JPH0562842B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a bias circuit that sets an operating point in an amplifier or a specific DC level in a feedback circuit, and particularly relates to a bias circuit that sets an operating point in an amplifier or a specific DC level in a feedback circuit. Regarding speeding up.

[Conventional technology]

As shown in FIG. 4, the bias circuit divides the power supply voltage V _CC with voltage dividing resistors 2 and 4, and when it is configured with a semiconductor integrated circuit, the power supply voltage V CC is applied to the terminal 6 formed at the voltage dividing point. A capacitor 8 is connected to remove noise and ripple components included in _CC .
A specific voltage, for example, voltage V _CC /2, generated at the voltage dividing point of the resistors 2 and 4 is taken out from the output terminal 12 via a buffer amplifier 10 that does not have an amplification gain Gv to achieve low impedance. , is supplied to an input bias of an amplifier (not shown) and a feedback bias for setting the DC level of a feedback circuit of the amplifier.

[Problem that the invention seeks to solve]

If the capacitor 8 added to such a bias circuit has a large capacitance and the time constant with the resistor is increased, the rejection ratio of noise and ripple components from the power supply can be increased, but the power supply will be reduced accordingly. There is a drawback that it takes time for the bias voltage output to rise after power-on, and that the steady voltage value is delayed.

Therefore, the present invention aims to improve the rejection ratio of noise and ripple components from the power supply and to speed up the rise of the bias voltage output.

[Means for solving problems]

The bias circuit of the present invention is a bias circuit in which a capacitor is connected to a connection point of a voltage dividing resistor that divides a specific voltage to generate a constant bias voltage, and this bias voltage is applied to an amplifier. a resistive voltage divider circuit having a voltage dividing point that instantaneously rises in response to the current voltage and is lower than the charging voltage by a constant voltage when the capacitor is charged to a steady voltage; a comparator that compares the voltage of the capacitor with the voltage of the capacitor; and a comparator connected through a resistor between the power supply line and the capacitor, when the charging voltage of the capacitor is lower than the voltage division point of the resistor voltage divider circuit. and a transistor that receives an output generated by the capacitor at its base and becomes conductive, and causes a charging current to flow through the resistor to the capacitor.

[Effect]

Therefore, in the present invention, when the power is turned on, the capacitor is supplemented with charging current by the precharge circuit, thereby shortening the charging time of the capacitor, thereby speeding up the rise of the bias voltage output.

Therefore, the capacity of the capacitor can be increased,
It is possible to increase the rejection ratio of noise and ripple components from the power supply.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the drawings.

First Embodiment FIG. 1 shows an embodiment of the bias circuit of the present invention, and the same parts as the bias circuit shown in FIG. 4 are given the same reference numerals.

As shown in Figure 1, this bias circuit includes:
A precharge circuit 2 that rapidly charges the capacitor 8 when the power is turned on, that is, when the voltage V _CC is applied.
0 is added. This precharge circuit 20
is a voltage dividing resistor 2 with a diode 22 interposed therebetween.
4 and 26 constitute a voltage dividing circuit.

In this example, the resistance values of voltage dividing resistors 2 and 4 are
R ₂ and R ₄ are set to R ₂ = R ₄ in order to obtain the midpoint bias of the power supply voltage V _CC , and the voltage dividing resistor 24,
Similarly, the resistance values R ₂₄ and R ₂₆ of No. 26 are set to R ₂₄ =R ₂₆ . At this time, the voltage dividing point potential at the cathode of the diode 22 is compared with the voltage dividing point potential of the voltage dividing resistors 2 and 4, and in a steady state, the voltage dividing point potential at the cathode of the diode 22 is
The value is set to a value that is 1/2 (V _F /2) lower than the forward drop V _F of .

In order to compare these voltage division point potentials, a voltage comparator 28 is installed with the cathode side of the diode 22 facing the inverting input terminal (-), and its comparison output is used as a switching element to flow charging current to the capacitor 8. It is added to the base of transistor 30. A current limiting resistor 32 is inserted on the collector side of the transistor 30, and when the resistor 32 is conductive, a precharge current flows through the capacitor 8.

Based on the above configuration, its operation will be explained with reference to FIG.

When the power is turned on, the potential at point C rises instantaneously, while the potential at point B becomes lower than the potential at point C because capacitor 8 is not charged, and comparator 28
The output becomes L (low potential) level. This output makes the transistor 30 conductive and the power supply voltage
A precharge current flows from V _CC to capacitor 8 through resistor 32 .

