JP2003281464A - Logarithmic transformation circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a logarithmic transformation circuit which expands a dynamic range without receiving the influence of the bias current of an operational amplifier. <P>SOLUTION: The logarithmic transformation circuit comprises a p-n junction type transistor Tr and the operational amplifier OP, has an input current I and the forward current Ie of the transistor Tr inputted to the input terminal of the operational amplifier OP and has the forward voltage of the transistor Tr determined by the output voltage of the operational amplifier OP. Compensation current injection circuits (OP2, R1, R2) for injecting a compensation current Ib' of nearly the same amplitude with a bias current Ib and of an inverse polarity to the input terminal independently from the operational amplifier OP to the input terminal of the operational amplifier OP. <P>COPYRIGHT: (C)2004,JPO

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 used in various control devices and measurement / monitoring devices for arithmetic processing of addition, subtraction, multiplication and division.

[0002]

2. Description of the Related Art Multipliers, dividers and the like use a configuration in which their input signals are logarithmically converted and operated. The dynamic range of the logarithmic conversion circuit is very important because the number of significant digits of the multiplier and the divider is the deciding factor of how many digits the logarithmic conversion circuit can accurately logarithmically convert.

Conventionally, this type of logarithmic conversion circuit has been based on the circuit configuration shown in FIG. That is, the logarithmic conversion circuit utilizes that the forward current (emitter current in the illustrated example) and forward voltage (base-emitter voltage in the illustrated example) of the PN junction transistor Tr have a logarithmic relationship, The current input from the input terminal Iin is supplied to the operational amplifier OP.
Is input to the inverting input terminal (−) of the operational amplifier OP through the resistor R, the non-inverting terminal (+) of the operational amplifier OP is grounded, and the emitter of the transistor Tr for logarithmic conversion is connected to the inverting input terminal (−). At the same time, the base of the transistor Tr is connected between the output of the operational amplifier OP and the output terminal Vout, a positive power source for outputting a forward current is applied to the collector of the transistor Tr, and the emitter current of the transistor Tr is It is configured to flow into the inverting input terminal (-) of the operational amplifier OP.

[0004]

However, in the conventional logarithmic conversion circuit as shown in FIG. 4, the bias current flowing in the operational amplifier OP cannot be ignored in a small signal region where the output level thereof is relatively small, and the The logarithmic relationship of the output is shifted. Therefore, there is a problem that the dynamic range is limited (narrowed). This will be described in more detail.

In FIG. 4, when the current input from the input terminal Iin (hereinafter referred to as "input current") is I, the emitter current of the transistor Tr is Ie, and the bias current flowing in the operational amplifier OP is Ib, this bias current Ib is obtained. Is expressed by equation (1). Ib = I + Ie (1) When the output voltage of the operational amplifier OP, that is, the base voltage of the transistor Tr that is branched and output before being output to the output terminal Vout is Vo, the emitter current I of the transistor Tr is I.
e is expressed by equation (2). Ie = k · exp (Vo) (k is a constant) (2) Here, if the logarithm of both sides is taken, the equation (2) is transformed as follows. Vo = log (Ie) + k '(k' is a constant) (2 ') When the emitter current Ie is eliminated from the equations (1) and (2'), the equation (2 ') becomes )become that way. Vo = log (-I + Ib) + k '... (3)

A graphical representation of the relationship of equation (3) is shown in FIG.
It becomes a curve like A of. That is, in the region of (-I) >> Ib, the characteristic is asymptotic to Vo = log (-I) (a straight line having a slope of 45 degrees), and in the region of (-I) << Ib, Vo
= Log (-Ib) asymptotically (becomes horizontal). That is, in the region of (-I) <Ib, the error of logarithmic conversion becomes large and it cannot be put to practical use.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above problems and to provide a logarithmic conversion circuit which is not affected by the bias current of an operational amplifier.

[0008]

In view of the difficulty in designing a circuit so that the bias current of an operational amplifier becomes zero, the present invention has the problem of injecting a compensation current for canceling the bias current. Solve the problem.

