JP2793343B2 - Bias circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to a bias circuit for flowing a constant DC current to the collector (emitter) of a transistor constituting a part of a high-frequency circuit. It is used for high-frequency amplification circuits, frequency conversion circuits, oscillation circuits and the like of semiconductor integrated circuits.

(Prior Art) Conventionally, a circuit as shown in FIG. 4 has been used for a common emitter type high frequency amplifier circuit of a semiconductor integrated circuit. Here, 1 is a high-frequency signal input terminal, 2 is an output terminal, 3 is a transistor with a common emitter, and 9 is a load resistor. 5 is a bias resistor for applying a DC bias to the transistor 3, 6 is a high-frequency bypass capacitor,
5 ₁ a resistor for correcting the voltage drop across the resistor 5 by the base current of the transistor 3, the transistor 3 and a transistor 4 is in the direct current form a current mirror. When the area ratio between the emitters of the transistor 3 and the transistor 4 is N: 1, a current N times the reference current source 7 flows through the transistor 3. In this case the resistor 5 ₁ is set to approximately N times the value of the resistor 5. Actually, since the collector current of the transistor 4 is reduced by the base current I _B of the transistor 3 and 4 than the constant current source 7, a resistor 5 ₁ may be corrected by setting slightly larger.

(Problem to be Solved by the Invention) In the conventional bias circuit, the transistor 3 and the transistor 4 are DC current mirrors in FIG. 4, but the AC signal from the terminal 1 is bypassed by the capacitor 6. , Applied only to transistor 3,
When the input signal from the terminal 1 increases, the signal cannot be ignored, and the collector (emitter) DC current of the transistor 3 increases due to the nonlinearity of the PN junction between the base and the emitter of the transistor 3. . This situation is shown in the graph in FIG. 5 by the curve b and FIG. For further resistor 5 ₁ which is N times the high resistance of the resistor 5,
Increased thermal noise voltage out resistance 5 ₁ is not suitable for applications requiring low-noise characteristics. Particularly when implemented in a semiconductor integrated circuit, the capacitance value of the capacitor 6 can not be too large, it is impossible to sufficiently drop the thermal noise generated by the resistor 5 ₁ capacitor 6. Resistor 5, although the increase of the current when the resistance 5 ₁ to a sufficiently large value can be reduced, the transistor 3, the resistor by the base current of the transistor 4 5, resistor 5 ₁ minute by the power supply voltage of the voltage drop Vcc
Therefore, there is a problem that thermal noise is further increased. The increase in the current consumption has a problem that the life of the battery is shortened in a system using a battery as a power supply. Further, when the current fluctuates, the operating point of the circuit fluctuates, so that it is difficult to operate the circuit normally.

SUMMARY OF THE INVENTION An object of the present invention is to provide a high-frequency circuit bias circuit suitable for a semiconductor integrated circuit operable at a low power supply voltage with low noise, without fluctuation of the collector current of a biased element due to a high-frequency input signal.

[Constitution of the Invention] (Means and Actions for Solving the Problems) The present invention provides a first transistor which is supplied with a high-frequency signal to a base and constitutes a part of a high-frequency circuit, a first transistor, a base and an emitter. , A resistor connected between the base and the collector of the second transistor, a capacitor connected to the collector of the second transistor for bypassing a high-frequency signal, and a second transistor And a constant current source connected to the collector of the first and second transistors for supplying an added value of a current substantially equal to the base current of the first and second transistors and a reference current.

That is, in the present invention, in order to eliminate the above-mentioned conventional disadvantage, a pair of transistors having a base and an emitter connected to each other are used, and a high-frequency bypass capacitor and a reference constant current source are connected to the collector of one of the second transistors. A resistor is connected between the base and the collector of the second transistor, so that the pair of transistors operates as a current mirror both DC and AC. Furthermore, the above 1
To correct the decrease in the base current I _B due to the collector current of the transistor pairs, by adding a circuit for adding a current approximately equal to the reference current source to the base of the current of a pair of transistors. Then, the other first transistor of the pair of transistors is used as a part of the high-frequency circuit.

In this way, since the negative feedback is applied to the base of the second transistor in particular, the collector (emitter) current of the first transistor does not fluctuate, and a low-voltage power supply circuit based on the reference current is used for base current correction. As a result, low-noise, small-capacitance capacitor values, etc. can be achieved, which facilitates integration into an integrated circuit.

(Embodiment) An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram of the same embodiment. Here, the same reference numerals are used for portions corresponding to FIG. In FIG. 1, the transistor 4 is a transistor that forms a pair with the transistor 3, and the area ratio between the emitters of the transistor 3 and the transistor 4 is N: 1. Transistors 3 and 4 have their bases and emitters connected to each other and their emitters grounded, and their bases are connected via a resistor 5 to a high-frequency bypass capacitor 6, a collector of transistor 4, a reference current source 7, and a current source for correcting base current. 8 is connected. Since the base and the emitter of the transistor 3 and the transistor 4 are connected to each other, N: 1
Current flows at the ratio of The AC component of the collector current of the transistor 4 is smoothed by the capacitor 6, and the voltage across the capacitor is applied to the base of the transistor 4 via the resistor 5, so that the DC component of the collector current of the transistor 4 becomes equal to the reference current. , A negative feedback is applied from the resistor 5. Since the transistors 3 and 4 are current mirrors, the DC component of the collector current of the transistor 3 is always N times the reference current regardless of the AC signal voltage at the input terminal 1 (DC a in FIG. 5). .

