JPS5834045B2 - Voltage controlled variable gain circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a voltage controlled variable gain circuit used in a noise reduction system, etc., and in particular to a voltage controlled variable gain circuit that is equipped with a bias circuit that is highly accurate and can provide a stable idling current despite fluctuations in temperature and power supply. This invention relates to a controlled variable gain circuit.

A voltage-controlled variable gain circuit for a noise reduction system has a class B push-pull circuit using complementary transistors.

A small idling current must be passed through this class B push-pull circuit in order to prevent crossover distortion that occurs near the zero crossing of the signal.

In this case, if the bias circuit for passing the idling current is not appropriately designed, various problems will occur.

For example, if the idling current is too large, loss or thermal runaway will occur, and if the idling current is too small, crossover distortion will occur.

FIG. 1 shows a bias circuit commonly used in a class B push-pull circuit using complementary transistors.

This circuit is excellent as a bias circuit, but its only drawbacks are poor temperature characteristics and bias voltage dependence on resistance ratio.

First, to clarify the temperature characteristics, in audio power amplifiers, the idling current in the output stage is often selected to be a few fractions of the current in a single driver stage.

Further, since the transistor 11 of the bias circuit is a small signal transistor, the current is normally smaller than the saturation current.

If you try to apply idling current under such conditions,
The resistor 12.
You have to choose 13.

The idling current has no temperature dependence at VB=2
VBE, which is smaller than VB
If selected, it will have a positive temperature coefficient, resulting in crossover distortion at low temperatures and increased loss at high temperatures.

Such a phenomenon becomes more pronounced as the ratio of the current of one stage of the driver to the idling current increases.

This is a very big problem with the voltage-controlled variable gain circuit used in the noise reduction device known as the dbx noise reduction device, which has a much larger ratio of driver single-stage current to idling current than that of acid power amplifiers. .

As a means to improve the temperature characteristics, in the circuit shown in FIG. 1, a resistor 14 is inserted into the collector of the transistor 11, and a resistor 12. There is a method of determining the value of 13 and subtracting a certain amount of voltage drop from that voltage through the resistor 14 to obtain it as a bias voltage.

According to this method, a low bias voltage can be obtained without reducing the temperature dependence of the bias voltage, so that a good temperature dependence of the idling current can be obtained.

However, although good temperature dependence can be obtained in this circuit, there is a drawback that dependence on the power supply becomes worse.

In other words, since the voltage drop across the resistor 14 depends on the current of the driver single stage, the power supply dependence becomes more pronounced as the ratio of the driver single stage current and idling current increases, similar to temperature dependence. Voltage-controlled variable gain circuits used in noise reduction devices have little practical application.

Furthermore, the disadvantage that the bias voltage of the bias circuit depends on the resistance ratio becomes apparent in integrated circuits.

In other words, in individual component circuits, the variation in saturation current of transistors is large, and setting the idling current by adjusting the resistor is essentially unavoidable, and the merit of not adjusting it is small, so it is not a big problem, but in integrated circuits In this case, no adjustment is strongly desired.

The relative accuracy of resistors within an integrated circuit is relatively good, with resistors having similar values being within the range of ±2%.

However, in the former bias circuit, the accuracy of the resistance ratio acts exponentially, so even with an accuracy of ±2, the variation is more than three times as large.

The present invention has been made in view of the above problems, and provides a bias circuit suitable for integrated circuits, particularly a bias circuit that has good temperature dependence and can flow an idling current with high precision, thereby reducing noise. The present invention aims to provide a voltage controlled variable gain circuit suitable for systems.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 2 shows the configuration of a bias circuit according to an embodiment of the voltage controlled variable gain circuit of the present invention.

Transistor 21 has its base and collector shorted, its collector connected to terminal 22, and its emitter connected to the collector of transistor 23.

That is, transistors 23.21 are connected in cascade.

Output terminals 24 and 25 are connected to the emitters of the transistors 21 and 23, respectively, and a terminal 26 is connected to the emitter of the transistor 23.

On the other hand, a so-called diode-connected transistor 27 whose base and collector are short-circuited is connected to the collector of the transistor 21, and a diode-connected transistor 28 is connected between the emitter of the transistor 27 and the base of the transistor 23. connected in series.

