JP3018435B2 - Servo circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a servo circuit, and in particular, an upper limit and a lower limit of a servo operation range are set to predetermined values, and a range between the lower limit and the lower limit is defined. It is suitable for use in a servo circuit configured to perform a normal servo operation.

<Summary of the Invention> A servo circuit according to the present invention is a servo circuit including a variable gain amplifier and an amplitude limiter for defining an upper limit and a lower limit of a range in which the variable gain amplifier varies a gain. By configuring the temperature coefficient of the amplifier and the temperature coefficient of the upper limit value defining signal and the lower limit value defining signal of the gain variable range given to the amplitude limiter to have opposite polarities, the temperature characteristic of the gain variable range is reduced. This is a servo circuit in which the temperature characteristic of the gain control range is eliminated with a simple circuit configuration.

<Prior Art> A servo in which the gain of a variable gain amplifier is changed by changing a control voltage or a control current supplied to a variable gain amplifier (hereinafter referred to as VCA) to control the gain of a signal to be processed. Circuits are known. In such a servo circuit, if the VCA control characteristic has a temperature characteristic, or if the upper limit and lower limit of the control voltage or control current have a temperature characteristic, the gain control range of the entire servo circuit has a temperature characteristic. I will.

FIG. 5 is a configuration diagram showing an example of a conventionally used closed loop servo circuit. In Figure 5, together with the processing signals S ₁ given is supplied to VCA20 the processed signal input terminal T _1, a control signal input terminal T control signal S ₂ given to ₂ the first level detection circuit 21 Supplied. on the other hand,
A VCA output signal S ₃ is derived to an output terminal T ₃ and
The signal is supplied to the second level detection circuit 22. Then, the output signals S ₄ and S _{5 of} the first and second level detection circuits 21 and 22 are output.
Are added by the adder 24 such that their polarities are opposite to each other. Sum output S ₆ of the adder 24 is led to the control circuit 23. The control circuit 23, based on the polarity of the applied signal, to derive the signal S ₇ for controlling the gain of VCA20 the limiter 25. For example, when each level detection output S ₄ , S ₅ is applied to the adder 24 with the polarity shown, if the addition output S ₆ is positive, VC
Control signal S _7, such as to raise the gain of A20 are derived from the control circuit 23. Thus, the gain of VCA20 is improved, the magnitude of the signal applied to the adder 24 gradually increases the signal S ₅ having a negative polarity, the servo converges where addition output becomes zero.

Above limiter 25, together with the upper limit value defining signal S ₈ from the upper limit value generator 26 is provided, though the lower limit value defining signal S ₉ is supplied from the lower limit value generator 27, the limiter 25 these upper limit And its lower limit, its output signal S
Limit ₁₀ to change.

Limiter output signal S ₁₀ output from the limiter 25
Is supplied to the temperature characteristic canceling circuit 28. The temperature characteristics canceling circuit 28, those provided in order to cancel the temperature characteristic of the control characteristics of the VCA20, the signal S ₁₀ outputted from the limiter 25 from the temperature characteristics canceling circuit 28 as a gain control signal S ₁₁ The output is output to the VCA 20.

Although FIG. 5 shows a case where the apparatus is configured as a closed loop, the same operation is performed in a case where the apparatus is configured as an open loop as shown in FIG.

<Problem to be Solved by the Invention> The conventional servo circuit configured as described above is a VCA2
Since the circuit 28 for canceling the temperature characteristic of the control characteristic of 0 is required, the circuit scale, power consumption, cost, and the like increase.

In addition, since it is not preferable that the signals S ₈ and S ₉ output from the upper limit value generator 26 and the lower limit value generator 27 have temperature characteristics, these upper limit value defining signal S ₈ and lower limit value defining signal S ₉
It is necessary to use a constant voltage circuit (or a constant current circuit) in order to cancel out the temperature characteristics of the above, but this complicates the circuit configuration. The circuit configuration can be simplified by configuring the limiter circuit 25 using elements such as a diode and a zener diode.However, since the limiter circuit using these elements has temperature characteristics, the characteristics may vary depending on temperature changes. Is inconvenient, or the operation varies.

The present invention has been made in view of the above problems, and has as its object to realize a servo circuit having no temperature characteristic in a gain control range with a simple circuit configuration.