Current source 8 for the base current compensation transistor 3, a current approximately equal to the base current of the transistor 4, so to offset the base current I _B, the collector current of the transistor 4 is approximately equal to the reference current 7. The above transistor 4
Can be made sufficiently small by increasing the value of N (for example, 10), so that the capacitance value of the capacitor 6 does not need to be so large, and the capacitor 6 can be easily incorporated in the semiconductor integrated circuit.

1, when the AC signal at the input terminal 1 increases, for example, the base current and the collector current of the transistors 3 and 4 increase, but the constant current sources 7 and 8
Since only be obtained constant current from the base current I _B is reduced direction, because it this was applied with negative feedback acts in the collector current decreases direction of the transistor 4, therefore the collector current of the transistor 4 is kept constant Is done. That is, since the transistors 3 and 4 are current mirrors, the collector current of the transistor 3 is also constant.

On the other hand, when viewing FIG. 1 in terms of AC, the AC component at the upper end of the capacitor 6 is bypassed by the capacitor 6, so that
Negative feedback does not occur in the base direction of the transistors 3 and 4 in an AC manner, and the AC change in the portion of the load 9 and the output terminal 2 is performed as usual regardless of the constant collector DC current of the transistor 3.

According to the circuit of FIG. 1, as shown in the graph of FIG. 5b, in the conventional technique, the DC current sharply increases when the input signal voltage or the thermal voltage VT of the transistor (about 26 mV at room temperature) is exceeded. However, in the present invention, as shown in the graph of FIG. 5A, the collector DC current of the transistor 3 is always constant. In other words, this means that the variation of the collector DC current of the transistor 3 due to the base AC signal of the transistor 3 is negatively fed back to the base of the transistor 4 through the resistor 5 so that the transistor 3 having a mirror relationship with the collector current of the transistor 4 Is constant.

In addition, instead of using a large resistance value as in the past for base current correction, a method is used in which a current almost equal to the base current flows to cancel out, so thermal noise generated by the resistance is small and a low-noise amplifier circuit can be made. Further, since the voltage drop due to the base current of the bias resistor can be reduced, a circuit that operates with a low power supply voltage (for example, 1.0 V) can be made. Further, since the capacitance value of the high-frequency bypass capacitor is not so large, it can be easily built in the semiconductor integrated circuit.

In application of the second figure the present invention, 10 operates as a frequency converter, a high-frequency signal of frequency f ₁ to the input 1, input 11 receives the local oscillation signal from a signal source 30 of frequency f ₂ to the input 12 ,
An intermediate frequency signal of “f ₁ −f ₂ ” is output to output ₂ . Reference numeral 8 denotes a specific circuit example of a current source for correcting a base current. The transistor 20 and the resistor 23, the current source 16 is a circuit for applying a bias to the bases of the transistors 18 and 19, and the transistor 21 and the transistor 18 are differential. Work as an amplifier,
Negative feedback is applied so that the collector current of the transistor 17 becomes equal to the current of the current source 15.

Since the collector of transistor 18 is connected to the base of transistor 17, the collector current of transistor 18 is equal to the base current of transistor 17. Since the transistors 18 and 19 have an emitter area ratio of 1: M and operate as a 1: M current mirror, the collector current of the transistor 19 is M times the base current of the transistor 17. For example, when the current sources 7 and 15 are set equal, the base currents of the transistor 4 and the transistor 17 become equal. Therefore, if "M = N + 1", the collector current of the transistor 19 becomes It is equal to the sum of the base current. Further, the resistor 37 is for adjusting the load conditions of the collectors of the transistors 3 and 4 to improve the pairing of the transistors 3 and 4 with respect to the AC signal.

FIG. 3 shows an example in which the present invention is applied to an oscillation circuit 40, in which 31 is a crystal oscillator, 32 and 33 are oscillation capacitors, 34 is a DC bypass inductor, and 35 and 36 are current mirror circuits.
In this example, the emitters of the transistors 3 and 4 are not grounded in AC, but are grounded only in DC, which is different from other examples.

[Effects of the Invention] As described above, according to the present invention, the collector current of a biased element (transistor) due to a high-frequency input signal can be made constant, and low noise, low power supply voltage, and suitability for IC integration can be obtained. And a bias circuit having the same.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of one embodiment of the present invention, FIGS. 2 and 3 are circuit diagrams of application examples of the circuit, FIG. 4 is a conventional bias circuit diagram, FIG. FIG. 4 is a characteristic diagram of FIG. 4, and FIG. 6 is a characteristic diagram of FIG. 1 input terminal, 2 output terminal, 3,4 pair transistor (emitter area ratio N: 1), 5 bias resistor, 6 high frequency bypass capacitor, 7 reference current source, 8 ……………………………………………………………………………………………………… …………………………………………………………………………………………………………………. ... crystal oscillator, 32, 33 ... oscillation capacitor, 34 ... inductor, 40 ... oscillation circuit.

Claims (1)

(57) [Claims] A first transistor for supplying a high-frequency signal to a base and forming a part of a high-frequency circuit; a second transistor having the first transistor and a base and an emitter connected to each other; A resistor connected between the base and collector of the transistor, a capacitor connected to the collector of the second transistor for bypassing a high-frequency signal, and a base current of the first and second transistors connected to the collector of the second transistor A constant current source for supplying an added value of a current substantially equal to a reference current and a reference current.