501of the transistor504is increased, resulting in lowering the voltage at the node503. This is because the voltage drop indicated as the product of the resistance502and the drain current501becomes large. Therefore, the voltage at the gate bias point106which has the same voltage as the voltage503decreases in the RF amplifying transistor104. Consequently, the effect of the temperature compensation is achieved. Thus, the bias circuit shown inFIG. 6has a relation of drain current and threshold voltage change shown inFIG. 8and a relation of drain current and temperature change shown nFIG. 9.

Furthermore, Japanese Laid Open Patent Application (JP-P2001-168699A) discloses a technique to maintain a stable operation in spite of the temperature change of the threshold voltage Vthof the transistor in a MOSFET for power supply. In the conventional technique, resistances19,17, and16, diodes21and22, and a zenar diode20are connected with the gate of MOSFET1, and the threshold voltage change compensation can be achieved in the temperature change.

However, there are the following problems in the above-mentioned conventional techniques. That is, generally, when the threshold voltage change ΔVthhas occurred in the transistor, the change in the characteristics of the transistor occurs if the bias voltage is not changed by the threshold voltage change ΔVthof the transistor for the RF amplification. The circuit ofFIG. 1has the compensation effect to the temperature change but does not have the compensation effect to the threshold voltage change. Therefore, the drain current103decreases, which cause a change in characteristic as the threshold voltage Vthbecomes shallow.

Generally, a temperature coefficient of an epitaxial resistance formed on a GaAs substrate has a positive coefficient. In the bias circuit ofFIG. 4, When the rise of temperature is caused, the values of a drain current302of transistor305and resistance303become large and a product of the drain current302and the resistance303becomes large. That is, a voltage drop across resistance303becomes large. As a result, as the temperature increases, the gate bias106of the transistor104for the RF amplification becomes low to decrease the drain current103, resulting in the degradation of the RF characteristic. In this way, in case of the bias circuit ofFIG. 4, there is not a compensation effect to the temperature change.

Also, when the gain of the transistor104should be kept constant to the temperature change, it is needed to decrease the drain current103with the temperature decrease. In the bias circuit ofFIG. 5, the temperature change compensation effect is insufficient because the bias circuit has only a function to keep the drain current103constant to the temperature change.

In the bias circuit ofFIG. 6, three current paths for the drain current501of the transistor504, for a current505flowing through the diode506and the resistance509, and for a current510flowing through the resistance511and the diode513are needed. Therefore, the consumption current becomes large.

In the technique disclosed in Japanese Laid Open Patent Application (JP-P2001-168699A), it is possible to compensate for the threshold voltage change to the temperature change but it not possible to compensate for the threshold voltage change to change on the manufacturing.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a bias circuit which can carry out both of compensation of a threshold voltage change and compensation of a temperature change.

In an aspect of the present invention, a bias circuit which applies a bias voltage to a control terminal of a first active element for RF signal amplification, includes a threshold voltage change compensation circuit and a first temperature compensation circuit. The threshold voltage change compensation circuit contains a second active element and compensates the bias voltage based on a change in threshold voltage of the first active element by using the second active element. The first temperature compensation circuit is connected between the control terminal and the voltage change compensation circuit and configured to compensate a change in the bias voltage based on a temperature change.

Here, the first and second active elements may be transistors. In this case, the first and second active elements may be voltage controlled-type transistors, or current controlled-type transistors. The second active element preferably has a same threshold voltage as the first active element.

Also, the threshold voltage change compensation circuit preferably includes a first resistance connected with the second active element.

In this case, the threshold voltage change compensation circuit may further include a second temperature compensation circuit configured to compensate a first voltage of a first node between the first resistance and the second active element based on the temperature change. The second temperature compensation circuit may include a first diode circuit containing at least one diode.

