JPH06223211A - Logarithmic amplifier - Google Patents

Abstract

PURPOSE:To attain high speed operation and to improve temperature characteristics in a logarithmic amplifier for outputting the logarithmic transformation value of an input signal. CONSTITUTION:The logarithmic amplifier is provided with a resistor ladder 1 for dividing a reference voltage VREF, plural comprators 2-1 to 2-4 for comparing plural voltage-divided points obtained by the ladder 1 with an input voltage VIN, plural switch circuits 3-1 to 3-4 controlled by the outputs of these comparators 2-1 to 2-4, and plural constant current sources I0 connected to the circuits 3-1 to 3-4 and constituted so that the sum of currents obtained by the switching of the circuits 3-1 to 3-4 is outputted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a logarithmic amplifier, and more particularly to a logarithmic amplifier capable of high-speed operation and having a temperature coefficient which is substantially negligible.

[0002]

2. Description of the Related Art Conventionally, a logarithmic amplifier having a structure as shown in FIG. 6 is known. This logarithmic amplifier
A first operational amplifier 102 having an inverting input terminal connected to an input terminal 101 via an input resistor _Ra , an output terminal connected via a diode, and a non-inverting input terminal connected to GND;
A second operational amplifier 103 having a non-inverting input terminal connected to the output terminal of the first operational amplifier 102, an inverting input terminal connected to the current source I _REF, and a diode connected to the output terminal.
And the non-inverting input terminal is connected to the output terminal of the second operational amplifier 103, and the inverting input terminal is connected in series to output resistors R _b and R.
connected to a node _c, the output resistance R _b the output end, and a third operational amplifier 104 which is connected to the R _c output terminal 105.

In the logarithmic amplifier constructed as above, when the input voltage V _IN is applied to the input terminal 101 and the input current I _IN (I _IN = V _IN / R _a ) is input, the following equation (1)
The logarithmically converted output voltage V _OUT represented by
Obtained from 5. V _OUT = (1 + R _c / R _b ) · kT / q · ln (I _REF / I _IN ) (1)

Further, as a logarithmic amplifier capable of high-speed operation, as shown in FIG. 7, a unit circuit 113 composed of a full-wave detector 111 and an amplifier / limiter 112 is connected in multiple stages to output the output of the full-wave detector 111. It is known that by summing, an approximate logarithmic conversion output I _OUT is obtained with respect to the input voltage V _IN . A unit circuit including the full-wave detector 111 and the amplifier / limiter 112 in FIG.
A specific circuit configuration of 113 is shown in FIG.

[0005]

By the way, the conventional logarithmic amplifier shown in FIG. 6 has a feature that a logarithmic conversion output can be obtained with high accuracy with respect to the input current I _IN , but it cannot operate at high speed. However, there is a drawback that the temperature coefficient is large. The temperature characteristic is proportional to the absolute temperature T, as is clear from the equation (1). In order to cancel this, it suffices to provide the output resistance R _b or R _c with an appropriate temperature coefficient, but it is generally not desirable to form a resistor having such an appropriate temperature coefficient monolithically. It is possible.

The logarithmic amplifier shown in FIGS. 7 and 8 can operate at high speed, but the accuracy is not high because it is an approximate output of logarithmic conversion, the input voltage range is small, and it is difficult to correct the temperature characteristic. There is a drawback that In this logarithmic amplifier, the input voltage range and temperature characteristics can be improved by installing an attenuator with an appropriate temperature coefficient in front of the input terminal, but an attenuator with such characteristics is realized monolithically. Requires a special manufacturing process.

As described above, the conventional logarithmic amplifier has a problem that it is difficult to operate at high speed, or even if it can operate at high speed, it is difficult to compensate the temperature characteristic.

The present invention has been made to solve the above problems in the conventional logarithmic amplifier, and an object of the present invention is to provide a logarithmic amplifier which is fast and has a small variation in temperature characteristic.

[0009]

In order to solve the above problems, the present invention provides a reference voltage source and a reference voltage source connected to the reference voltage source in a logarithmic amplifier which outputs a logarithmic conversion value of an input signal. A resistor ladder for dividing the voltage, N comparator circuits for comparing the input signal voltage with the potentials of the N voltage dividing points obtained by the resistor ladder, and N comparators controlled by the outputs of the N comparator circuits. Switch circuit and N constant current sources connected to the N switch circuits, the constant current source circuit including the N constant current sources and the switch circuit is switched by the output of the comparison circuit. The output current is the sum of the electric currents obtained by the above.