As a result, the capacitor 8 is rapidly charged as shown in section a of the charging characteristic X shown in FIG. In this charging, the potential V _B at point B becomes a potential V _C (=V _CC /2−V _F /2) lower than the midpoint potential V _CC /2 by 1/2 of the forward drop V _F of the diode 22. is carried out until it is reached.

Further, at this time, due to the conduction of the transistor 30, the resistor 32 is connected in parallel to the resistor 2, and for example, the resistance value of the resistor 2 is set to R ₂ ,
Let the resistance value of the resistor 32 be R ₃₂ , and when R ₂ > R ₃₂ ,
R ₂ · R ₃₂ / (R ₂ + R ₃₂ ) < R ₃₂ , and the charging current of the capacitor 8 can be made larger, and the potential V _B at point B is smaller than the midpoint potential V _CC /2.
It is possible to quickly reach a potential V _C lower by 1/2 of the forward drop V _F of 2, that is, V _F /2.

As a result, the output of the comparator 28 becomes H (high potential) level, the transistor 30 becomes non-conductive, and from this point on, the charging of the capacitor 8 is stopped by the resistor 2.
The charging is carried out only through the charging section b of the charging characteristic X in FIG. 2. In this case, since the capacitor 8 is precharged in advance, the time for transition to a steady voltage is correspondingly faster, and the precharge circuit 20
When charging only with resistor 2 without installing
Compared to the charging voltage V _y ) due to the charging characteristic Y shown in the figure,
The total time to transition to steady bias voltage output is faster by _tP .

Note that if the voltages in sections a and b of charging characteristic X shown in FIG. 2 are V _xa and V _xb , respectively, these are as follows.
It is as follows.

V _xa ≒ R ₄ / (R ₀ + R ₄ )・V _CC・{1−exp(−t/C ₈・R ₀ )} ……(1) V _xb ≒V _y =R ₄ /(R ₂ +R ₄ )・V _CC・{1−exp(−t/C ₈・R ₂ )} ……(2) However, in formulas (1) and (2), R ₄ is the resistance value of resistor 4, and C ₈ is the capacitor. 8 capacitance, R ₀ is resistance 2,
Combined resistance value of 32 parallel circuits {=R ₂・R ₃₂ /(R ₂
+R ₃₂ )}.

Second Embodiment FIG. 3 shows a specific embodiment of the precharge circuit 20, in which the voltage comparator 28 is composed of a differential amplification type comparator consisting of transistors 34, 36, a resistor 38, and a constant current source 40. , the transistor 30 includes transistors 42, 44 and a resistor 4.
It consists of 6 Darlington connection circuits. In Figure 3, B and C are point B in Figure 2,
Corresponds to point C. Therefore, according to such a circuit, rapid rise of the bias voltage output can be easily realized with an extremely simple configuration.

[Effects of the Invention] As described above, according to the present invention, when the power is turned on, the precharge circuit replenishes the capacitor with charging current, shortens the capacitor charging time, and speeds up the rise of the bias voltage output. By being able to do this, the following effects can be obtained.

(a) A stable bias voltage output can be obtained at the same time as the power is turned on.

(b) Precharging can speed up the voltage rise, making it possible to increase the capacitance of the capacitor and the resistance value of the voltage dividing resistor, increasing the rejection ratio of noise and ripple components from the power supply. .

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the bias circuit of the present invention, and FIG. 2 is a graph showing its charging characteristics.
FIG. 3 is a circuit diagram showing a specific circuit configuration of the precharge circuit, and FIG. 4 is a circuit diagram showing a conventional bias circuit. 2, 4, 24, 26... voltage dividing resistor, 8... capacitor, 20... precharge circuit, 22... diode, 28... voltage comparator, 30... transistor, 32... resistor.

Claims (1)

[Claims] 1. A bias circuit that generates a constant bias voltage by connecting a capacitor to a connection point of a voltage dividing resistor that divides a specific voltage, and applies this bias voltage to an amplifier, the circuit comprising: a resistive voltage divider circuit having a voltage dividing point that instantaneously rises in response to the voltage and is lower by a constant voltage than the charging voltage when the capacitor is charged to a steady voltage; a comparator that compares the voltage of the capacitor with a voltage of the capacitor; 1. A bias circuit comprising: a transistor that receives an output generated by the transistor at its base, conducts it, and supplies a charging current to the capacitor through the resistor.