Alternatively, the bias circuit may further include a second temperature compensation circuit interposed between the threshold voltage change compensation circuit and the first temperature compensation circuit to compensate a first voltage of a first node between the first resistance and the second active element based on the temperature change. The second temperature compensation circuit may include a first diode circuit containing at least one diode. In either case, it is preferable that the diode in the first diode circuit is a Schottky diode or a diode with a negative temperature coefficient.

Also, the first temperature compensation circuit preferably includes a second diode circuit containing at least one diode. The first temperature compensation circuit may further include a second resistance connected with the second diode circuit. It is preferable that the diode in the second diode circuit is a Schottky diode or a diode with a negative temperature coefficient.

Also, a second node between the second resistance and the second diode circuit is connected with the control terminal, and the bias circuit may further include an additional circuit comprising a third resistance and a third active element connected with the third resistance, connected with the first temperature compensation circuit on a lower voltage side of the first temperature compensation circuit.

In this case, the additional circuit may further include a third temperature compensation circuit configured to compensate a second voltage of a third node between the third resistance and the third active element based on the temperature change. The third temperature compensation circuit may include a third diode circuit containing at least one diode. It is preferable that the diode in the second diode circuit is a Schottky diode or a diode with a negative temperature coefficient. Also, it is preferable that the second and third active elements have a same threshold voltage as the first active element.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, a bias circuit of the present invention will be described in detail with reference to the attached drawings.

FIG. 10is a circuit diagram showing the configuration of the bias circuit according to the first embodiment of the present invention. The bias circuit has both of a compensation function of threshold voltage change and a compensation function of temperature change.

It should be noted that an amplifier151shown inFIG. 10has the same configuration as those of shown inFIGS. 1,4and6. That is to say, the amplifier151includes a capacitor107, a RF amplifying transistor104(a first active element for RF signal amplification), and a resistance108. A RF signal is supplied to one end of the capacitor107. A gate bias point106connected with a control terminal of the transistor104is connected between the other end of the capacitor107. The resistance108is connected with the gate bias point106. A voltage101is applied to the drain of the RF amplifying transistor104through the resistance102, and a voltage105is applied to the source of the RF amplifying transistor104.

As shown inFIG. 10, the bias circuit is connected with the gate bias point106of the RF amplifying transistor104in the amplifier151through the resistance108. The bias circuit has high impedance enough for a high frequency signal by the resistance108, and includes a temperature compensation circuit153and a threshold voltage change compensation circuit152for compensation of a threshold voltage change.

The temperature compensation circuit153has n (n is a positive integer) diodes118(118ato118n) and a resistance120, which are connected in series. A voltage121is applied to the cathode of the diode118through the resistance120.

The threshold voltage change compensation circuit152supplies a voltage to the amplifier151through the temperature compensation circuit153. The threshold voltage change compensation circuit152includes a transistor113(a second active element) which has the same DC characteristic as the RF amplifying transistor104(having the same threshold voltage as the RF amplifying transistor104), a resistance111connected with the drain of the transistor113, and a resistance114connected with the gate of the transistor113. A voltage109is applied to the drain of the transistor113through the resistance111. A voltage115is applied to the gate of the transistor113through the resistance114. A voltage116is applied to the source of the transistor113.

The anode of the first one of the n diodes118in the temperature compensation circuit153is connected with a node112between the resistance111and the drain of the transistor113in the threshold voltage change compensation circuit152. In addition, m (m is a positive integer) diodes117(117ato117m) are connected in series between the voltage109and the voltage119at the cathode of the end one of the diodes118. The diodes117compensate a voltage change at the node112due to a change of a drain current110flowing through the resistance111and the transistor113, i.e., a voltage change supplied from the threshold voltage change compensation circuit152to the temperature compensation circuit153, depending on a temperature change. Thus, the diodes117functions as a second temperature compensation circuit. More specifically, the diodes117are connected in series between the node112and the anode of the first one of the diodes118.