In the logarithmic amplifier configured as described above, the reference voltage is divided by the resistance ladder to extract the potential of the voltage dividing point so as to have an exponential relationship, and the potential of the voltage dividing point and the input voltage are compared by the comparison circuit. By comparing and switching the constant current source by the output of the comparison circuit to obtain the total output of the constant current source, it is possible to obtain the output current approximate to the logarithmic conversion of the input voltage.

The comparison circuit and the switch circuit of the constant current source can operate at high speed. Since the comparison circuit and the switch circuit are operated in parallel, the entire logarithmic amplifier can be operated at high speed. In addition, good temperature characteristics can be obtained by making the temperature coefficients of the reference voltage source and the constant current source small enough to be ignored.

[0012]

EXAMPLES Next, examples will be described. FIG. 1 is a circuit configuration diagram showing a basic embodiment of a logarithmic amplifier according to the present invention. In the figure, 1 is a resistor ladder, and 4 resistors R
1 and one resistor R3 connected in series, and four resistors R2 connected between each connection point A, B, C, D and GND of the series connected circuit. The reference voltage V _REF is applied to one end, and the other end is connected to GND.

Reference numerals 2-1, 2-2, 2-3, and 2-4 are comparators, and the potentials V _A and V of the connection points A, B, C, and D of the resistor ladder 1 are provided at one input terminals of each of them. _B , V _C , and V _D are applied, and the input voltage V _IN is applied to the other input end. 3-1, 3-2, 3-3, 3-4 are constant current sources I
₀ switch circuit, comparators 2-1 and 2-
It is controlled by the output of 2, 2-3, 2-4,
The output terminals of the switch circuit are connected in common and output terminal 4
It is connected to the.

In the logarithmic amplifier constructed as described above, the potential V of the voltage dividing points A, B, C and D having an exponential relationship obtained by dividing the reference voltage V _REF by the resistance ladder 1.
_A , V _B , V _C , and V _D and the input voltage V _IN are compared by comparators 2-1, 2-2, 2-3, and 2-4, respectively, and each comparator 2-1, ... Switch circuit 3 by the output of 4
-1, 3-2, 3-3, 3-4 are controlled. By obtaining the total output of the constant current source I ₀ by switching these switch circuits 3-1, ... 3-4, the output current I _OUT approximated to the logarithmic conversion of the input voltage V _IN can be obtained. The input / output characteristics of this logarithmic amplifier are shown in FIG.

In the above-mentioned basic embodiment, for the sake of simplicity of explanation, the voltage dividing points are shown as four points A to D, but the logarithmic conversion error can be reduced by increasing this. You can

Next, a specific embodiment of the logarithmic amplifier according to the present invention will be described with reference to FIG. This embodiment is shown in FIG.
The comparator and the switch circuit in the basic embodiment shown in (4) are configured by an emitter coupling circuit in which the emitters of two transistors are connected. That is, the potentials V _{A to} V _{D of the} connection points (voltage dividing points) A to D of the resistance ladder 1
Are applied to the bases of the respective transistors Q1 and Q
3, Q5 and Q7 and transistors Q2, Q4, Q6 and Q8 to which the input voltage V _IN is applied to the bases are respectively provided, and the emitters of the transistors Q1 and Q2 are commonly connected through resistors to form a constant current source I ₀ . Connect, likewise transistor Q
The emitters of Q3, Q4, the emitters of transistors Q5, Q6, and the emitters of transistors Q7, Q8 are commonly connected via resistors and connected to a constant current source I ₀ . The collectors of the transistors Q1, Q3, Q5, Q7 are connected to the power source V _CC , and the transistors Q2, Q4,
The collectors of Q6 and Q8 are commonly connected and connected to the current-voltage conversion circuit 11 via the output terminal 4.

In the logarithmic amplifier configured as described above,
The voltage dividing point potentials V _A , V _B , V _C , and V _D obtained by dividing the reference voltage V _REF with the resistor ladder 1 are respectively expressed by the following equation (2).
It is represented by (5). V _D = V _REF / β (2) V _C = V _REF / β ² (3) V _B = V _REF / β ³ (4) V _A = V _REF / β ⁴ (5) where β = (α + 1) / α, and α is a constant that determines the ratio of resistance values, and the resistance values R ₁ , R ₂ , R 3 of the resistors R ₁ , R ₂ , R ₃ , R
₃ and the following equations (6) and (7). R ₃ = (α + 1) R ₁ (6) R ₂ = α (α + 1) R ₁ (7)