Further, a series circuit of a diode-connected transistor 41 and a resistor 42 is connected between the base and emitter of the transistor 23.

Such a bias circuit is connected within a class B push-pull circuit as shown in FIG.

In other words, the circuit shown in FIG. 3 is generally a complementary 8E P P (Single Ended Pu) circuit as a class B push-pull circuit using complementary transistors.
5hPull) circuit, and an NPN transistor 33 and a PNP) transistor 3 are connected between the 10B power supply and the -B power supply.
4 series circuits are connected.

Further, a current source 31 is connected to the 10B power source, and an NPfl transistor 32 which acts as a single stage of 5 drivers is connected to the -B power source.

In the bias circuit, the collector of the transistor 21 is connected to the current source 31, the emitter of the transistor 23 is connected to the transistor 32, the emitter of the transistor 21 is connected to the base of the NPN transistor 33, and the emitter of the transistor 23 is connected to the PNP transistor 34. are built into the base of each unit so as to be connected to each other.

In the bias circuit connected in this way, the current flowing through the transistors 27 and 28 is bypassed to the emitter of the transistor 23 through the series circuit of the transistor 41 and the resistor 42, and a highly accurate idling current is obtained without depending on hFE. is obtained.

Furthermore, since the diode-connected transistor 41 and the resistor 42 have opposite temperature characteristics, there is an advantage that the overall temperature dependence can be improved.

FIG. 4 shows another embodiment of the bias circuit used in the voltage controlled variable gain circuit of the present invention.

This circuit is obtained by adding a voltage level shifting circuit consisting of three NPN transistors 51, 52, and 53 to the circuit shown in FIG.

Specifically, the transistor 51 is a diode-connected transistor, and has a collector connected to the terminal 22 and an emitter connected to the base of the transistor 21.

The transistor 52 has a collector connected to the emitter of the transistor 51, an emitter connected to the terminal 25, and a base connected to the base of the transistor 23.

Transistor 53 is diode-connected, with its collector connected to the emitter of transistor 27 and its emitter connected to the collector of transistor 28.

Transistor 51 is used for voltage level shifting, transistor 52 acts as a current source, and transistor 53 is used for compensating the voltage between the base and emitter of transistor 51 for voltage level shifting.

In such a bias circuit, as in the case described with reference to FIG. , the ambient temperature dependence of the idling current is reduced.

Furthermore, according to this bias circuit, transistor 51.
By adopting the configuration in which a voltage level shifting circuit consisting of transistors 21 and 23 is provided, there is also an effect that the idling current can be varied widely without changing the emitter area ratio of the transistors 21 and 23.

FIG. 5 shows a voltage controlled variable gain circuit according to an embodiment of the present invention, which is equipped with the aforementioned bias circuit.

This voltage variable gain circuit is used in a dbx noise reduction device.

In the figure, a signal input terminal 55 is connected to an operational amplifier 51 via a resistor 56.

The output terminal of the operational amplifier 57 is an output stage transistor 670.
connected to the base.

This output stage transistor 61 has an emitter connected to the -BB power supply via a resistor 58, and a collector connected to a terminal 26 and an output terminal 25 of a bias circuit 61 having the configuration shown in FIGS. 2 and 4. .

The terminal 22 of the bias circuit 61 is connected to the 10B power supply, and the output terminal 24 is connected to the emitter of a PNP transistor 62 whose base is grounded and whose collector is connected to the input terminal of the operational amplifier 57.

Transistor 62 constitutes a first feedback loop of operational amplifier 57.

The collector of the power output stage transistor 1 is an NPN transistor 6 whose base is connected to the control terminal 66 and whose collector is connected to the input terminal of the operational amplifier 57.
It is connected to the emitter of 3.

This transistor 63 constitutes a second feedback loop of the arithmetic tank 1 receiver 57.

Further, the output terminal 24 of the bias circuit 61 has a base connected to the control terminal 66 and a collector connected to the operational amplifier 68.
The emitter of the PNP transistor 64 is connected to the input terminal of the PNP transistor 64.

