JPS5834968B2 - 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.

That is, the output characteristic of the voltage variable gain circuit with respect to the control voltage is linear within a predetermined range of the control voltage, and becomes nonlinear when the control voltage is smaller or larger than the range.

Therefore, in order to widen the gain control range, it is desirable to set the operating midpoint of the circuit to be located in the center of the linear region.

However, if the above-mentioned idling current is too small, the midpoint of the operation will drop and approach the nonlinear region, making crossover distortion likely to occur.If the idling current is too large, the midpoint of the operation will rise and approach the nonlinear region, causing the output waveform to become asymmetric between positive and negative. Become.

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 explain 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,
Collector of transistor 11 which is the bias voltage.

The emitter voltage vB is the base of the transistor 11.

Resistor 12.13 must be chosen to be less than twice the emitter voltage VBE.

The idling current has no temperature dependence at vB-2
This is true only when VBE is chosen, and if B is chosen smaller than that, it will have a positive temperature coefficient, causing crossover distortion at low temperatures and increasing 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.

Therefore, this is a much bigger problem in the voltage-controlled variable gain circuit used in the noise reduction device known by the trade name dbx noise reduction device, which has a much larger ratio of the driver single-stage current to the idling current than the power amplifier. .

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. The voltage-controlled variable gain circuit used in noise reduction devices cannot be put to practical use.

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 the 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 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, since the accuracy of the resistance ratio acts exponentially in the bias circuit described above, even with an accuracy of ±2%, the variation is three times or more.

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.

The base and collector of the transistor 21 are short-circuited, the collector is connected to the terminal 22, and the emitter is connected to the collector of the 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 and the base of the transistor 23. connected in series.

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 a complementary-8EPP (Single Ended Pu5h
A NPN transistor 33 and a PNP transistor 34 are connected between the 10B power supply and the -B power supply.
series circuit is connected.

In addition, the 10B power source has a current source 31. -B power supplies are each connected to an NPN transistor 32 which acts as one stage of a driver.

The collector of the bias circuit 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 I-disk 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 with the above connection, most of the current in the first stage of the driver flows through the transistors 21 and 23, and approximately 1/h of the current in the first stage of the driver flows through the transistors 27 and 28.
pEL doesn't flow.

However, hFE is the DC current amplification factor of the transistor 23.

In this case, if transistors 21 and 23 have the same characteristics,
The base-emitter voltages of the transistors 21.23 are equal, and the voltage between the collector of the transistor 27 and the emitter of the transistor 28, which are precisely diode-connected, is taken out at the output terminal 24.25.

Therefore, 5EPP is connected to the output terminals 24 and 25.
If the characteristics of the transistor 33 and the PNP transistor 34 are the same, the idling current flowing through these transistors 33 and 34 is exactly 1/hF of the current of one stage of the driver.
It can be set as E.

That is, the value of the idling current and its dependence on ambient temperature fluctuations depend on hFE of the transistor 23.

In general, hFE has a value of about 200 to 300, and its dependence on ambient temperature fluctuations and power supply voltage fluctuations is very small.

Therefore, the idling current becomes extremely small and has little dependence on ambient temperature and power supply voltage fluctuations.

Note that if it is difficult to match the characteristics of the NPN transistor 33 and the PNP transistor 34, either one of the transistors 27 and 28 may be replaced with a diode-connected PNP transistor.

In addition, the selection of idling current is determined by the ratio of hFE to the saturation current of each transistor (in the integrated circuit, the emitter area ratio).
It can be arbitrarily determined by

The accuracy and temperature dependence of the idling current by this bias circuit correspond to those of the transistor hFE.

Therefore, in a circuit with an extremely small idling current, such as the voltage controlled variable gain circuit used in the dbx noise reduction device, much better accuracy and power dependence than conventional circuits can be obtained.

FIG. 4 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 the dbx noise reduction device.

In the figure, a signal input terminal 55 is connected to an operational amplifier 57 via a resistor 56.

The output terminal of operational amplifier 57 is connected to the base of output stage transistor 67.

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

The terminal 22 of the bias circuit 61 is connected to the +B power supply, and the output terminal 24 is connected to the emitter of a PNP transistor 2 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 51.

On the other hand, the collector of the output stage transistor 7 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 the second feedback loop of the operational amplifier 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 a 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 l-range screen 5 whose base is grounded and whose collector is connected to the input terminal of the operational amplifier 68.

A resistor 69 is connected between the input and output terminals of the operational amplifier 68, and the signals supplied through the transistors 64 and 65 are added together and sent to the output terminal -70.

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.
Idling current is passed through 63, 64, and 65.

The idling current varies depending on the control voltage applied to the terminal 66, but when the control voltage is set to ground potential, it is approximately 1
Selected around μA.

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 1.5VBB, 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.

If the idling current decreases, crossover distortion will occur, and if it increases, an asymmetrical output waveform will occur depending on the control signal, causing a phenomenon as if the control signal has leaked to the output.

On the other hand, the present invention using a bias circuit as shown in FIG. 2 has good temperature dependence and power supply dependence, so the above drawbacks are eliminated and a high performance voltage controlled variable gain circuit can be achieved. can be provided.

That is, according to the present invention, it is possible to realize a voltage-controlled variable gain circuit for a noise reduction device that has low dependence on temperature and power supply, can always provide an idling current with high accuracy, and has a wide gain variable range. .

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 drawings]

FIG. 1 is a diagram showing the configuration of a conventional bias circuit, FIG. 2 is a diagram showing the configuration of a bias circuit used in the voltage controlled variable gain circuit of the present invention, and FIG. 3 is a diagram showing the configuration of the bias circuit of FIG. 2.
Diagram showing the connections when installed in a class push-pull circuit, No. 4
The figure is a diagram showing an 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 source, 32...
... Drive stage transistors, 33 and 34...
...5EPP connected transistor, 61...
...Bias circuit, 62-64...Transistor, 57.68...Operation amplifier.

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 bias output terminal connected to each emitter of said first and third transistors, and a second bias output terminal connected to each emitter of said second and fourth transistors and coupled to an output terminal of said amplifier. a bias circuit having a bias output terminal, and the bias circuit includes a fifth transistor whose base and collector are commonly connected, and a fifth transistor having the same polarity as the fifth transistor and whose collector is connected to the emitter of the fifth transistor. a sixth transistor connected;
a series connection circuit of at least two diodes that is forward biased by a portion of the drive current flowing through the fifth and sixth transistors, and a voltage across the series connection circuit; means for applying current to the bases of each of the transistors and for causing all of the current flowing in the series connection circuit to flow to the base of the sixth transistor, and for applying current to the base of each of the fifth and sixth transistors from the emitters of the fifth and sixth transistors, respectively. A voltage controlled variable gain circuit configured to take out a second bias output terminal.