For instance, in order to keep a gain of an integrated circuit constant, a bias is set in such a way that a gain is maximum at a high temperature and the drain current103is restricted with the decrease of the temperature. When the threshold voltage change is caused, the voltage at the gate bias point106(the gate voltage V106) in the RF amplifying transistor104is shifted by only an amount according to the threshold voltage change to keep the drain current103constant. Such a transistor will be described below. The threshold voltage change compensation circuit152compensates the threshold voltage change ΔVth, in which the voltage109is applied through the resistance111with a resistance value R111to the transistor113which has the same DC characteristic as the transistor104. The resistance value R111of the resistance111is set to satisfy the relation of ΔVth=R111*ΔId110(the amount of the change of current110according to the threshold voltage change).

In the temperature compensation circuit153, a characteristic shown inFIG. 2is used in which a forward voltage Vfof the diodes118ato118nof the diode118increases with the decrease of the temperature. The number n of the diodes118ato118nis selected so that a relation of ΔVf*n=ΔV106is satisfied, i.e., the change of forward voltages Vfof the diodes due to the temperature (ΔVf) is equal to the change of the voltage V106of the gate bias point106in the transistor104which is necessary to restrict the drain current with the decrease of the temperature. According to the above-mentioned characteristic of the diode, a voltage drop from the set voltage at the node112becomes large at a low temperature. As a result, the voltage119becomes low, which is the same voltage as the gate bias point106in the transistor104. That is, it is possible to lower the voltage V106of the gate bias point106with the decrease of the temperature. On contrary, when the temperature rises, the voltage drop from set voltage at the node112becomes small, so that the voltage V106of the gate bias point106can be raised. In this way, the gain change of the transistor104due to the temperature change can be restrained.

In general, a resistance value changes with a certain coefficient to a temperature change. The temperature coefficient is a positive value to the temperature change usually in the resistance using a GaAs epitaxial layer. Therefore, the temperature change causes that the resistance value R111of the resistance111changes by ΔR111. Moreover, the drain current110(Id110) of the transistor113has a temperature change of ΔId110. For instance, when the temperature rises, the resistance value R111of the resistance111becomes large, so that the voltage drop as a product of the drain current110and the resistance value becomes large. Therefore, the voltage becomes low at the node112between the temperature compensation circuit153and the threshold voltage change compensation circuit152. In this way, the voltage at the node112changes as ΔV112=ΔR111(Id110+ΔId110)+ΔId110(R111+ΔR111) due to the temperature change. This means that the above-mentioned simple connection of the threshold voltage change compensation circuit152and the temperature compensation circuit153cannot satisfy both compensation functions for the threshold voltage change and the temperature change.

For this reason, in order to compensate the change of the voltage V112at the node112(ΔV112) due to the temperature change, the number m of the diodes117ato117mis determined so as to meet ΔV112=ΔR111*(Id110+ΔId110)+ΔId110*(R111+ΔR111)≈ΔVf*m. The diodes117ato117mare connected in series between the voltage109and the voltage119, specifically, between the node112and the anode of the first one of the diodes118in the first embodiment.

By adopting the above-mentioned circuit structure, the bias circuit can achieve compensation functions for both of the threshold voltage change and the temperature change.

In the application of the bias circuit in the first embodiment, the number of diodes m should be preferably small from the viewpoint of a chip area. This implies that the change of the voltage V112at the node112due to the temperature change should be also preferably small, from the above-mentioned relation of ΔV112=ΔR111*(Id110+ΔId110)+ΔId110*(R111+ΔR111)≈ΔVf*m. When the Id-Vg characteristic of the transistor113changes approximately linearly to the threshold voltage change, the value of the resistance111is uniquely determined from the relational of the drain current change due to the threshold voltage change, ΔId110*R111=ΔVth. Accordingly, ΔR111and (R111+ΔR111) are also uniquely decided. Therefore, it is sufficient to reduce (Id110+ΔId110) in order to reduce ΔV112. Consequently, it is preferable that the gate bias point of the transistor113is biased to a vicinity of the threshold voltage Vth.

