JPH05343921A - Linear detection circuit - Google Patents

Abstract

PURPOSE:To allow excellent linearity for the detection circuit by correcting the linearity of a diode at an input side of an operational amplifier so as to make an AC voltage at an input terminal and a DC voltage at an output terminal equal to each other. CONSTITUTION:Since an operational amplifier 4 is operated to make a voltage between a noninverting input terminal (b) and an inverting input terminal (c) zero, a voltage across a resistor 3 is equal to a voltage across a resistor 7 to be fed back. Components 1, 5 are resistors whose resistance is equal to each other and components 2, 6 are diodes whose characteristic is equal to each other. Thus, the voltage between the terminals b, c is equal, resulting that the potential between an input terminal (a) and an output terminal (d) is equal. Then, the linearity of the diode is corrected by the operational amplifier 4 and the entire linearity is improved. Furthermore, since an AC input is rectified till the input reaches the operational amplifier 4, the circuit is used up to a UHF band when a diode for a UHF band is employed. Furthermore, the resistors 1, 5 are used for correcting the dispersion in the characteristic of the diodes 2, 6 and then the linearity of 40dB or over is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a detection circuit in the medium wave radio frequency band, and more particularly to a linear detection circuit which requires linearity from the voice frequency to the UHF band.

[0002]

2. Description of the Related Art Conventionally, there is a detector circuit using a diode and an operational amplifier called an ideal detector circuit (Hideo Tsunoda: Practical operational amplifier circuit, Tokyo Denki University Press, 1
July 983), an example of the circuit is shown in FIG. 3 (a). This circuit is an ideal diode for half-wave rectification, which is also called an inverting ideal diode.

Assuming that the input voltage v _i is a positive half cycle in the figure, the output voltage is negative because the operational amplifier A (351) is an inverting amplifier circuit. Therefore,
Since a forward voltage is applied to D _{1 and} it is turned on, a negative feedback action acts on the inverting terminal and the output voltage becomes a constant voltage of about -0.7 V (forward voltage drop). Due to this voltage, the diode D ₂ is turned off, so v ₀ is completely 0.

Next, when v _i changes to a negative half cycle, the output of A becomes positive. What is important here is that since D ₂ does not turn on until the output voltage of A reaches the forward voltage drop of D ₂ (about + 0.7V), negative feedback is not applied to operational amplifier A, The gain during this period A becomes an extremely large gain A ₀ (10 ¹ or more) of the operational amplifier itself. Therefore, considering the voltage (+ 0.7V) at which D ₂ is turned on as the input voltage,
This means that an extremely small input voltage of 0.7V / A ₀ is enough. Therefore, if the input voltage is even higher than 0.7V / A ₀ , D ₂ will be in the ON state and negative feedback will be applied, resulting in an inverting amplifier circuit with a closed loop gain of R ₂ / R ₁ as shown in Fig. 3 (b). It becomes half-wave rectification of the ideal diode as shown.

If a detection circuit is constructed using such an ideal diode, it is preferable that the operational amplifier has a low frequency due to the frequency characteristic of the operational amplifier. It cannot be used in the medium wave frequency band. Further, in order to assemble the circuit into a full-wave rectification type as an ideal detection circuit, two operational amplifiers are required and the circuit configuration becomes complicated.

[0006]

Conventionally, attempts have been made to use an ideal diode using an operational amplifier and a diode in a detection circuit in the medium wave radio frequency band.
This is to connect a diode to the feedback loop of the amplifier and try to correct the linearity of the diode. However, the problem at this time is the operational amplifier frequency characteristic as described above, and it is premised that the operational amplifier has a sufficient amplification degree even at a high frequency and that the phase characteristic is good.

In the detection circuit which can be used for high frequency, there is a circuit in which a diode, an operational amplifier, a transistor circuit and the like are combined. However, these circuits have a complicated circuit structure and cannot be obtained unless precise adjustment is made. There are problems such as.

FIG. 4 shows the above-described circuit example, and Table 1 shows the names of the parts used and the numbers of the parts corresponding to the part numbers of the constituent elements (Tamotsu Inaba: Selected analog practical circuit collection, CQ publisher, 198).
January 1997, page 197), this circuit is a transistor (T
_r1 −T _r3 ) is a wide band amplifier, and rectifying diodes (D ₁ , D ₂ ) are put in a negative feedback loop to form a level detection circuit having a band of about 10 MHz and good linearity. However, the circuit configuration is complicated.

[0009]

[Table 1]

Therefore, an object of the present invention is to solve these drawbacks and to provide a linear detection circuit which has a simple circuit configuration, requires no adjustment, is inexpensive, and can obtain a wide range of linearity from low frequency to high frequency. To do.

[0011]

To achieve this object, the linear detection circuit of the present invention inputs a DC voltage obtained by rectifying an AC input by a diode to one or both of a positive input terminal and a negative input terminal of an operational amplifier. Then, the output from the output terminal of the operational amplifier is fed back to the negative input terminal of the operational amplifier via the diode of the same kind as the diode to correct the linearity of the diode, and the detection output is taken out from the output terminal of the operational amplifier. It is characterized by being configured as described above.