<Means for Solving the Problems> A servo circuit according to the present invention includes a variable gain amplifier in which a logarithmic value of a gain changes with a certain temperature coefficient with respect to an absolute temperature;
An amplitude limiter for defining an upper limit and a lower limit of a range in which the variable gain amplifier varies the gain, and an upper limit value defining signal and a lower limit value defining signal of a gain variable range given to the amplitude limiter, Gain control range determining means for changing the temperature control with a temperature coefficient having a polarity opposite to the temperature number.

<Operation> Since the temperature coefficient of the variable gain amplifier and the temperature coefficient of the amplitude limiter for defining the upper and lower limits of the variable gain range in the variable gain amplifier have opposite polarities,
When the characteristics of the variable gain amplifier and the temperature characteristics of the variable gain range set in the amplitude limiter change due to the temperature change, these changes cancel each other out. Therefore, the temperature characteristic of the gain limit range of the entire servo circuit becomes zero, and a circuit for canceling the temperature characteristic of the gain control range becomes unnecessary.

<Embodiment> FIG. 1 is a configuration diagram of a closed loop servo circuit showing an embodiment of the present invention.

As is clear from FIG. 1, the servo circuit of the embodiment
The overall circuit configuration is substantially the same as the conventional servo circuit shown in FIG. That is, the processing signals S ₁ given is supplied to VCA1 for processing the signal input terminal T _1, the control signal input control signal S _2, which is supplied to the terminal T ₂ is supplied to the first level detection circuit 2 . On the other hand, the VCA outputs the output S _3, while being led to the output terminal T _3, is supplied to the second level detecting circuit 3. Then, the output signals S ₄ and S _{5 of} the first and second level detection circuits 2 and 3 are added by the adder 4 so that the polarities thereof are opposite to each other. Output of the adder 4
S ₆ is led out to the control circuit 5.

The control circuit 5 controls the VC based on the polarity of the given signal.
A signal S ₇ for controlling the gain of A1 deriving the limiter 6. For example, when the level detection outputs S ₄ and S ₅ are applied to the adder 4 with the illustrated polarity, if the addition output S ₆ is positive, a control signal S ₇ that increases the gain of the VCA 20 is derived. Thus, the gain of VCA1 is improved, the magnitude of the signal applied to the adder 4 is negative polarity signal gradually increases, servo where addition output S ₆ becomes zero converges.

The limiter 6 is provided with an upper limit value defining signal S ₈ from an upper limit value generator 8 of the gain control range determining means 7 and a lower limit value defining signal S ₉ from a lower limit value generator 9.
The limiter 6 limits the output signal S ₁₀ , that is, the gain control signal S ₁₁ in FIG. 5, to change between these upper and lower limits.

Limiter output signal S ₁₀ output from the limiter 6
Is derived to VCA1 as a gain control signal. As a result, the VCA 1 performs an operation of varying the gain within the operation range determined by the gain control range determining means 7.

By the way, as shown in FIG. 2, the gain control method in the VCA1 generally changes the ratio of the output currents flowing through the transistors Q1 and Q2 of the differential amplifier circuit 11 by the control voltage varying means 12 to change the operating state of the circuit. Is used.

In the circuit for controlling the gain in this manner, if the control voltage of the control voltage varying means 12 is Vc, the transistor Q
The ratio between the output currents I ₁ and I ₂ of Q1 and Q2 is expressed by the following equation. That is, I ₁ / I ₂ = exp (qVc / kT) (1)

In general, the gain of VCA1 constitute a circuit to be proportional to I ₁ / I _2. Therefore, assuming that the proportionality constant is k, the gain A of VCA1 is expressed by the following equation: A = k · exp (qVc / kT) (2) Therefore, it can be seen that the logarithmic value of the gain of a VCA of the type shown in FIG. 2 is inversely proportional to the absolute temperature T.

On the other hand, as a means for generating a voltage directly proportional to the absolute temperature T, for example, a method as shown in the circuit diagram of FIG. FIG. 3 shows first to fourth constant current sources 13a, 13b, 13c, 13d, and first and second transistors Q
3, Q4, a resistor R1 connected between the emitter of the transistor Q4 and ground, and a resistor R2 connected between the constant current source 13d and ground, etc., are configured as a current mirror circuit, and the output voltage is It is taken out from both ends of the resistor R2.

