JPH08212276A - Logarithmic amplifier - Google Patents

Abstract

PURPOSE: To realize stable operation of the circuit and to obtain high speed responsiveness by making an operation amplifier have a composite amplifier constitution of FET input type operation amplifier and a bipolar input type operation amplifier. CONSTITUTION: The input terminal 10 of the logarithmic amplifier is connected to the negative input terminal 11a of the FET input type operation amplifier 11. In addition, the input terminal 10 is also connected to the negative input terminal 22a of the bipolar input type operation amplifier 22 through a capacitor 16. The output terminal 22c of the bipolar input type operation amplifier 22 is connected to an output terminal 31 at a first stage through a resistor 30. Then, output voltage (positive voltage) generated at the output terminal 22c of the operation amplifier 22 is inputted to an operation amplifier 42 at a second stage and also to a transistor 33. Then, the higher the voltage applied to the emitter 33a of the transistor 33 is, the lower the resistance value between the emitter and a collector and the larger a feedback quantity through a capacitor 32.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a logarithmic amplifier,
In particular, the present invention relates to a logarithmic amplifier which has been speeded up.

[0002]

2. Description of the Related Art A current signal having a very wide range, such as an output signal of a photodetector for reading a high gradation image, may be logarithmically amplified in order to compress the range and facilitate the handling of the signal. Many.

Conventionally, as one of the logarithmic amplifiers for performing this logarithmic amplification, as shown in Japanese Patent Laid-Open No. 2-277181, an operational amplifier, a logarithmic conversion element connected to a feedback circuit of this operational amplifier, and this logarithm. An oscillation prevention circuit including a conversion element and a capacitor connected in parallel,
A control circuit is known that controls the oscillation prevention circuit so that the amount of feedback through the oscillation prevention circuit is reduced as the input current becomes smaller according to the input current to the operational amplifier.

This type of logarithmic amplifier increases the amount of feedback of high frequency components when the input current is large and the operation tends to be unstable, and the input current is small and the operation of the logarithmic amplifier is stable, but there is no problem. When the response characteristic becomes a problem, the feedback amount of the high frequency component is suppressed, whereby the stable operation of the circuit and the improvement of the high-speed response can both be achieved.

[0005]

In order to further improve the high speed response in the conventional logarithmic amplifier, it is necessary to configure the operational amplifier itself with a high speed. A bipolar input type is known as an operational amplifier excellent in high speed, but in the above conventional logarithmic amplifier having a circuit configuration that reduces the feedback amount via the oscillation prevention circuit as the input current becomes smaller, Since the input bias current causes an error, in actuality, the FET input type operational amplifier with a small input bias current must be used from the viewpoint of accuracy, and the bipolar input type operational amplifier cannot be used. Due to such circumstances, it has been difficult for the conventional logarithmic amplifier to obtain sufficient high-speed response.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a logarithmic amplifier which realizes a stable operation of a circuit and has a sufficient high-speed response.

[0007]

A first logarithmic amplifier according to the present invention is, as described in claim 1, an operational amplifier, a logarithmic conversion element connected to a feedback circuit of the operational amplifier,
An oscillation prevention circuit including a capacitor connected in parallel with the logarithmic conversion element, and an input current to the operational amplifier, the amount of feedback through the oscillation prevention circuit is reduced as the input current decreases. In a logarithmic amplifier including a control circuit for controlling the oscillation prevention circuit, the operational amplifier has a composite amplifier configuration of an FET input type operational amplifier and a bipolar input type operational amplifier.

In the above structure, more preferably, the input stage of the bipolar input type operational amplifier is provided with an FET as the input stage.

The second logarithmic amplifier according to the present invention is
According to a third aspect of the present invention, to an operational amplifier, an oscillation preventing circuit including a logarithmic conversion element connected to a feedback circuit of the operational amplifier, a capacitor connected in parallel with the logarithmic conversion element, and the operational amplifier. In a logarithmic amplifier including a control circuit for controlling the oscillation prevention circuit so as to reduce the amount of feedback through the oscillation prevention circuit as the input current becomes smaller, in accordance with the input current of It is a bipolar input type operational amplifier provided with an input stage.