Specifically, a PN junction transistor,
An operational amplifier through which a bias current flows during operation, an input current and a forward current of the transistor are input to an input terminal of the operational amplifier, and a forward voltage of the transistor is determined by an output voltage of the operational amplifier. In the logarithmic conversion circuit described above, a compensation current injection circuit for injecting a compensation current having substantially the same amplitude as the bias current but a reverse polarity to the input terminal independently of the operational amplifier is provided at the input terminal of the operational amplifier. Characterize. The compensation current injection circuit includes, for example, a compensation operational amplifier whose electrical characteristics are the same as or similar to those of the operational amplifier, and a first resistor and a second resistor having the same resistance value. The output terminal of the operational amplifier is feedback-connected to the input terminal of the compensating operational amplifier via the first resistor, and
It is configured to be connected to the input terminal of the operational amplifier via the second resistor. In such a logarithmic converter, the bias currents flowing in the operational amplifier are canceled even in a small signal region, so that there is no shift in the logarithmic relationship between the input and output, and the dynamic range can be expanded.

Considering that the bias current of the operational amplifier may be smaller than the bias current of the compensating operational amplifier and the input current may be relatively small, the input amplifier and the output terminal of the operational amplifier may be connected to each other. In addition, it is desirable that a unidirectional element that conducts electricity in the opposite direction to the transistor be inserted and connected in parallel with the transistor. The unidirectional element may be a diode or a PN junction compensating transistor that conducts electricity in the opposite direction to the transistor. When a compensation transistor is used, the transistor and the compensation transistor are complementary PN junction transistors, a positive power source for outputting a forward current is connected to the transistor, and the compensation transistor is connected to the compensation transistor. By connecting a negative power supply for outputting a reverse current, a logarithmic conversion circuit that operates for both positive and negative polarities of the input current can be obtained.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS <First Embodiment> FIG. 1 is a block diagram of a logarithmic conversion circuit according to a first embodiment of the present invention. This logarithmic conversion circuit has a basic configuration of the conventional logarithmic conversion circuit shown in FIG. 4, and at the inverting input terminal (-) of the operational amplifier OP, a compensation current Ib having substantially the same amplitude as the bias current Ib and a reverse polarity is obtained. Is provided independently of the operational amplifier OP.

The compensation current injection circuit includes a compensation operational amplifier O.
It is comprised including P2, the 1st resistor R1, and the 2nd resistor R2. A power supply is connected to the compensating operational amplifier OP2, but it is omitted here. The output terminal of the compensating operational amplifier OP2 is feedback-connected to the inverting input terminal (−) of the compensating operational amplifier OP2 via the first resistor R1 and also connected to the inverting input terminal (−) of the compensating operational amplifier OP2 via the second resistor R2. It is also connected to the inverting input terminal (-).

The bias current is the operational amplifier for compensation OP2.
Also flows. If this bias current is Ib2 and the output voltage of the compensation operational amplifier OP2 is V2, the first and second
Compensation current I when resistors R1 and R2 have the same resistance value
b'is represented by Formula (4).

If an IC having the same electrical characteristics (including temperature characteristics) in the same case as the operational amplifier OP is used as the compensating operational amplifier OP2, Ib 'becomes almost equal to Ib. At this time, equations (1) and (3) are as follows. If the relationship of equation (6) is expressed in a graph, (Ib-Ib ') ≈
Since 0 and (Ib-Ib ') << Ib, the result is as shown in B of FIG. That is, in the region of (-I) >> Ib-Ib ', it is asymptotic to Vo = log (-I) and becomes a straight line with a slope of 45 degrees. Further, in the region of (-I) << Ib-Ib ', it gradually approaches (becomes horizontal) Vo = log (Ib-Ib'). The critical value at which the curve starts to slope is the value on the horizontal axis shown. As described above, according to the logarithmic conversion circuit of this embodiment, it is understood that the dynamic range is widened as compared with the conventional logarithmic conversion circuit with a simple configuration.

<Second Embodiment> In the logarithmic conversion circuit of the first embodiment, when Ib '> Ib and the input current I is small, the output voltage Vo has a negative potential. Since the logarithmic conversion circuit is basically based on the inversion operation of the negative potential input and the positive potential output, when the output voltage Vo becomes the negative potential, the transistor Tr
Does not operate, negative feedback is not applied to the operational amplifier OP. Then, the gain of the operational amplifier OP becomes infinite, and as a result, the output voltage Vo changes to the negative power supply voltage, which may result in a large error output. Therefore, as shown in FIG. 2, a compensation diode D is inserted and connected in parallel with the transistor Tr, and the output voltage is limited by the forward characteristic of the compensation diode D to avoid this error. .

