JPH0634247B2 - Variable time constant differentiator - Google Patents

Description

The present invention relates to a time constant variable type differentiator suitable for an active filter circuit in which operational amplifiers are connected in multiple stages.

[Technical background]

FIG. 5 shows an example of a conventional differentiator, which is a capacitor C ₀
It is composed of an operational amplifier A ₀ to which a signal is input via a resistor and a resistor R ₀ connected between its input and output terminals. Further, when configuring a time constant variable type differentiator, the resistor R ₀ must be a variable resistor.

When using this differentiator to construct an active filter circuit in which operational amplifiers are connected in multiple stages, eliminate external resistors as much as possible to make a semiconductor integrated circuit into a single chip and reduce the size. ing. However, resistors that cannot be integrated into a semiconductor integrated circuit are hybridized to a ceramic substrate or the like, or components such as a variable resistor are generally attached externally to a printed circuit board to further reduce the shape. Is required.

[Problems to be Solved by the Invention]

A conventional differentiator is composed of a resistor, a capacitor, and an operational amplifier, and in order to make a time constant variable type differentiator, an external variable resistor must be used, and it is necessary to use a semiconductor integrated circuit. Is not preferable. When an active filter circuit is configured using these differentiators, if an external variable resistor is used, it is mounted on a printed circuit board, so there is the drawback that the size becomes large, and the manufacturing cost rises. It becomes a factor.

Also, there is a differentiator using a function device, but its circuit configuration is extremely complicated, and when an active filter circuit is configured, it has a drawback of being extremely large.

An object of the present invention is to provide a time constant variable type differentiator which can be easily integrated into a semiconductor integrated circuit and is suitable for an active filter circuit.

[Means for solving the problem]

The time constant variable type differentiator of the present invention comprises an operational amplifier, and a negative feedback circuit of the operational amplifier comprises an integrator composed of a variable conductance amplifier and a capacitor connected to the output stage thereof.

[Embodiment] FIG. 1 is a circuit diagram showing an example of a differentiator according to the present invention.

In FIG. 1, the input terminal 1 is connected to the positive phase input terminal of the operational amplifier A ₁ , the output terminal thereof is connected to the output terminal 2 and the positive phase input terminal of the transconductance amplifier A ₂ is connected. , The capacitor C ₁ is connected between the output terminal of the transconductance amplifier A ₂ and the ground, and is also connected to the negative phase input terminal of the operational amplifier A ₁ , and the transconductance amplifier A ₂ is connected to the integrator by the capacitor C ₁ . And acts as a negative feedback circuit of the operational amplifier A ₁ . A differential output is obtained from the output terminal 2.

The differentiator according to the present invention will be described based on the block diagram of FIG.

The transfer function V ₂ / V _{1 in} the block diagram of FIG. 3 is expressed by the following equation.

(However, V ₁ is an input voltage, V ₂ is an output voltage, and α and β are variables.) In order to form a time constant variable type differentiator, the transfer function needs to have the following relationship. There is.

V ₂ / V ₁ = sC / gm (2) Therefore, the following equation is obtained from the equations (1) and (2).

To satisfy the equation (3), α and β are α = ∞ and β = gm / s
It is necessary to be C, and by configuring the transfer function of equation (3), a differentiator with a variable time constant can be formed.

That is, blocks 3 to 5 in the block diagram shown in FIG.
Can replace the variable α (α = ∞) of the block 4 with an operational amplifier A ₁ having a substantially infinite gain,
The variable β (β = gm / sC) of the block 5 is the capacitor C
₁ and a variable conductance amplifier A ₂ whose transconductance can be changed. The adder / subtractor 3 can be constructed by connecting to the positive and negative phase input terminals of the operational amplifier A ₁ used in the block 4, so that the differentiator shown in FIG. 1 is formed.

FIG. 4 shows the input / output waveforms of the differentiator, and by varying the mutual conductance of the variable conductance amplifier A ₂ of the differentiator of FIG. ₁ , the variable conductance of the capacitor C ₁ and the variable conductance amplifier A ₂ is changed. It is shown that the time constant τ due to is variable.

FIG. 4 (a) shows the waveform of the input pulse.
Figure (b) shows the output waveform. As the transconductance of the variable conductance amplifier A ₂ is increased, the differential waveform has a time constant τ such as P ₁ , P ₂ , and P _3.
Indicates that a large differential waveform is output.

FIG. 2 shows another embodiment of the time constant variable type differentiator according to the present invention, which is composed of a positive phase type operational amplifier, and is composed of a negative phase variable conductance amplifier A ₂ . The differential output waveform is the inverse of the differentiator in FIG.

[Effect]

The time constant variable type differentiator of the present invention can be formed by an extremely simple circuit, and its mutual conductance can be easily changed by adjusting the operating current of the variable conductance amplifier. Time constant τ
Has an advantage that it can be easily adjusted, and since it is composed of an operational amplifier and a variable conductance amplifier, there is an advantage that a semiconductor integrated circuit can be easily formed.

If the time constant variable type differentiator of the present invention is used, the shape of the filter can be made extremely small even if a multi-stage active filter circuit is configured, and the low band or band formed by connecting in multiple stages. It is extremely effective for a pass-through active filter circuit.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a time constant variable type differentiator according to the present invention, and FIG. 2 is a circuit diagram showing another embodiment of a time constant variable type differentiator according to the present invention. FIG. 3 is a block diagram for explaining a time constant variable type differentiator of the present invention, FIG. 4 is a diagram showing operation waveforms of the differentiator, and FIG. 5 is an example of a conventional differentiator. FIG. 1: input terminal, 2: output terminal, A ₁ : operational amplifier,
A ₂ : Variable conductance amplifier, C ₁ : Capacitor

Claims (1)

[Claims] 1. A time constant characterized by comprising an operational amplifier, an integrator comprising a variable conductance amplifier and a capacitor connected to its output stage, and applying negative feedback to the operational amplifier via the integrator. Variable type differentiator.