The third logarithmic amplifier according to the present invention is
A logarithm comprising an operational amplifier, a logarithmic conversion element connected to a feedback circuit of the operational amplifier, and an oscillation prevention circuit having a capacitor connected in parallel with the logarithmic conversion element as described in claim 4. In the amplifier, the operational amplifier has a composite amplifier configuration of an FET input type operational amplifier and a bipolar input type operational amplifier, and is different from a feedback circuit in which the logarithmic conversion element is connected to the bipolar input type operational amplifier. It is characterized by the provision of a minor feedback circuit.

The minor feedback circuit mentioned above refers to a feedback circuit related only to the bipolar input type operational amplifier.

Further, in the third logarithmic amplifier, more preferably, as described in claim 5, in accordance with the input current to the operational amplifier, the feedback amount via the oscillation prevention circuit is reduced as the input current becomes smaller. A control circuit is provided to control the oscillation prevention circuit so as to reduce it.

[0013]

In the first logarithmic amplifier of the present invention having the above structure, the high speed response is improved by using the bipolar input type operational amplifier excellent in high speed.
On the other hand, by using the FET input type operational amplifier with a small input bias current, it is possible to prevent a large error from occurring.

In the first logarithmic amplifier, if the input stage of the bipolar input type operational amplifier is provided with the FET, the input stage causes the influence of the input bias current and the current noise of the bipolar input type operational amplifier. The influence is suppressed to a smaller extent, and errors and noise are more reliably prevented.

Further, in the second logarithmic amplifier of the present invention having the above structure, the high speed response is enhanced by using the bipolar input type operational amplifier excellent in the high speed characteristic. On the other hand, this bipolar input type operational amplifier is Since the input stage is provided with the FET, the input stage suppresses the influence of the input bias current and the influence of the current noise of the bipolar input type operational amplifier, and prevents the generation of a large error and noise.

On the other hand, in the third logarithmic amplifier of the present invention, similarly to the first logarithmic amplifier, the high speed response is enhanced by using the bipolar input type operational amplifier excellent in high speed. F bias current is small
By using the ET input type operational amplifier, it is possible to prevent a large error from occurring.

And in this third logarithmic amplifier,
Since a minor feedback circuit different from the feedback circuit to which the logarithmic conversion element is connected is provided for the bipolar input type operational amplifier, in the composite amplifier including the bipolar input type operational amplifier and the FET input type operational amplifier, a high frequency The frequency range of the bipolar input type operational amplifier for use in the device is narrowed and low noise is realized. Hereinafter, this point will be described with reference to FIGS. 6 and 7.

The open loop frequency characteristic of the bipolar input type operational amplifier when the minor feedback circuit is not provided is shown by the solid line in FIG. At this time, the pole frequency is, for example, 1 kHz. On the other hand, when the minor feedback circuit as described above is provided, the frequency characteristic of the bipolar input type operational amplifier changes as shown by the broken line in FIG. 6, and the pole frequency changes to, for example, 1 MHz.

Therefore, the open loop frequency characteristic of the composite amplifier composed of the low frequency FET input type operational amplifier and the high frequency bipolar input type operational amplifier is as follows.
When the minor feedback circuit is not provided, Fig. 7 (a)
If it is as shown in FIG. 3, it will be changed to that shown in FIG. 7B because the minor feedback circuit is provided. That is, the frequency range of the bipolar input type operational amplifier for high frequency is, for example, 1 kHz or more in the former case, but is narrowed to 1 MHz or more in the latter case.