Next, the operation of this embodiment will be described. The voltage dividing point potential V _A shown by the above equations (2) to (5)
V _D is compared with the input voltage V _IN to switch the constant current source I ₀ . The sum of the currents of the switched constant current sources I ₀ is output as the output current I _OUT , and the current-voltage conversion circuit
Converted to a voltage signal by 11. Where input voltage V
_IN is, if a _{V B <V IN <V C} , the transistor Q
2, Q4, Q7 are completely ON, and the transistors Q1, Q3, Q8 are completely OFF. The ratio of the collector currents of the transistors Q5 and Q6 is the voltage dividing point potential V _C.
And the value of the input voltage V _IN . Output current I
_OUT is the sum of the collector currents of the transistors Q2, Q4, Q6, and Q8, so that when the collector current of the transistor Q6 is I _C6 , it is expressed by the following equation (8). I _OUT = 2I ₀ + I _C6 (8)

By setting the emitter resistances of the transistors Q1 to Q8 to appropriate values, the input / output characteristics become as shown by the solid line a in FIG. As can be seen from this figure, the output current I _OUT changes stepwise with respect to the input voltage V _IN , and is closer to a straight line than the input / output characteristics (FIG. 2) of the basic embodiment shown in FIG. As a result, the logarithmic conversion error is reduced. In FIG. 4, the broken line b indicates the ideal input / output characteristic of the logarithmic amplifier.

In this embodiment, the temperature variation of the logarithmic conversion characteristic can be reduced by making the temperature characteristics of the reference voltage V _REF and the constant current source I ₀ small enough to be ignored. FIG. 5 shows a configuration example of a generating circuit for the reference voltage V _REF and the constant current source I ₀ . This configuration example includes a current source circuit 21, a current source temperature compensation circuit 22, and a constant current output circuit.
23, and a reference voltage generation circuit using the bandgap voltage
It consists of 24 and.

Further, in this embodiment, high speed operation is possible because the transistors are not saturated and constant current switching is performed in parallel.

[0022] Furthermore, in this embodiment, by adding a current of I _0/2 as an offset to the output current I _OUT, as shown by the solid line c in FIG. 4, the logarithmic conversion characteristic close to the ideal characteristics be able to.

[0023]

As described above on the basis of the embodiments,
According to the present invention, it is possible to realize a logarithmic amplifier that can operate at high speed and has good temperature characteristics.

[Brief description of drawings]

FIG. 1 is a block diagram showing a basic embodiment of a logarithmic amplifier according to the present invention.

FIG. 2 is a diagram showing input / output characteristics of the embodiment shown in FIG.

FIG. 3 is a circuit configuration diagram showing a specific embodiment of the present invention.

FIG. 4 is a diagram showing input / output characteristics of the embodiment shown in FIG.

5 is a circuit configuration diagram showing a configuration example of a reference voltage and constant current generating circuit in the embodiment shown in FIG.

FIG. 6 is a block diagram showing a configuration example of a conventional logarithmic amplifier.

FIG. 7 is a block configuration diagram showing another configuration example of a conventional logarithmic amplifier.

8 is a diagram showing a circuit configuration example of a conventional full-wave detector and an amplifier / limiter shown in FIG.

[Explanation of symbols]

 1 Resistance ladder 2-1, 2-2, 2-3, 2-4 Comparator 3-1, 3-2, 3-3, 3-4 Switch circuit 4 Output terminal 11 Current-voltage conversion circuit

Claims (3)

[Claims] 1. A logarithmic amplifier for outputting a logarithmic conversion value of an input signal, a reference voltage source, a resistor ladder connected to the reference voltage source for dividing the reference voltage, and N resistors obtained by the resistor ladder. N comparison circuits for comparing the potential of the pressure point and the input signal voltage, N switch circuits controlled by the outputs of the N comparison circuits, and N switch circuits connected to the N switch circuits. A constant current source, and configured to output a sum of currents obtained by switching the constant current source circuit including the N constant current sources and a switch circuit by the output of the comparison circuit. Logarithmic amplifier. 2. The comparison circuit and the switch circuit,
2. The logarithmic amplifier according to claim 1, wherein the logarithmic amplifier comprises an emitter coupling circuit in which the emitter of the transistor to which the potential of the voltage dividing point is applied to the base is connected to the emitter of the transistor to which the input signal voltage is applied to the base. 3. A logarithmic amplifier according to claim 1, wherein the temperature coefficients of the reference voltage source and the constant current source are made small enough to be ignored.