JPH029414Y2 - - Google Patents

Description

[Detailed Description of the Invention] (Technical Field) The present invention relates to an exponential function waveform generation circuit, and more particularly to an exponential function waveform generation circuit that can easily generate an index with an arbitrary time constant.

(Prior Art) Conventionally, an exponential function waveform generation circuit is used when generating a sweep signal or when performing process simulation in an analog manner. FIG. 1 is an electrical configuration diagram showing an example of a conventional exponential function waveform generating circuit. In the figure, E is the voltage source for initial value setting, R _S is the charging resistor, SW is the changeover switch, C is the capacitor whose one end is connected to the changeover switch SW, and R is the b contact of the changeover switch SW. U is a buffer amplifier that receives the voltage applied across the capacitor C. An overview of the operation of the circuit configured as described above is as follows.

First, when the changeover switch SW is connected to the a contact side, the initial value setting voltage source E is connected to the capacitor C, and the capacitor C is charged to the voltage value E. (These identification symbols will be used as they are below for the circuit constants of the resistor R and capacitor C.)
The charging time constant at this time is R _s ×C. After that, connect the changeover switch SW to the b contact side,
The capacitor C and the resistor R constitute a closed circuit, and the charge stored in the capacitor C is discharged through the resistor R. At this time, the voltage V across the capacitor C has a negative exponential waveform expressed by the following equation.

V=Eexp(-(t/τ))...(1) Here, τ is a discharge time constant, expressed as R×C. This voltage V is then buffered by a buffer amplifier U and output as it is to become the output of the circuit shown in the figure. When the circuit shown in the figure is repeatedly charged and discharged, a sweep signal is generated.

Consider the case where such a conventional circuit generates an exponential function with a time constant of several tens of minutes. The maximum capacitance of capacitor C is usually about 10 μF. Therefore, by using a capacitor with the maximum capacity, the time constant and the number
If you try to achieve 10 minutes, the value of the resistor used will be several tens of MΩ. When it comes to resistors of several tens of MΩ, stable ones are expensive and difficult to obtain. Furthermore, when a time constant of several tens of minutes is achieved, the discharge current becomes extremely small (for example, on the μA order), and countermeasures against leakage current during mounting are required. Furthermore, if the resistance is several tens of MΩ, stable and highly accurate resistors (metal film resistors, etc.) will have a large shape. Therefore, when using a relay or the like for purposes such as switching a resistor using an electrical signal, it has been difficult to secure a mounting space for the relay.

(Purpose of the invention) The present invention was made in view of the above points, and its purpose is to provide an inexpensive and compact exponential waveform generation circuit that can generate an exponential function waveform with a large time constant with a simple configuration. The aim is to realize this.

(Structure of the invention) The present invention achieves this purpose by using an operational amplifier,
A capacitor connected between the input and output of the operational amplifier, a gain adjuster that adjusts the gain of the output of the operational amplifier by a factor of G, and a discharge resistor connected between the output of the gain adjuster and the input of the operational amplifier. a charging circuit comprising a voltage source and a charging resistor connected in series to the voltage source; and when charging the capacitor, the charging circuit is connected to the input of the operational amplifier and the discharging circuit is opened. ,
The present invention is characterized by comprising a switch means for cutting off charging by the charging circuit and closing the discharging circuit when the capacitor is discharged.

(Embodiments) Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 2 is an electrical configuration diagram showing an embodiment of the present invention. Components that are the same as those in FIG. 1 are designated by the same numbers. In the figure, U ₁ is an operational amplifier, and U ₂ is a gain amplifier with a gain G that adjusts the output V ₀ of the operational amplifier U ₁ by a factor of G. Capacitor C is connected between the input and output of operational amplifier _U1 , and is connected to voltage source E,
A series circuit of a charging resistor R _s and a changeover switch SW is also connected between the input and output of the operational amplifier U ₁ . A changeover switch SW' is provided at the input section of the operational amplifier _U1 , and b is used as the input of the operational amplifier _U1 .
Either side (output side of gain regulator _U2 ) or side a (charging circuit side) is selected. The contacts of the changeover switch SW' are adapted to be switched in conjunction with the changeover switch SW. Further, the output voltage V ₀ is gain-adjusted by a factor of G by a gain adjuster U ₂ and then applied to the input of the operational amplifier U ₁ via a discharge resistor R.

The operation of the circuit configured as described above will be explained as follows.

First, it is assumed that the changeover switches SW and SW' are connected to the a contact side as shown in the figure. In this state, the capacitor C is charged with electric charge from the voltage source E, and the capacitor C is charged to the voltage value E. As a result, the output V ₀ becomes equal to E. Next, when the changeover switches SW and SW' are switched to the b contact side, the charge stored in the capacitor C starts discharging. At this time, if the current flowing through the discharge resistor R is i, then i is equal to the value obtained by dividing the output V ₀ ' of the gain regulator U ₂ by the resistance value R. Therefore, i is given by the following formula.

i=V ₀ '/R=GV ₀ /R (2) Next, the rate of change (dV ₀ /dt) of the output voltage V ₀ is determined. If the total amount of charge charged in the capacitor C is Q, then the relationship Q=CV ₀ holds true. From this, the following equation holds.