The noise characteristic of a logarithmic amplifier that handles a small amount of current is dominated by the current noise of an operational amplifier, and the current noise of a bipolar input type operational amplifier is generally larger than that of a FET input type operational amplifier. Therefore, as described above, by reducing the frequency range that the bipolar input type operational amplifier having a large current noise takes charge of, the noise reduction of the logarithmic amplifier can be achieved.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows a logarithmic amplifier which is a first embodiment of the present invention. As shown in the figure, the input terminal 10 of this logarithmic amplifier is an FET input type operational amplifier.
11 is connected to the minus input terminal 11a. On the other hand, the + input terminal 11b of the operational amplifier 11 is grounded. Two DC powers of + V volt and -V volt are supplied to the operational amplifier 11. The power supply lines 12 and 13 are grounded via noise prevention capacitors 14 and 15. Although not shown, the operational amplifier 11 is provided with an offset adjusting circuit.

The input terminal 10 is also connected to the negative input terminal 22a of the bipolar input type operational amplifier 22 via the capacitor 16. The + input terminal 22b of this operational amplifier 22
Is the output terminal 11c of the above FET input type operational amplifier 11.
It is connected to the. Also, the-input terminal 22a of the operational amplifier 22
Is connected to one end of a resistor 17, and the other end of the resistor 17 is grounded. This operational amplifier 22 has + V volt,-
Two DC powers of V volts are supplied. The power supply lines 24 and 25 are grounded via noise prevention capacitors 26 and 27. Although not shown, the operational amplifier
An offset adjustment circuit may be provided in 22 as well.

The output terminal 22c of the bipolar input type operational amplifier 22 is the output terminal of the first stage via the resistor 30.
It is connected with 31. Between the input terminal 10 and the output terminal 22c of the operational amplifier 22, an oscillation preventing capacitor 32 and a PNP transistor 33, which are connected in series, are connected. The emitter 33a and the collector 33b of the PNP transistor 33 are the output terminal 22c and the resistor of the operational amplifier 22, respectively.
It is connected to one end of 34. The other end of the resistor 34 is grounded. The base 33c of the PNP transistor 33 is connected to its collector 33b, and is further connected to one end of the capacitor 32.

On the other hand, the input terminal 10 and the output terminal 31 of the first stage
Between PNP transistor having exponential function characteristics
35 is connected. The emitter 35 of the transistor 35
The a and collector 35b are connected to the output terminal 31 and the input terminal 21, respectively, and the base 35c is grounded.

When the current signal Iin is input to the input terminal 10 of the logarithmic amplifier configured as above, the current signal Iin is input.
Output voltage (positive voltage) corresponding to the logarithmic value of in is the output terminal
Occurs on 31.

The circuit of the second stage formed between the output terminal 31 of the first stage and the output terminal 41 of this logarithmic amplifier is a circuit for temperature compensation of the logarithmic conversion circuit of the first stage. Is.
The emitter 43a of a PNP transistor 43 having the same characteristics as the feedback transistor 35 is connected to the output terminal 31 of the first stage. The collector 43b of the transistor is the + input terminal of the second stage operational amplifier 42 together with the base 43c.
It is connected to 42a. This transistor 43 plays a role of an input impedance of the second-stage operational amplifier 42, and a transistor having the same characteristic as that of the first-stage feedback transistor 35 is used as the transistor 43, so that the temperature which will be described later is reduced. Together with the compensating element 44, it contributes to the temperature compensation of the first-stage logarithmic conversion circuit. The + input terminal 42a is connected to a −V volt power source via a resistor 45.

As with the operational amplifier 22, the operational amplifier 42 is +
Two power sources of V volt and -V volt are connected,
The power supply lines 46 and 47 are grounded via capacitors 48 and 49, respectively. A resistor 50 is connected between the-input terminal 42b and the output terminal 42c of the operational amplifier 42. The-input terminal 42b is grounded via the temperature compensating element 44.

A signal (positive voltage) logarithmically converted by the first-stage circuit is added to the second-stage circuit configured as described above.
Is inputted, the fluctuation of the output value due to the temperature change is corrected, and an appropriately amplified signal is outputted from the output terminal 41.