The collector of the output stage transistor 67 is connected to the emitter of an NPN transistor 5 whose base is grounded and whose collector is connected to the input terminal of the operational amplifier 68.

The operational amplifier 68 has a resistor 69 connected between its input and output terminals, adds the signals supplied via the transistors 64 and 65, and sends the sum to the output terminal 10.

The gain of this voltage controlled variable gain circuit is variably controlled by a control signal supplied to the control terminal 66.

The bias circuit 61 includes a transistor 62 that controls the gain.
63.64.65 to pass idling current.

The idling current varies depending on the control g4 voltage applied to the terminal 66, but when the control voltage is set to ground potential, it is selected to be around 1 μm.

On the other hand, the current flowing through the output stage transistor 67 is approximately 1 mA.

Therefore, considering that the conventional circuit shown in FIG. 1 is used as the bias circuit 61, it is necessary to select the bias voltage VB to be about 1.5VBE, and in this state, the temperature change of about ±50°C from room temperature is When you think about it, the change in idling current can actually reach several tens of times.

A decrease in idling current causes crossover distortion, and an increase in idling current causes leakage of control signals.

On the other hand, in the present invention using a bias circuit as shown in FIGS. 2 and 4, a series-connected transistor and a resistor are used to bypass the current flowing through the series-connected circuit.

Therefore, temperature dependence or power supply dependence is improved, the above-mentioned drawbacks are eliminated, and a high-performance voltage-controlled variable gain circuit can be provided.

As described above, according to the present invention, it is possible to realize an excellent voltage-controlled variable gain circuit for a noise reduction device that can provide a highly accurate idling current and is less dependent on temperature and power source.

Furthermore, the fact that the bias voltage does not depend on the resistance ratio is advantageous when integrating the circuit, and it is possible to eliminate the need for adjustment.

[Brief explanation of the drawing]

FIG. 1 shows the configuration of a conventional bias circuit, FIG. 2 shows the configuration of a bias circuit used in the voltage controlled variable gain circuit of the present invention, and FIG. 3 shows the configuration of the bias circuit of FIG.
Diagram showing the connections when installed in a class push-pull circuit, No. 4
This figure shows another configuration of the bias circuit used in the voltage controlled variable gain circuit of the present invention, and FIG. 5 is a diagram showing one embodiment of the voltage controlled variable gain circuit of the present invention. 21 and 23...cascade-connected transistors, 27.28...diode-connected transistors, 31...current sources, 32...drive stage transistors, 33 and 34...8EPP-connected Tran: Instagram.

Claims (1)

[Claims] 1. An amplifier that amplifies a human power signal, and a first feedback circuit that includes a first transistor whose collector is connected to the input side of the amplifier and returns the output current of the amplifier to the input;
A second transistor including a second transistor whose collector is connected to the input side of the amplifier and provided between the input and output terminals of the amplifier so as to feed back a current in a direction opposite to the direction of the feedback current of the first feedback circuit. a first output circuit including a feedback circuit, a third transistor configured to output an output current of the amplifier; and a first output circuit configured to output a current in a direction opposite to the current of the first output circuit. a second output circuit including a fourth transistor; a summing circuit connected to the collectors of the third and fourth transistors to add the currents drained by the first and second output circuits;
means for applying a gain control voltage between the bases of the first and second transistors and between the bases of the third and fourth transistors, and supplying an idling current to the first to fourth transistors; a first transistor connected to each emitter of the first and third transistors such that
and a second bias output terminal connected to each emitter of the second and fourth transistors and coupled to an output terminal of the amplifier, the bias circuit having a base and a fifth transistor whose collectors are commonly connected, a sixth transistor whose collector is connected to the emitter of the fifth transistor and whose polarity is the same as that of the fifth transistor, and the fifth and sixth transistors. at least 2 forward biased by a portion of the drive current applied
a series connection circuit of diodes, means for applying a voltage across the series connection circuit to the bases of each of the fifth and sixth transistors; a series circuit of a diode and a resistor connected between the base and emitter of the transistor, and configured to take out the first and second bias output terminals from the emitters of the fifth and sixth transistors, respectively. A voltage controlled variable gain circuit.