1. Field of the Invention
The present invention relates generally to an amplifier circuit. More specifically, the invention relates to an amplifier circuit including a bipolar transistor connected in cascode.
2. Description of the Related Art
In an application requiring amplification in a wide band at high frequency, such as an RF (Radio Frequency) receiver circuit of a moving body in a mobile communication, for example, amplifier circuits employing transistors connected in cascode have been employed frequently.
FIG. 2 is a circuit diagram of a typical amplifier circuit employing cascode connected transistors. The amplifier circuit as illustrated in FIG. 2 is called as a negative feedback type amplifier circuit (hereinafter referred to as "first prior art"). As shown in FIG. 2, a base of a first transistor Q1 is connected to an input terminal 1 and an emitter is grounded. A collector of the transistor Q1 is connected to an emitter of a second transistor Q2. A collector of the second transistor Q2 is connected to an output terminal 2, and in conjunction therewith, to a power source Vcc via a load Z1, resistors R12 and R11. The base of the second transistor Q2 is connected to a junction point between the resistors R11 and R12, and, in conjunction therewith, to the ground via a capacitor C. On the other hand, the base of the first transistor Q1 is connected to a junction point between the load Z1 and the resistor R12 via a feedback resistor R13.
In this circuit, a negative feedback is applied to the base of the first transistor Q1 via the resistor R13 so as to stabilize circuit operation.
FIG. 3 is a circuit diagram showing an amplifier with a cascode connected transistors, called as a current mirror type amplifier circuit (hereinafter referred to as "second prior art). In this prior art, bias current is stabilized using a current mirror circuit. As shown in FIG. 3, a base of a first transistor Q1 is connected to an input terminal 1, and an emitter is grounded. A collector of the first transistor Q1 is connected to an emitter of a second transistor Q2. A collector of the second transistor Q2 is connected to an output terminal 2 and to a power source Vcc via a load Z1. A base of the second transistor Q2 is connected to the power source Vcc via a resistor R21 and grounded via a capacitor C.
The emitter of the first transistor Q1 and an emitter of a third transistor Q3 forming a current mirror circuit are grounded. The collector of the third transistor Q3 is connected to the power source Vcc via a resistor R22. A collector and a base of the third transistor Q3 are connected via resistors R23 and R24. A junction between the resistors R23 and R24 is connected to the base of the first transistor.
In the shown circuit, by bias resistors R22 to R24, a current flowing through a current mirror circuit, namely a collector current Ic of the first transistor Q1, is stabilized. Then, feedback of FIG. 2in which a bias current of the amplifier circuit is supplied using the current mirror circuit, is not employed to enable driving at lower voltage than the circuit of FIG. 2.
In the circuit employed in the moving body of the mobile communication and so forth, it has been demanded to lower a driving voltage down to a voltage near an extreme, e. g. approximately 1 V for low driving voltage and low power consumption. If lowering of driving voltage and reduction of power consumption are introduced into the prior art set forth above, stable circuit operation cannot be assured. For instance, if fluctuation is caused in direct current amplification h.sub.FE of the transistor due to fluctuation in the manufacturing process, significant fluctuation is caused in the collector current Ic flowing through the circuit due to lowering of voltage and lowering of current.
In the circuit of the first prior art shown in FIG. 2, it becomes necessary to increase feedback amount to the base of the first transistor Q1 in order to stabilize the circuit operation. For achieving this, it becomes necessary to increase the collector current Ic or to increase resistance of the resistors R11 and R12. When higher resistance is provided for the resistors R11 and R12, higher power source voltage becomes necessary. This is contradictory to the demand for lower current and lower voltage.
In case of the circuit of the second prior art shown in FIG. 3, an attempt for establishing stability of operation of the current mirror circuit constructed with the transistors Q3 and Q1, is made by employing the bias resistors R22 to R24. However, in order to operate the circuit stably with respect to fluctuation of the direct current amplification h.sub.FE, for example, a current higher than or equal to a predetermined current has to be applied to the third transistor Q3. Increasing the current of the third transistor which does not directly contribute to amplifying operation of the circuit, is not desirable for increasing of current consumption.