JPH024520Y2 - - Google Patents

Description

[Detailed Description of the Invention] (Technical Field) The present invention relates to a sweep signal generation circuit, and more particularly to a sweep signal generation circuit that generates a sweep signal by repeatedly generating an exponential function.

(Prior Art) Sweep signal generation circuits are used in various cases, for example, for horizontal and vertical sweeping of CRTs. As the sweep signal, a triangular wave, a sawtooth waveform, a negative exponential waveform, or the like is used. FIGS. 2 and 3 are electrical configuration diagrams showing a conventional example of a sweep signal generation circuit whose output waveform is a negative exponential function waveform. In FIG. 2, U is an operational amplifier, C is a capacitor connected between the input and output of the operational amplifier U, and SW is a changeover switch whose one end is connected to the virtual ground point of the operational amplifier U. the switch
The b contact of the SW is connected to one end of the discharge resistor R, and the a contact is connected to the initial value setting voltage source E. Rs is a charging resistor connected in series with the voltage source E. The other ends of the discharging resistor R and the charging resistor Rs are both connected to the output of the operational amplifier U.

In the circuit configured as described above, assuming that the contact of the changeover switch SW is now on the a side as shown in the figure, the capacitor C is charged to the voltage value of the voltage source E. After that, when the changeover switch SW is switched to the b contact side, the voltage source E and the charging resistor are connected.
The series circuit (charging circuit) of Rs is disconnected, and a discharging resistor R is connected in parallel to the capacitor C instead. As a result, the resistor R and the capacitor C form a discharge circuit, and the output V ₀ is attenuated as shown by the following equation.

V ₀ =Eexp(−(t/τ)) (1) Here, E is the initial value charged in the capacitor C, and the identification symbol of the voltage source E is used as it is as the voltage value. τ is a discharge time constant, and if the identification symbols are used as they are as circuit constants for the resistor R and capacitor C (the same shall apply hereinafter), τ is equal to CR. After a predetermined period of time has elapsed, when the changeover switch SW is switched to the a-contact side again, the discharged capacitor C is again rapidly charged with electric charge from the charging circuit at the charging time constant CRs. When such operations are repeated at regular intervals, the output V ₀ becomes a sweep signal as shown in the figure.

According to such a conventional circuit, the input-referred drift of the operational amplifier U appears as a fluctuation in the output _V0 , making it impossible to obtain a high-quality sweep signal.
Further, since the initial value setting voltage source E is floating with respect to the ground potential, an independent power source is required. Therefore, the cost is high and it is inconvenient to handle.

In the conventional example shown in FIG. 3, the initial value setting voltage source is grounded. Components that are the same as those in FIG. 2 are designated by the same numbers. In the figure, E' is a voltage source for initial value setting whose one end is grounded, and the voltage value -E
This voltage is applied to the input of the operational amplifier U via the charging resistor Rs' and the changeover switch SW' which operates in conjunction with the switch SW. Changeover switch SW,
Assuming that SW' is on the a-contact side as shown in the figure, the circuit shown in the figure becomes an inverting amplifier with resistor Rs' as the input resistance and resistor Rs as the feedback resistor, and the resistances of Rs and Rs' If we make the values equal, the steady-state output V ₀ will be equal to E, and the capacitor C will be charged to this value E. And the changeover switch
When SW and SW' switch to the b contact side, the circuit shown in the figure constitutes a discharge circuit, and its output V ₀ decreases exponentially as shown by equation (1). and,
The above-described charging operation and discharging operation are performed at regular intervals, and the output V ₀ becomes a sweep signal as shown in the figure.

In the circuit shown in FIG. 3, since the initial value setting voltage source E' is grounded, there is no need to prepare an independent power source as in the example shown in FIG. Therefore, although the number of changeover switches increases by one, it is superior to the circuit shown in FIG. 2 in terms of cost and handling. However, this circuit has twice the voltage drift as compared to the circuit shown in FIG. Generally, in this type of circuit, an operational amplifier U with a small offset current is used, so that current drift caused by the offset current flowing through the resistor Rs' does not pose a problem. However, such a low offset current type operational amplifier usually has a temperature drift of at least several μV/°C, and cannot stably obtain the output voltage V ₀ at the initial value setting.

(Purpose of the invention) The present invention was made in view of the above points, and its purpose is to realize a sweep signal generation circuit that eliminates the influence of temperature drift of the operational amplifier and stabilizes the output. be.

(Structure of the invention) The invention that achieves this object includes an operational amplifier having a capacitor connected between its input and output, a discharging resistor connected between the input and output of the operational amplifier via an on-off switch, and a discharging resistor connected between the input and output of the operational amplifier. a differential amplifier that compares the output of the operational amplifier with a reference voltage and outputs a difference signal; and a second differential amplifier that operates in conjunction with the on/off switch to feed back the output of the differential amplifier to the input side of the operational amplifier. and an on/off switch, and is characterized in that the output of the operational amplifier is used as its output.