In the circuit of FIG. 3, when the ratio of the emitter area of the transistor Q3 to the emitter area of the transistor Q4 is 1: n, the output current I _out flowing through the resistor R2 is I _out = I (2kT / q) · 1n・ N｝ / R1 (3)

Therefore, the upper limit value or lower limit value V _LIMIT of the control voltage taken out from the resistor R2 is V _LIMIT = (R2 / R1) · (2kT / q) · 1n · n (4), and the output voltage V _LIMIT is absolute. It is proportional to the temperature T.
Therefore, the circuits of FIG. 3 are used as the upper limit value generator 8 and the lower limit value generator 9 in FIG. 1, and the voltage V _LIMIT output from these circuits is used as the upper limit value and the lower limit value of the control voltage of VCA1. If used, the temperature characteristics of the gain control range of the entire servo circuit cancel each other to become zero. Therefore, according to the present invention, a servo circuit having no temperature characteristic in the gain control range can be realized with a simple circuit configuration. Note that, in the case of the circuit shown in FIG. 1, an example in which the output signal of the VCA 1 is directly input to the level detection circuit 3 has been described, but the signal may be input to the level detection circuit 3 after some signal processing. Good. Further, the characteristics of the control circuit 5 and the level detection circuits 2 and 3 can be freely set.

Note that the servo circuit of the present invention can be favorably applied to an open-loop type servo circuit as shown in the circuit diagram of FIG.

Further, in the above embodiment, the upper limit value supplied to the limiter 6 defining signal S ₈ and the lower limit value defining signal S ₉ showed the case of the voltage, these signals S ₈ and S ₉ are supplied in the form of current You may make it. When the current is supplied in the form of a current, the output current I _{out in} FIG. 3 may be directly taken _out and supplied to the limiter 6.

When the servo circuit of the embodiment is formed into an IC, a circuit for generating a voltage (current) directly proportional to the temperature as shown in FIG. 3 is generally incorporated in the IC in many cases. Since it can be used, it is possible to configure a servo circuit having no temperature characteristics without substantially increasing the circuit scale.

<Effect of the Invention> As described above, according to the present invention, the temperature coefficient of the variable gain amplifier and the temperature coefficient of the amplitude limiter for defining the upper and lower limits of the variable gain range in the variable gain amplifier are opposite to each other. When the characteristics of the variable gain amplifier and the temperature characteristics of the variable gain range set in the amplitude limiter change due to a temperature change,
These changes can be mutually canceled to make the temperature characteristic of the gain control range of the entire servo circuit zero.
Therefore, a circuit for canceling the temperature characteristics of the gain control range can be eliminated, and the circuit configuration of the servo circuit having no temperature characteristics of the gain control range can be simplified. Further, since the circuit configuration is not simplified by using elements such as a zener diode and a diode as a limiter of the control voltage of the variable gain amplifier, it is necessary to prevent a characteristic variation due to a variation in temperature coefficient of these elements. can do.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a closed-loop servo circuit showing an embodiment of the present invention, FIG. 2 is a circuit diagram showing an example of a gain control method in the circuit of FIG. 1, and FIG. 3 is set to a limiter. FIG. 4 is a circuit diagram showing an example of a circuit for generating a voltage that determines a variable gain range to be applied, FIG. 4 is a configuration diagram showing an example applied to an open-loop servo circuit, and FIGS. FIG. 5 is a block diagram of a closed-loop servo circuit, and FIG. 6 is a block diagram of an open-loop servo circuit. 1 Variable gain amplifier 2 First level detection circuit 3 Second level detection circuit 4 Adder 5 Control circuit 6 Limiter 7 Gain control range Decision means, 8: Upper limit value generator, 9: Lower limit value generator

Claims (1)

(57) [Claims] A variable gain amplifier in which a logarithmic value of a gain changes with a certain temperature coefficient with respect to an absolute temperature; an amplitude limiter for defining an upper limit and a lower limit of a range in which the variable gain amplifier changes a gain; A gain control range determining means for changing an upper limit value defining signal and a lower limit value defining signal of the gain variable range given to the amplitude limiter with a temperature coefficient having a polarity opposite to a temperature coefficient of the variable gain amplifier. A servo circuit characterized in that: