JPH0685589A - Logarithmic conversion circuit - Google Patents

Abstract

PURPOSE:To provide the logarithmic conversion circuit with a small scale circuit so as to suppress an output more than a logarithmic function for the signal of a fine area. CONSTITUTION:An input signal A is amplified by differential amplifier circuits 11 and 12 in first and next steps and an output step amplifier circuit 13 and turned to an output signal B, and an operating current 11 of the differential amplifier circuit 11 in the first step is controlled by a detection signal B' of this output signal B. Thus, logarithmic conversion is while suppressing the fine area. The scale is just made possible for the feedback part of the conventional circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a logarithmic conversion circuit, and more particularly to a karaoke machine, stereo set, VTR.
The present invention relates to a logarithmic conversion circuit used for an echo adding circuit (a circuit for generating an echo signal from an original signal related to sound and adding the echo signal to the original signal for output) and the like in audio equipment such as.

[0002]

2. Description of the Related Art Most of today's audio equipment has a so-called karaoke function for mixing and synthesizing a voice input from a microphone with another reproduction signal, etc. Functions are also essential. The echo signal for this purpose is usually generated by delaying an original signal, which is a sound such as voice or music converted into an electric signal, for a predetermined time. Further, in order to improve the reverberation effect, not only the original signal but also the echo signal is attenuated and fed back to generate a signal mixed with the original signal, and delay and mixing of this signal are repeated.

In such an echo adding circuit,
From the viewpoint of securing a dynamic range and reducing the cost of an expensive delay circuit, a compression circuit may be placed before the delay circuit and a decompression circuit may be placed after the delay circuit. A logarithmic conversion circuit is suitable as this compression circuit. In Figure 5,
The block diagram of the conventional logarithmic conversion circuit is shown. Here, 1 is an operational amplifier, and 2 is an exponential conversion circuit.

The exponential conversion circuit 2 is inserted into the negative feedback of the operational amplifier 1 and exponentially converts the output signal B of the operational amplifier 1 to generate the feedback signal C. The operational amplifier 1 receives the input signal A on the non-inverting input side,
The inverting input receives the feedback signal C and outputs the output signal B. With this configuration, the input signal A is converted according to the logarithmic function that is the inverse function of the exponential function, and the output signal B that is logarithmically converted is output.

FIG. 6 shows a concrete example of the exponential conversion circuit 2. The first stage differential amplifier circuit 2a, the second stage differential amplifier circuit 2b, and the output stage amplifier circuit 2c are connected in this order. Then, the output signal B for the logarithmic conversion circuit is received as an input signal of this circuit, and the operating current I2 of the differential amplifier circuit 2b at the next stage is controlled according to the detection signal B'of this circuit. Since the amplification factor of the transistor changes exponentially with respect to its operating current, the differential amplification circuit 2b
The amplification factor at changes with the value of the signal B exponentially. As a result, the signal B is exponentially converted to generate the signal C. This signal C is the feedback signal C for the logarithmic conversion circuit.

[0006]

In such a conventional logarithmic conversion circuit, a logarithmic function conversion is realized by feeding back the exponentially converted signal of the output signal. Therefore, the circuit scale is large, and adjustments such as oscillation prevention are troublesome. However, if an adjusting circuit, a compensating circuit, etc. are added as a measure against oscillation, the circuit scale will increase and the cost will increase.

This hinders the original purpose of reducing the cost of other circuits such as the delay circuit by using the logarithmic conversion circuit as the compression circuit. Moreover, the increase in the circuit scale is also inconvenient because it is against the demand for downsizing of devices and equipment that employ the circuit. Further, such a conventional logarithmic conversion circuit has its conversion characteristic maintained even for a minute value of an input signal. Therefore, when the delay circuit of the echo adding circuit in the above-mentioned application example is mainly composed of a digital memory, for example, a fixed amount of quantization error due to A / D conversion in the preceding stage of the delay circuit is generated. This is a problem because undesired noise due to the noise becomes relatively large with respect to a minute value of the input signal.

Intuitively, as a countermeasure against this, it is conceivable to add a circuit before the logarithmic conversion circuit for suppressing the value of the minute range of the input signal, but this causes an increase in the circuit scale and the above-mentioned. Inconvenience will cause further inconvenience. An object of the present invention is to solve such a problem of the prior art, and when the value or level of the input signal is very small, the output is more than when the logarithmic function which is the original conversion function is followed. This is to realize a logarithmic conversion circuit in which the value or level of a signal is suppressed by a small-scale circuit.

[0009]

The structure of the logarithmic conversion circuit of the present invention which achieves such an object is such that it receives an input signal,
In a logarithmic conversion circuit that outputs an output signal obtained by logarithmically converting the input signal, a pair of differential transistors and a current control circuit connected downstream of the differential transistor and flowing an operating current of the differential transistor are provided. And a voltage signal according to a difference between the input signal and a predetermined reference value.
Amplifier circuit, a second amplifier circuit that receives the voltage signal and outputs a current signal having a current value corresponding to the value of the voltage signal, and receives the current signal and amplifies the current signal at a predetermined amplification factor. And a third amplifier circuit for generating the output signal, and the value of the operating current is controlled according to the value or level of the output signal.

[0010]

In the logarithmic conversion circuit of the present invention having such a configuration, the value or level of the output signal is fed back to the operating current of the differential transistor in the first stage amplifier circuit. Generally, the amplification factor of the transistor changes exponentially with respect to the operating current, but such a circuit whose function is set on the feedback side generally follows a logarithmic function which is its inverse function. Therefore, the value of the input signal or the like is logarithmically converted to generate the output signal.

When the value of the input signal and the like become minute and the value of the output signal and the like also become minute, the operating current decreases due to the feedback, but the operating current is small in the first-stage amplifier circuit. Then, the operating resistance of the differential transistor is not negligible. Therefore, when the value of the input signal or the like is very small, the amplification factor becomes particularly small, and the value of the output signal or the like is further suppressed as compared with the conversion based on the original logarithmic function. Also, regarding the circuit scale, the overall scale is almost the same as the circuit scale of the conventional exponential conversion circuit, and is considerably smaller than the conventional one.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the logarithmic conversion circuit having the structure of the present invention will be described below with reference to the circuit diagram of FIG.
Here, 11 is a first stage differential amplifier circuit, 12 is a next stage differential amplifier circuit, 13 is an output stage amplifier circuit, and 14 is a detection circuit. The differential amplifier circuit 11 includes a current mirror, a pair of differential transistors Q1 and Q2 that are supplied with the same collector current by the current mirror, and a current control circuit that controls the operating current I1 of the pair of differential transistors in this order. It is connected between the power source Vcc and the ground GND. The base of one transistor Q1 of the pair of differential transistors receives the input signal A through the resistor R1, and the base of the other transistor Q2 receives a predetermined reference voltage.

Similarly, the differential amplifier circuit 12 includes a current mirror, a pair of differential transistors Q3 and Q4, and a constant current circuit for flowing an operating current I2 of the pair of differential transistors. It is connected between.
The base of the transistor Q4 receives the collector voltage of the transistor Q1 on the input signal A side of the differential amplifier circuit 11, and the base of the transistor Q3 receives the reference voltage. The output stage amplifier circuit 13 receives the current signal A ′ output from the differential amplifier circuit 12 as an input, amplifies the current signal A ′, and converts the current signal A ′ into a voltage signal to generate and output an output signal B.

The detection circuit 14 detects the amplitude level of the output signal B by detecting the output signal B and generates a detection signal B '. The detection signal B ′ is fed back to the current control circuit of the differential amplifier circuit 11 as a control signal, so that the operating current I1 is controlled according to the level of the output signal B.