FIG. 11is a graph showing the voltage change of the gate bias point106(the gate voltage V106) of the RF amplifying transistor104when the bias circuit shown inFIG. 10is used. Specifically,FIG. 11shows a calculation result at 25° C. under the following conditions, that is, the drain current110of the transistor113Id110=8 mA, the resistance value of the resistance111connected with the drain of the transistor113R111=40Ω, the number m of the diodes117for compensation of the voltage at the node112is 2, the number n of diodes118in the temperature compensation circuit153is 2, the resistance value R120of the resistance120in the temperature compensation circuit153is 1.2 KΩ, the temperature change range is from −40° C. to +110° C., and the threshold voltage change is from −0.2V to +0.2V. The result was obtained that the gate voltage V106is offset so as to compensate for the threshold voltage change, such that the gate voltage lowers with the temperature decrease in any threshold voltage.

FIGS. 12 and 13show the change of the drain current103in the transistor104in this case. InFIG. 12, the drain currents103are plotted to the threshold voltage change, and inFIG. 13, the drain current103are plotted to the temperature change.FIG. 12indicates that the bias circuit shown inFIG. 10operates so that the drain current103becomes constant even if the threshold voltage changes. Also,FIG. 13indicates that the bias circuit shown inFIG. 10operates so that the drain current103is lowered with the temperature decrease. In this way, it is possible to compensate for both of the threshold voltage change and the temperature change by using the bias circuit in the first embodiment.

Next, an effect for the RF characteristic will be described, when the bias circuit in the first embodiment is used. S21characteristics (a small signal gain) were calculated and compared, in which both the conventional circuit shown inFIG. 5and the bias circuit in the first embodiment in the above conditions are applied to a two-stage amplifier of the 38 GHz band. The conventional circuit has both compensation functions for the threshold voltage change and the temperature change. The temperature change is in the rage of −40° C. to +110° C., and the threshold voltage change is in a range of ±0.2V.

In the conventional circuit shown inFIG. 5, ΔS21=5.64 dB, and in the bias circuit in the first embodiment, ΔS21=2.88 dB. The results show that the S21characteristic change can be restrained to about ½ in the bias circuit, which greater improvement can be achieved comparing with the conventional circuit.

In this way, according to the bias circuit in the first embodiment, the temperature compensation circuit153, in which the n diodes118and a resistance120are connected in series, and the threshold voltage change compensation circuit152, in which the bias voltage is applied to the drain of the transistor113, which has the same DC characteristic as the RF amplifying transistor104, through the resistance111. The temperature compensation circuit153is connected with the threshold voltage change compensation circuit152through the m diodes117ato117mconnected in series and provided the node between the drain of transistor113and the resistance111and the anode of the first one of the n diodes118ato118nin order to compensate the temperature change of the resistance111. Therefore, by setting the resistance value R111of the resistance111to meet ΔVth=R111*ΔId110, when the drain current110of the transistor113changes by ΔId110, it is possible to change the voltage V106of the gate bias point106of the transistor104by ΔVtheven when the threshold voltage changes. Thus, the compensation is in effective. Also, in case of the temperature change, the number n of the diodes118ato118nconnected in series is set to meet ΔVf*n≈ΔVgs, namely, such that the change of the forward voltages Vfof the diodes is equal to the change of the Vgsof the transistor104necessary for compensation of the temperature change. In addition, the number m of the diodes is determined to meet the relation of ΔR*(Id+ΔId)+Δid*(R+ΔR)≈ΔVf*m, since the resistance111connected with the drain of the transistor113and the drain current Id of the transistor113have temperature dependence. Thus, the voltage applied to the temperature compensation circuit153, configured by the n diodes118ato118nand the resistance120is compensated by using the m diodes117ato117m.

In this way, the temperature change of the resistance111and the transistor113used in the circuit152is compensated by the m diodes117ato117m. Thus, the bias circuit has both functions of compensation for both of the threshold voltage change and the temperature change, resulting in improvement of the characteristic change and increase of a production yield of IC.