<Third Embodiment> A transistor may be used instead of the compensating diode D. An example of the circuit configuration in this case is shown in FIG. In the example of FIG. 3, a negative power supply is connected to the collector of the PN junction compensation transistor.
With this configuration, negative feedback is applied even when the positive potential is input and the negative potential is output. If phase-correcting (complementary) transistors are selected for these two transistors Tr and Tr2, a logarithmic conversion circuit that operates for both positive and negative polarities of the input current I can be obtained.

As described above, according to this embodiment, the dynamic range of the logarithmic conversion circuit can be expanded with a simple and inexpensive circuit structure, and the accuracy of multiplication and division operations can be improved.

It should be noted that the above-mentioned embodiments show suitable examples for carrying out the present invention, and do not limit the scope of the present invention. The present invention has a PN junction transistor Tr and an operational amplifier OP through which a bias current flows during operation. The input current and the forward current of the transistor Tr are input to the input terminal of the operational amplifier OP, and the transistor Tr The present invention can be applied to all logarithmic conversion circuits in which the forward voltage is determined by the output voltage of the operational amplifier OP. In the above embodiment, the forward current was described as an emitter current and the forward voltage was a base-emitter voltage, but the forward current may be a collector current. In this case, the collector of the transistor is connected to the non-inverting input terminal of the operational amplifier OP, the output terminal of the operational amplifier OP is connected to the emitter of the transistor, and the base of the transistor may be grounded.

[0019]

As is apparent from the above description, according to the present invention, it is possible to provide a logarithmic conversion circuit which is not affected by the bias current of the operational amplifier.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram of a logarithmic operation circuit according to a first embodiment of the present invention.

FIG. 2 is a circuit configuration diagram of a logarithmic operation circuit according to a second embodiment of the present invention.

FIG. 3 is a circuit configuration diagram of a logarithmic operation circuit according to a third embodiment of the present invention.

FIG. 4 is a circuit configuration diagram showing a conventional logarithmic operation circuit.

FIG. 5 is a graph showing the width of the dynamic range.

[Explanation of symbols]

OP operational amplifier OP2 Compensation operational amplifier I input current Ib bias current Ib 'compensation current Ie Emitter current R, R1, R2 resistors Output voltage of V2 compensation operational amplifier Vo output voltage Iin input terminal Vout output end

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 5J030 CB02 CC03 CC05 CC06                 5J090 AA01 AA47 CA32 CA81 FA10                       HA02 HA08 HA19 HA25 HN04                       KA01 KA12 MA23 MN01 NN11                       TA02                 5J500 AA01 AA47 AC32 AC81 AF10                       AH02 AH08 AH19 AH25 AK01                       AK12 AM23 AT02 NH04 NM01                       NN11

Claims (5)

[Claims] 1. A PN-junction transistor and an operational amplifier through which a bias current flows during operation. An input current and a forward current of the transistor are input to an input terminal of the operational amplifier, and a forward current of the transistor is provided. In the logarithmic conversion circuit in which the directional voltage is determined by the output voltage of the operational amplifier, a compensation current having substantially the same amplitude as the bias current but a reverse polarity is supplied to the input end of the operational amplifier independently of the operational amplifier. A logarithmic conversion circuit, characterized in that a compensation current injection circuit for injection is provided. 2. The compensating current injection circuit has a compensating operational amplifier whose electrical characteristics are the same as or similar to those of the operational amplifier, a first resistor and a second resistor having the same resistance value.
A resistor, the output terminal of the compensation operational amplifier is feedback-connected to the input terminal of the compensation operational amplifier via the first resistor, and the operational amplifier is also coupled via the second resistor. The logarithmic conversion circuit according to claim 1, wherein the logarithmic conversion circuit is configured to be connected to the input terminal of the. 3. A unidirectional element that conducts electricity in the opposite direction to the transistor is inserted and connected in parallel with the transistor between the input terminal and the output terminal of the operational amplifier. A logarithmic conversion circuit according to item 1 or 2. 4. The logarithmic conversion circuit according to claim 3, wherein the unidirectional element is a PN junction compensating transistor that conducts electricity in a direction opposite to that of the transistor. 5. The transistor and the compensation transistor are complementary PN junction transistors, a positive power supply for outputting a forward current is connected to the transistor, and the compensation transistor has a reverse power supply. The logarithmic conversion circuit according to claim 4, wherein a negative power source for outputting a directional current is connected.