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

FIG. 1 is an electrical configuration diagram showing an embodiment of the present invention. Components that are the same as those shown in FIGS. 2 and 3 are designated by the same numbers. U ₁ is an operational amplifier similar to U in FIGS. 2 and 3, and U ₂ compares the output V ₀ of the operational amplifier U ₁ with the reference voltage E of the reference voltage source Es,
This is a differential amplifier that outputs the difference signal. SW ₁ and SW ₂ are changeover switches that operate in conjunction with each other. The output of differential amplifier U ₂ is connected to changeover switch SW ₁
The charging resistor Rs is connected to the charging resistor Rs via the b contact. One end of the changeover switch SW ₂ is connected to the discharge resistor R.
The a-contact is connected to the output terminal. The operation of the circuit configured as described above will be explained as follows.

In charging mode, selector switch SW ₁ ,
The contacts of SW ₂ are all set on the b side as shown in the figure. In this state, the differential amplifier
U ₂ compares the output V ₀ and the reference voltage E, outputs a difference signal between V ₀ and E, works to make V ₀ equal to E, and supplies a charge that cancels the fluctuation of V ₀ to the resistor Rs.
into the input of operational amplifier U ₁ through. As a result, the initial value of the output V ₀ becomes the voltage E of the reference voltage source Es. According to the invention, the operational amplifier U ₁
Even if the output V ₀ fluctuates due to temperature drift, the output V ₀ is always fixed at the reference voltage E due to the negative feedback provided by the differential amplifier U ₂ .
For example, if the output V ₀ becomes larger than the reference value E, the output e of the differential amplifier U ₂ becomes positive,
This voltage is converted into a current of (e/Rs) by the resistor Rs. This current flows into capacitor C and reduces the charge, so that the output V ₀ decreases to match E. In this way, according to the circuit of the present invention,
The influence of the drift of operational amplifier U ₁ is eliminated.

According to the circuit shown in the figure, the drift of the output V ₀ approximately matches the amount of drift of the differential amplifier U ₂ . In other words, the performance of the circuit shown in the figure is that of a differential amplifier.
Determined by the drift characteristics of U ₂ . However, since the offset current characteristics of the differential amplifier _U2 do not matter, a bipolar type differential amplifier can be used. If a bipolar type is used, one with a temperature drift of about 0.1 to 0.2 μV/°C can be easily obtained and is inexpensive. Therefore, the drift characteristic can be improved by one order of magnitude or more compared to the conventional one. According to the present invention, it is not necessary to use a particularly low-drift operational amplifier _U1 as long as it has a small offset current. Therefore,
No need to use expensive differential amplifier U ₂
Even if this is added, overall costs can be significantly reduced.

The operation in the charging mode has been described above, but the operation in the discharging mode (sweep mode) is as follows. That is, both switches SW ₁ and SW ₂ are switched from the B contact side to the A contact side.
As a result, the differential amplifier U ₂ is removed from the feedback loop, a discharge circuit is formed by the capacitor C and the discharge resistor R, and the charge stored in the capacitor C begins to discharge, as shown in equation (1). decreases exponentially so that As a result, the output V ₀ also decreases exponentially.

By performing the above charging mode and discharging mode at regular intervals, a sweep signal as shown in the figure is obtained at the output V ₀ , and the sweep signal that generates a stable signal without the influence of drift can be obtained from the circuit shown in the figure. It can be used as a generating circuit.

(Effects of the invention) As explained in detail above, according to the invention,
By comparing the output voltage with a reference voltage for initial value setting and feeding back the result to the input side, it is possible to easily create a sweep signal generation circuit that eliminates the temperature drift of the operational amplifier and stabilizes the output. This configuration can be realized at low cost.

[Brief explanation of the drawing]

FIG. 1 is an electrical configuration diagram showing an embodiment of the present invention, and FIGS. 2 and 3 are electrical configuration diagrams showing a conventional example. U, U ₁ ... operational amplifier, U ₂ ... differential amplifier,
SW, SW′, SW ₁ , SW ₂ ... Selector switch, E,
E', Es...voltage source, R, Rs, Rs'...resistance, C...
...capacitor.

Claims (1)

[Scope of utility model registration request] An operational amplifier with a capacitor connected between its input and output, a discharge resistor connected between the input and output of the operational amplifier via an on/off switch, and a difference signal that compares the output of the operational amplifier with a reference voltage. A second on/off switch operates in conjunction with the on/off switch to feed back the output of the differential amplifier to the input side of the operational amplifier. Sweep signal generation circuit for output.