FIG. 14shows a circuit diagram of the bias circuit according to the second embodiment of the present invention. The bias circuit has both of the threshold voltage change compensation function and the temperature change compensation function as well as in the first embodiment. In the second embodiment, the temperature compensation circuit153is configured of diodes118and a resistance120and operates to lower the voltage119with the decrease of the temperature, as well as in the first embodiment. The threshold voltage change compensation circuit152is configured of a transistor113and a resistance111, and carries out the threshold voltage change compensation by a drain current change ΔId110of the transistor113due to the threshold voltage change and the resistance111.

In the second embodiment, as shown inFIG. 14, the diodes117are not connected between the node112and the anode of the first one of the diodes118, but between the voltage109and the node112in series. More specifically, the diodes117are connected between the voltage109and the resistance111. As a result, a voltage change at the node112due to the temperature change of the resistance111can be compensated by the diodes117, as well as in the bias circuit in the first embodiment. It should be noted that the configuration of the bias circuit other than the above-mentioned portion in the second embodiment is the same as that in the first embodiment. Therefore, the description is omitted by assigning the same reference numerals.

FIG. 15is a graph showing the voltage change of the gate voltage V106of the RF amplifying transistor104when the bias circuit shown inFIG. 14is used. More specifically,FIG. 15shows calculation results at 25° C. for the following conditions, that is, the drain current110Id110of the transistor113is 8 mA, the resistance value R111of the resistance111connected with the drain of the transistor113is 20 Ω, the number m of the diodes117for compensation of the voltage V112at the node112is 1, the number n of the diodes118in the temperature compensation circuit153is 3, and the resistance value R120of the resistance120in the temperature compensation circuit153is 1.8 KΩ. The temperature change and the threshold voltage change are same ranges as in the first embodiment. As shown inFIG. 15, the gate voltage V106is lowered as the temperature is lowered.

FIGS. 16 and 17show the change of the drain current103in the transistor104in this case. InFIG. 16, the drain current103is plotted for the threshold voltage change, and inFIG. 17, the drain current103is plotted for the temperature change.FIG. 16indicates that the bias circuit shown inFIG. 14operates so that the drain current103is kept constant to the threshold voltage change. Also,FIG. 17indicates that the bias circuit shown inFIG. 14operates so that the drain current103is lowered with the temperature decrease. In this way, it is possible to compensate both of the threshold voltage change and the temperature change by using the bias circuit in the second embodiment.

The calculation result of the S21characteristic change in the two-stage amplifier of the 38 GHz band indicates ΔS21=3.02 dB. The S21characteristic change is restrained to about ½ in the bias circuit in the second embodiment, comparing with the conventional circuit shown inFIG. 5. As a result, the bias circuit in the second embodiment has the same effect as that in the first embodiment.

FIG. 18shows a circuit diagram of a bias circuit in a third embodiment of the present invention. The bias circuit has both of the threshold voltage change compensation function and the temperature change compensation function, as well as in the first and second embodiments. In the third embodiment, the temperature compensation circuit153is configured of the diodes118and a resistance120which operates to lower the voltage119with the decrease of the temperature, as well as in the first embodiment. The threshold voltage change compensation circuit152is same as in the second embodiment.

In the third embodiment, a circuit154is further included. The circuit154has a transistor (a third active element)126, a resistance124, a resistance127, and diodes123. The circuit154is connected with on a lower voltage side of the temperature compensation circuit153, i.e., an opposite end of the resistance120to the voltage119. The transistor126is the same DC characteristic as the transistor104, i.e., has the same threshold voltage as the transistor104. A voltage122is applied to the drain of the transistor126through the resistance124and the m diodes123(123ato123m). A voltage129is applied to the source of the transistor126, and a voltage128is applied to the gate of the transistor126through the resistance127. A node121between the drain of the transistor126and the resistance124is connected to the lower voltage end of the temperature compensation circuit153(the opposite end of the resistance120to the voltage119).