Under such a structure, first, the input signal A which is a voltage signal is converted into a current signal through the resistor R1. In the differential amplifier circuit 11 receiving this, a voltage difference according to the current signal is generated as a difference between the base-emitter voltages of the transistors Q1 and Q2. Then, the differential amplifier circuit 12 converts the voltage difference into a differential current and outputs it as a current signal A ′. Then, the current signal A ′ is output as the output signal B which is a voltage signal by passing through the resistor R2 of the output stage amplifier circuit 13.

At this time, the amplification factor of the entire circuit from the input signal A to the output signal B is usually determined by the resistance ratio between the resistors R2 and R1 and the current ratio between the current I2 and the current I1. Determined. Since this amplification factor depends on the current ratio between the current I2 having a constant value and the current I1 controlled by the level of the output signal B, this entire circuit has its own output signal B depending on the level of its output signal B fed back. This is a circuit whose amplification factor changes. The change in the amplification factor is logarithmic because the transistors Q1 and Q2 change exponentially with respect to the operating current I1 and this functions on the feedback side.

Since the resistors R1 and R2 are fixed resistors, the resistance ratio between the resistors R2 and R1 is usually a constant value. However, the signal level of the input signal A becomes very small,
Along with this, the level of the output signal B also becomes very small, and when the operating current I1 further decreases, the transistors Q1 and Q2
The operating resistance of can not be ignored. With respect to the amplification factor, this operating resistor apparently acts as if it is connected in series with the resistor R1. Therefore, when the signal level of the input signal A is very small, the amplitude level of the output signal B is further suppressed to be smaller than the original logarithmic conversion based on the current ratio (see FIG. 4).

Although FIG. 2 shows an embodiment of another structure, its function and effect are the same as those of the above embodiment. Also,
FIG. 3 shows a detailed circuit diagram of the above embodiment. The scale of these circuits is almost the same as that of the feedback section of the conventional circuit, that is, that of the exponential conversion circuit (see the circuit of FIG. 1 or 2 and the circuit of FIG. 6 for comparison). By the way, the application of these circuits is not limited to the echo adding circuit.

[0019]

As can be understood from the above description, in the logarithmic conversion circuit of the present invention, when the value or level of the input signal is very small, the logarithmic function which is the original conversion function is followed. The value or level of the output signal is further suppressed. Therefore, it is possible to reduce the influence of quantization noise and the like without adding a special circuit or the like for suppressing a minute area of the signal. As a result, this logarithmic conversion circuit can be realized by a small-scale circuit corresponding to the scale of only the feedback portion of the conventional circuit.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of a logarithmic conversion circuit having the configuration of the present invention.

FIG. 2 is a block diagram of another embodiment of the logarithmic conversion circuit having the configuration of the present invention.

FIG. 3 is a detailed circuit diagram of an embodiment of a logarithmic conversion circuit having the configuration of the present invention.

FIG. 4 is a conversion characteristic diagram of the logarithmic conversion circuit having the configuration of the present invention.

FIG. 5 is an example of a conventional logarithmic conversion circuit.

FIG. 6 is an example of a conventional exponential conversion circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Operational amplifier 2 Exponential conversion circuit 11, 12 Differential amplification circuit 13 Output stage amplification circuit 14 Detection circuit

Claims (1)

[Claims] 1. A logarithmic conversion circuit for receiving an input signal and outputting an output signal obtained by logarithmically converting the input signal, comprising: a pair of differential transistors; A first amplifier circuit including a current control circuit for flowing the voltage signal to generate a voltage signal according to a difference between the input signal and a predetermined reference value; and a current corresponding to the value of the voltage signal upon receiving the voltage signal. A second amplifier circuit that outputs a current signal of a value, and a third amplifier circuit that receives the current signal and amplifies the current signal at a predetermined amplification factor to generate the output signal, A logarithmic conversion circuit, wherein the value of the operating current is controlled according to the value or level of the output signal.