[0012]

That is, according to the circuit of the present invention, in the detection circuit in which the diode and the operational amplifier are combined, the circuit operates so as to correct the linearity of the diode inserted on the input side of the operational amplifier, so that the input terminal of the circuit is The applied AC voltage value becomes equal to the DC voltage value output to the output end of the circuit, and this detection circuit shows good linearity.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the accompanying drawings. FIG. 1 (a) illustrates a half-wave rectification circuit which is the basis of the circuit of the present invention. In FIG. 1 (a), a resistor 1, a diode 2 and a resistor 3 form a half-wave rectifier circuit. The voltage across the resistor 3 is applied to the positive input terminal b of the operational amplifier 4. The resistor 5, the diode 6, and the resistor 7 are elements 1 to 3
A signal is fed back from the output terminal of the operational amplifier 4 by the same half-wave rectifier circuit as above, and the voltage across the resistor 7 is applied to the negative input terminal c of the operational amplifier 4.

Next, the operation of this circuit will be described. Since the operational amplifier has a property of operating so that the potential difference between the positive terminal b and the negative terminal c becomes zero, the resistor 3 which is a rectified output is used.
Becomes equal to the voltage across the resistor 7 fed back. Since the elements 1 and 2 and the elements 5 and 6 use the same type of resistors and diodes, the fact that the potentials of the terminals b and c are equal means that the potentials of the input terminal a of the rectifier circuit and the output terminal d of the operational amplifier are equal. Are also equal,
This means that the AC voltage value at the input end a and the DC voltage value at the output end d are equal. This means that the linearity of the diode is corrected by the operational amplifier, which means that this circuit becomes a detection circuit with good linearity.

The resistors 1 and 5 are for correcting variations in the characteristics of the diodes 2 and 6, and by connecting the resistors 1 and 5, linearity of 40 dB or more can be obtained. Further, since this circuit is rectified before being input to the operational amplifier 4, it is possible to provide a linear detection circuit which can be used up to the UHF band by using a diode for the UHF band.

FIG. 1B shows a full-wave rectification linear detection circuit, which is obtained by adding a diode 8 to the configuration of FIG. 1A and connecting it to the negative input terminal c of the operational amplifier 4.

The circuit shown in FIG. 2 is an application of the full-wave rectification linear detection circuit shown in FIG. 1 (b) to an ammeter with a digital display, in which a toroidal core 9 is wound from a feeder supplying a high frequency. The high frequency current is picked up by the CT (current transformer) and input to the linear detection circuit. Then, a DC voltage proportional to the average value of the high frequency current appears at the output of the operational amplifier 4. When this DC voltage is passed through the square root circuit 10, it is converted into an effective value. Thus, the RMS current and the average current can be displayed on the digital multimeter (DMM) 11. In FIG. 2, the switch 12 switches and displays the average value and the effective value of the input high frequency signal.

The linear detection circuit of the present invention has been described above.
Although the third embodiment has been described, the present invention is not limited to this, and it will be apparent that various modifications and changes can be made within the scope of the invention.

[0019]

According to the linear detection circuit of the present invention described above, for example, the following new functions utilizing this circuit are possible. That is, firstly, as in the embodiment shown in FIG. 2, the measured value of the high frequency current can be displayed digitally, and the linearity of the detection circuit is ensured. Therefore, in the embodiment, the range of 0.1 to 10 amperes is 100 times. Can be measured with an accuracy of 2%. Conventionally, the high-frequency ammeter was all analog display because the linearity of the detector could not be obtained. Second, half-wave rectification can be changed to full-wave rectification by adding one diode as shown in FIG. Conventionally, in order to perform full-wave rectification, a balanced circuit such as a transformer was required for the AC input, but in the present invention, full-wave rectification can be performed with an unbalanced circuit. Further, in the case of performing full-wave rectification in the ideal detection circuit, it is necessary to add a single operational amplifier and also to form a complicated circuit configuration. Third, since the present invention has already been rectified by the time of input to the operational amplifier,
If a diode for the UHF band is used, the linear detection circuit can be used up to the UHF band.

[Brief description of drawings]

FIG. 1 is a diagram for explaining the principle of the present invention, in which (a) is a half-wave rectifier circuit and (b) is a full-wave rectifier circuit.

FIG. 2 is a block diagram of a digital high-frequency ammeter using the circuit of the present invention.

FIG. 3 shows a configuration (a) of the conventional inverting ideal diode and its input / output waveform diagram (b).

FIG. 4 is a block diagram of a conventional detection circuit that can be used for high frequencies.

[Explanation of symbols]

 1, 3, 5, 7 Resistance 2, 6, 8 Diode 9 Toroidal core 10 Square root circuit 11 Digital multimeter 12 Switching device 13 Multiplying IC circuit

Claims (2)

[Claims] 1. An operational amplifier, wherein a DC voltage obtained by rectifying an AC input with diodes (2, 8) is input to either or both of a positive input terminal (b) and a negative input terminal (c) of an operational amplifier (4), The output from the output terminal of is fed back to the negative input terminal of the operational amplifier through the diode (6) of the same type as the diode to correct the linearity of the diode, and the detection output is taken out from the output terminal of the operational amplifier. A linear detection circuit characterized by being configured. 2. The DC detection circuit according to claim 1, wherein the rectified DC voltage is obtained from both ends of one resistor, a diode, and the other resistor in series connection of the other resistor.