The circuit154has the transistor126, the resistance124, and the diodes123, and the threshold voltage change compensation circuit152has the transistor113, the resistance111, and the diodes117. Here, the voltage122, the voltage128, and the voltage129in the circuit154are set so as to operate almost equally in a DC manner to the threshold voltage change compensation circuit152.

The transistor113and the transistor126have the same DC characteristic. Therefore, when the threshold voltage is changed, the voltage at the node112(V1112) and the voltage of the node121(V121) are changed by a same amount. Therefore, the absolute value |V112−V121| of the voltage difference between the voltage V112and the voltage V121is constant, so that the voltage drop in the resistance120is continuously constant. Therefore, the amount of the change of the voltage119is equal to the amount of the change of the voltage (V112) or the voltage (V121) As a result, the compensation effect by the threshold voltage change compensation circuit152can be directly reflected to the voltage119, that is, to the gate voltage V106of the transistor104. Consequently, the bias circuit in the third embodiment achieves a higher efficiency of the threshold voltage compensation than those in the first and second embodiments.

Similarly, the absolute value |V112−V121| of the voltage difference at the temperature change is kept constant. Therefore, the compensation effect by the temperature compensation circuit153can be directly reflected to the gate voltage V106of the transistor104. It should be noted that the amplifier151in the third embodiment is same as that in the first embodiment.

FIG. 19is a graph showing the voltage change of the gate voltage V106of the RF amplifying transistor104when the bias circuit shown inFIG. 18is used. More specifically,FIG. 19shows calculation results at 25° C. for the following conditions, that is, the drain current110Id110of the transistor113is 8 mA, the resistance value R111of the resistance111connected with the drain of the transistor113is 20 Ω, the number m of the diodes117for compensation of the voltage at the node112is 1, the number n of the diodes118in the temperature compensation circuit153is 3, the resistance value R120of the resistance120in the temperature compensation circuit153is 1.2 KΩ, the drain current Id125Of the transistor126is 8 mA, the resistance value R124of the resistance124connected with the drain of the transistor126is 20 Ω, the number m of the diodes123for compensation of the voltage V123is 1, and the temperature change and the threshold voltage change are same range as in the first embodiment. As shown inFIG. 19, the gate voltage V106is lowered as the temperature is lowered.

FIGS. 20 and 21show the change of the drain current103in the transistor104in this case. InFIG. 20, the drain current103is plotted for the threshold voltage change, and inFIG. 21, the drain current103is plotted for the temperature change.FIG. 20indicates that the bias circuit in the third embodiment operates so that the drain current103is kept constant to the threshold voltage change. Moreover,FIG. 20indicates that the bias circuit in the third embodiment operates so that the drain current103is lowered with the temperature decrease.

The calculation result of the S21characteristic change in the two-stage amplifier of the 38 GHz-band is ΔS21=2.86 dB. The S21characteristic change can be restrained to about ½ in the bias circuit in the third embodiment, comparing with the conventional circuit shown inFIG. 5. As a result, the bias circuit in the third embodiment has the same effect as those in the first and second embodiments. In addition, the bias circuit in the third embodiment is superior in the threshold voltage change compensation effect to those in the first and second embodiments.

FIG. 22shows a circuit diagram of the bias circuit according to the forth embodiment of the present invention. The bias circuit has both of the threshold voltage change compensation function and the temperature change compensation function, as well as in the first to third embodiments.

In the first to third embodiments, the voltage of the gate bias point106in the transistor104is lowered with the temperature decrease so that the transistor has a characteristic that the gain is kept constant. However in the fourth embodiment, the voltage of the gate bias point106in the transistor104is raised with the temperature decrease so that the transistor has a characteristic that the gain is kept constant.