The output voltage (positive voltage) generated at the output terminal 22c of the operational amplifier 22 is the second operational amplifier.
The signal is input to 42 as well as to the transistor 33. Here, the higher the voltage applied to the emitter 33a of the transistor 33, the lower the resistance value between the emitter and the collector, which increases the amount of feedback through the capacitor 32 and stabilizes the circuit operation. On the other hand, when the voltage applied to the emitter 33a of the transistor 33 is decreased, the resistance value between the emitter and the collector is increased.
The amount of feedback through the capacitor 32 is reduced and the frequency response characteristic of the circuit is improved.

In the logarithmic amplifier of the first embodiment, since the FET input type operational amplifier 11 having a small input bias current is used, it is possible to prevent the occurrence of a large error as in the conventional case and, on the other hand, it is excellent in high speed operation. By using the bipolar input type operational amplifier 22, the high speed response is also improved. For example, in the conventional logarithmic amplifier using only the FET input type operational amplifier, the input current Iin
However, it was confirmed that the frequency characteristic of this logarithmic amplifier according to the present invention was improved to about 1.5 MHz.

Next, a second embodiment of the present invention will be described with reference to FIG. In addition, in FIG.
Elements that are the same as the elements inside are given the same numbers, and duplicate descriptions thereof are omitted (the same applies hereinafter).

The logarithmic amplifier according to the second embodiment is different from the logarithmic amplifier according to the first embodiment in that an input stage by an FET 60 is added to the bipolar input type operational amplifier 22. is there. That is, the FET is connected to the negative input terminal 22a of the bipolar input type operational amplifier 22.
The source 60b of 60 is connected, and the gate 60c of this FET 60
Is connected to the input terminal 10 via the capacitor 16. The drain 60a of the FET 60 is connected to the V volt power source, and the drain 60b is connected to the -V volt power source through the resistor 61.

In the above structure, the input stage composed of the FET 60 suppresses the influence of the input bias current of the bipolar input type operational amplifier 22 and the influence of the current noise to a smaller extent, thereby further reliably preventing the generation of the error and the noise. .

Next, a third embodiment of the present invention will be described with reference to FIG. The logarithmic amplifier of the third embodiment does not include an FET input type operational amplifier, but instead has a FE for the bipolar input type operational amplifier 22.
The input stage by T60 and 62 is provided. That is, the negative input terminal 22 of the bipolar input type operational amplifier 22.
A FET 60 similar to that of the second embodiment is connected to a, and the gate 60c of this FET 60 is a direct input terminal.
Connected to 10.

Further, the bipolar input type operational amplifier 22
The source 62a of the FET 62 is connected to the + input terminal 22b. This source 62a is connected to a -V volt power supply through a resistor 63, while the drain 62b of FET 62 is + V.
Connected to the volt power supply. The gate 62c of the FET 62 is grounded.

In the above structure, the high speed response is enhanced by using the bipolar input type operational amplifier 22 which is excellent in high speed operation. On the other hand, the input stage of the bipolar input type operational amplifier 22 has the input stage composed of the FETs 60 and 62. Since the input stage is provided, the influence of the input bias current of the bipolar input type operational amplifier 22 and the influence of the current noise are suppressed to a small level, and large errors and noise are prevented from occurring.

Next, a fourth embodiment of the present invention will be described with reference to FIG. The logarithmic amplifier of the fourth embodiment is basically provided with a minor feedback circuit 70 for the bipolar input type operational amplifier 22 as compared with the logarithmic amplifier of the first embodiment described above. The points are different. This minor feedback circuit 70 has a feedback path 71
And voltage dividing resistors 72 and 73.

Due to the provision of the minor feedback circuit 70, the logarithmic amplifier has the bipolar input type operational amplifier 22 for high frequency, which has relatively large current noise, as described above with reference to FIGS. 6 and 7. The frequency range to be carried becomes narrow, and low noise is realized.

When the minor feedback circuit 70 is provided,
When the capacitance of the capacitor 16 is C1 and the resistances of the resistors 74, 75 and 17 are R1, R2 and R3 respectively, the time constant determined by C1 / R3 and the attenuation of the output of the FET input type operational amplifier 11 determined by R1 / R2 It is necessary to change the rate etc. appropriately according to the amplification rate by the minor feedback.

Next, a fifth embodiment of the present invention will be described with reference to FIG. Compared with the logarithmic amplifier of the fourth embodiment, the logarithmic amplifier of the fifth embodiment has
Basically, it is different in that a control circuit including a transistor 33 for controlling a feedback amount via the capacitor 32 is not provided.

Also in this case, as in the logarithmic amplifier according to the fourth embodiment, the minor feedback circuit 70 including the feedback path 71 and the voltage dividing resistors 72 and 73 is provided, so that a high frequency bipolar input type is provided. The frequency range of the operational amplifier 22 is narrowed, and low noise is realized.

[Brief description of drawings]

FIG. 1 is an electric circuit diagram showing a logarithmic amplifier according to a first embodiment of the present invention.

FIG. 2 is an electric circuit diagram showing a logarithmic amplifier according to a second embodiment of the present invention.

FIG. 3 is an electric circuit diagram showing a logarithmic amplifier according to a third embodiment of the present invention.

FIG. 4 is an electric circuit diagram showing a logarithmic amplifier according to a fourth embodiment of the present invention.

FIG. 5 is an electric circuit diagram showing a logarithmic amplifier according to a fifth embodiment of the present invention.

FIG. 6 is an explanatory diagram illustrating changes in frequency characteristics of a bipolar input type operational amplifier.

FIG. 7 is an explanatory diagram for explaining changes in frequency characteristics of a composite amplifier including a bipolar input type operational amplifier and an FET input type operational amplifier.

[Explanation of symbols]

11 FET input type operational amplifier 16 Capacitor 22 Bipolar input type operational amplifier 32 Oscillation prevention capacitor 33 Feedback amount control transistor 60, 62 Bipolar input type operational amplifier input stage FET 70 Minor feedback circuit 71 Feedback path 72, 73 Voltage dividing resistor

Claims (5)

[Claims] 1. An oscillation prevention circuit comprising an operational amplifier, a logarithmic conversion element connected to a feedback circuit of the operational amplifier, a capacitor connected in parallel with the logarithmic conversion element, and an input current to the operational amplifier. According to the above, in a logarithmic amplifier including a control circuit that controls the oscillation prevention circuit so as to reduce the amount of feedback through the oscillation prevention circuit as the input current decreases, the operational amplifier is a FET input type operational amplifier, A logarithmic amplifier characterized by having a composite amplifier configuration with a bipolar input type operational amplifier. 2. The logarithmic amplifier according to claim 1, wherein the input stage of the bipolar input type operational amplifier is provided with an input stage of an FET. 3. An oscillation prevention circuit comprising an operational amplifier, a logarithmic conversion element connected to a feedback circuit of the operational amplifier, a capacitor connected in parallel with the logarithmic conversion element, and an input current to the operational amplifier. According to the above, in a logarithmic amplifier including a control circuit that controls the oscillation prevention circuit so as to reduce the amount of feedback that passes through the oscillation prevention circuit as the input current decreases, A logarithmic amplifier, which is a provided bipolar input type operational amplifier. 4. A logarithmic amplifier including an operational amplifier, a logarithmic conversion element connected to a feedback circuit of the operational amplifier, and an oscillation prevention circuit including a capacitor connected in parallel with the logarithmic conversion element, The operational amplifier has a composite amplifier configuration of an FET input type operational amplifier and a bipolar input type operational amplifier, and a minor feedback circuit different from the feedback circuit to which the logarithmic conversion element is connected is provided for the bipolar input type operational amplifier. A logarithmic amplifier characterized in that 5. Depending on the input current to the operational amplifier,
5. The logarithmic amplifier according to claim 4, further comprising a control circuit for controlling the oscillation prevention circuit so as to reduce the amount of feedback through the oscillation prevention circuit as the input current becomes smaller.