V ₀ =Q/C When both sides of this equation are differentiated with respect to time t, V ₀ decreases, so dV ₀ /dt=-(1/C)(dQ/dt) Here, the rate of change of charge Q dQ Since /dt is equal to the current i flowing through the resistor R, the following equation holds true.

dV ₀ /dt=-(1/C)i...(3) From equations (2) and (3), the following equation holds true.

dV ₀ /dt=-(V ₀ G)/CR...(4) Solving equation (4) for V ₀ yields the following.

V ₀ =Eevp(-G/CR)t (5) In equation (5), the voltage source E is given as an initial value. As is clear from equation (5), V ₀ has a negative exponential waveform that decreases with time. Furthermore, as is clear from equation (5), the discharge time constant is τ=CR/G (6). From equation (6), it can be seen that the time constant τ can be changed by changing the gain G of the gain adjuster U ₂ while leaving the capacitor C and the resistor R unchanged.

In equation (6), when the gain G is made smaller than 1, the time constant τ becomes larger than CR. That is, by making the gain G sufficiently smaller than 1, the time constant τ can be made sufficiently large. According to this invention,
The gain G can be increased without increasing the value of the discharge resistor R.
By changing , the discharge time constant can be equivalently increased. Therefore, it is not necessary to use a resistor R having a large resistance value. As a result, since it is not necessary to use a resistor with a large resistance value, the shape can be made small and the mounting space can be reduced. Furthermore, since it is not necessary to set the resistance value to several tens of MΩ, no special measures against leakage current are required, and the configuration is simplified. Furthermore, since it is not necessary to use a resistor with a large resistance value, an inexpensive resistor can be used.

Note that it is desirable that the gain of the gain adjuster U ₂ can be varied by an external gain switching signal as shown in the figure. For example, if a D/A converter is used as the gain adjuster U ₂ , the output V ₀ is input to the reference input terminal, and gain switching is performed by inputting data as a gain switching signal to the data input terminal. A voltage V ₀ ' corresponding to the gain switching signal is output from the analog output. Therefore, according to the present invention, since relays and the like are not required, the mounting space is significantly reduced and electrical handling becomes easier.

FIG. 3 is an electrical configuration diagram showing another embodiment of the present invention, in which the charging circuit is operated by changeover switches SW ₁ and SW ₂ that operate in conjunction with each other. Furthermore, the polarity of the voltage source E' is also reversed from that of the circuit shown in FIG.

The potential on the virtual ground side of operational amplifier U ₁ is switched
The switch SW 1 is connected to SW ₁ and SW ₂ , and the b contact of the switch SW ₁ is connected to a voltage source E' having a voltage value -E via a resistor _Rs . The b contact of the switch SW ₂ is connected to one end of a resistor R _s ', and the other end of the resistor R _s ' is connected to a capacitor C. That is, the resistance R _s ′ is the switch
Connected in parallel to capacitor C via SW ₂ . Further, the a contact of the switch _SW1 is connected to the discharge resistor R. The operation of the circuit configured in this way will be explained as follows.

During charging, the contacts of the changeover switches SW ₁ and SW ₂ are both on the b side as shown in the figure. As a result, -E
_{A voltage of}
Its output V ₀ will be positive. Therefore, operational amplifier U ₁
A capacitor C connected between the input and output of is charged to _V0 .

Next, when the changeover switches SW ₁ and SW ₂ are switched to the a contact side, the charge in the capacitor C is discharged, but
The operation at this time is exactly the same as the circuit shown in FIG. 2, so the explanation will be omitted.

(Effects of the invention) As explained in detail above, according to the invention,
When charging and discharging a capacitor connected between the input and output of an operational amplifier, the output voltage is gain-adjusted by a factor of G and is fed back to the input side of the operational amplifier via a discharging resistor. Since the discharge time constant can be set large, it is possible to realize an inexpensive and compact exponential function waveform generation circuit that can generate an exponential function waveform with a large time constant with a simple configuration. Great effect.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of a conventional circuit, FIG. 2 is an electrical configuration diagram showing one embodiment of the present invention, and FIG. 3 is an electrical diagram showing another embodiment of the invention. U, U ₁ ... operational amplifier, V ₂ ... gain adjuster,
SW, SW′, SW ₁ , SW ₂ ... changeover switch, R,
R _s , R _s ′...Resistor, C...Capacitor, E, E'...
...voltage source.

Claims (1)

[Scope of utility model registration request] An operational amplifier, a capacitor connected between the input and output of the operational amplifier, and an output of the operational amplifier connected to G
a discharge circuit consisting of a gain adjuster that doubles the gain; a discharge resistor connected between the output of the gain adjuster and the input of the operational amplifier; a voltage source and a charging resistor connected in series to the voltage source; a charging circuit; and a switch means that connects the charging circuit to the input of the operational amplifier and opens the discharging circuit when charging the capacitor, and cuts off charging by the charging circuit and closes the discharging circuit when discharging the capacitor. An exponential function waveform generation circuit consisting of.