In the forth embodiment, a circuit which has the threshold voltage change compensation function is the same as that in the third embodiment, and includes the threshold voltage change compensation circuit152, which is composed of the transistor113, the resistance111, and the diodes117, and a circuit154, which includes the transistor126, the resistance124, and the diodes123. The node112is connected with the nodes121through the resistance120and the diodes118in order as shown inFIG. 22. The amplifier151is the same as in the first embodiment.

The bias circuit in the forth embodiment has the compensation effect function and operates in the same way as in the third embodiment when the threshold voltage changes. At the high temperature, the voltage drop by the diodes118decreases, so that the voltage119decreases. At a low temperature, the voltage drop by the diodes118increases, so that the voltage119increases.

FIG. 23is a graph showing the voltage change of the gate voltage V106of the RF amplifying transistor104when the bias circuit shown inFIG. 22is used. More specifically,FIG. 23shows calculation results at 25° C. under the following conditions, that is, the drain current Id110of the transistor113is 8 mA, the resistance value R111of the resistance111connected with the drain of the transistor113is 20 Ω, the number m of the diodes117for compensation of the voltage at the node112is 1, the number n of the diodes118in the temperature compensation circuit153is 1, the resistance value R120of the resistance120in the temperature compensation circuit153is 2.2 KΩ, the drain current Id125of the transistor126is 8 mA, the resistance value R124of the resistance124connected with the drain of the transistor126is 20 Ω, the number m of the diodes123for compensation of the voltage at the node121is 1, and the temperature change and the threshold voltage change are same range as in the first embodiment. As shown inFIG. 23, the gate voltage V106is lowered as the temperature is lowered.

FIGS. 24 and 25show the change of the drain current103in the transistor104in this case. InFIG. 24, the drain current103is plotted for the threshold voltage change, and inFIG. 25, the drain current103is plotted for the temperature change.FIG. 24indicates that the bias circuit in the forth embodiment operates so that the drain current103is kept constant with the threshold voltage change. Moreover,FIG. 25indicates that the bias circuit in the forth embodiment operates so that the drain current103is lowered with the temperature decrease. As a result, the bias circuit in the forth embodiment has the same effect as that in the third embodiments.

FIG. 26shows a circuit diagram of the bias circuit according to the fifth embodiment of the present invention. The bias circuit has both the threshold voltage change compensation function and the temperature change compensation function, as well as in the first to forth embodiments.

In the first to forth embodiments, a voltage-controlled type transistor is used. However, in the fifth embodiment, a current-controlled type transistor is used.

In the fifth embodiment, the threshold voltage change compensation circuit152is the same as that in the first embodiment. With the m diodes117for the compensation of the voltage at the node112and an amplifier151, the bias circuit in the fifth embodiment is the same as that in the first embodiment.

In the fifth embodiment, the transistor104is the current-controlled type transistor. Therefore, the voltages106and119are mutually short-circuited, and the current flowing through the diodes118is flown through the transistor104. That is to say, the circuit for the temperature compensation includes only the diodes118without the resistance120. In the fifth embodiment, the voltage V112at the node112is changed as the threshold voltage changes. A current change corresponding to the voltage change compensates the transistor104. Further, with the temperature change, the voltage119changes and the current flowing through the transistor104is changed. Thus, the RF characteristic of the transistor104can be compensated.

FIG. 27is a graph showing a base current characteristic to the temperature change of the RF amplifying transistor when the bias circuit shown inFIG. 26is used. More specifically,FIG. 27shows calculation results at 25° C. under the following conditions, that is, a collector current IC110is 8 mA, the resistance value R111of the resistance111connected with the collector of the transistor113is 20 Ω, the number m of the diodes117for compensation of the voltage V112is 2, the number n of the diodes118is 1, and the temperature change and the threshold voltage change are same range as in the first embodiment. As shown inFIG. 27, the base current is lowered as the temperature is lowered, and the effect of the threshold voltage change compensation is present. As a result, the bias circuit in the fifth embodiment has the same effect as that in the first to third embodiments.