JPH0522862Y2 - - Google Patents

Description

[Detailed explanation of the idea]

(Field of Industrial Application) The present invention relates to a sweep circuit in an oscilloscope or the like, and particularly relates to a sweep circuit capable of stabilizing the baseline (resting point) of a sawtooth wave. (Prior Art) Generally, an oscilloscope is provided with a sweep circuit for generating a sawtooth wave, and has a configuration as shown in FIGS. 1 to 3. In the case shown in Fig. 1, the baseline (resting point) of the sawtooth wave at the output (OUT) is determined by the forward voltage drops of diode D ₁₀ and transistor Q ₁₀ , and the base-emitter voltage V of transistor Q ₁₀ . It has become determined by _BE and.
The collector of the transistor Q ₁₀ is connected to the input AI (constant voltage) of the high-amplification inverting amplifier 20, while the emitter of the transistor Q ₁₀ is connected to the square wave oscillator G. In the above configuration, when changing the sweep time (=slope of the sawtooth wave), the capacitance of the capacitor Ct provided between the cathode of the diode _D10 and the input section AI is changed, or the resistance value of the resistor Rt is changed. A wide sweep range can be obtained by switching the capacitor Ct and the resistor Rt at the same time. Here, when the resistance value of resistor Rt is changed, the current flowing through resistor Rt changes, and the charging time of capacitor Ct changes.
The current to the resistor Rt is supplied through the transistor _Q10 even when the sawtooth wave is at rest. However, in the conventional method shown in Figure 1,
When the current flowing through the resistor Rt is changed, the V _BE of the transistor _Q10 also changes, so there is a drawback that the baseline of the sawtooth wave changes as the sweep time changes. Furthermore, in the conventional example shown in FIG. 2, an integrating capacitor Ct is connected between the input terminal and the output terminal of the inverting amplifier 20, and two diodes 21 and 22 whose respective same poles are connected are connected. The common terminal side of each of the independent terminals is connected to the input gate, and the terminal on the diode 21 side is connected to the output terminal of the inverting amplifier 20, while the terminal on the diode 22 side is connected to the input terminal of the inverting amplifier 20. It is connected. The input terminal of the inverting amplifier 20 can be connected to a constant current source 24 via a switch 23, and the switch 23 can be controlled in synchronization with the output pulse from the square wave oscillator 25. ing. In the above circuit, when the circuit is in a standby state, the diodes 21 and 22 are in a conductive state, and voltage feedback is applied to the inverting amplifier 20, so that the output voltage can be stabilized, and the baseline can be stabilized. It works like this. In the conventional method shown in FIG. 2 described above, when the sweep time is varied over a wide range, the baseline changes due to variations in the characteristics of each element, so the current I of the current source 24 is changed. By setting the current larger than the current flowing through the time constant setting resistor Rt, the fluctuation thereof was suppressed. However, this method has the problem that it is no longer possible to lower the impedance of the square wave oscillator 25 or to lower the resistance value of the time constant setting resistor Rt, making it impossible to handle high-speed sweeps. Furthermore, in the case shown in FIG. 3, a current control circuit 26 is used in place of the diode 21, but although the baseline stability is high,
This method had drawbacks such as a complicated circuit and an increased number of parts, and could not be adopted. (Purpose of the invention) The present invention was made in view of the above points, and its purpose is to provide a sweep circuit for an oscilloscope, etc. that can stabilize the baseline with a simple configuration. be. (Structure of the invention) The sweep circuit in an oscilloscope, etc. according to the invention is an oscilloscope, etc. in which a transistor is inserted between the Miller integrating circuit and the square wave oscillator so as to be able to change the bias voltage. The sweep circuit includes a detection transistor of the same type as the transistor arranged to detect a current passing through the transistor, and a detection transistor for controlling a bias voltage of the transistor in accordance with a detection signal from the detection transistor. The detection transistor has its emitter connected to the input terminal of the inverting amplifier, and its collector and base connected to the constant voltage power supply via a resistor.
The drain of the FET is connected to a constant voltage power supply, the source is connected to another constant voltage power supply via a resistor, and the base of the transistor is connected to the base of the transistor via a diode. It is characterized by (Embodiment) A sweep circuit in an oscilloscope or the like according to the present invention will be explained based on FIGS. 4 to 6 as an embodiment. In the figure, a signal from an input terminal IN is inputted to the anode of a diode _D1 and the emitter of _a transistor _Q1 through a resistor R1. A capacitor Ct is connected between the cathode of the diode D ₁ and the collector of the transistor Q ₁ .
The collector of the transistor Q ₁ is connected to the input terminal of the inverting amplifier 1, and the cathode of the diode D ₁ is connected to the output terminal of the inverting amplifier 1.
Q ₂ is an active element, in the above example, a detection section for detecting the current passing through the transistor Q ₁ , that is, the collector current, and Q ₃ controls the bias voltage of the transistor Q ₁ by the signal from the detection section Q ₂ . This is a control unit for In the illustrated example, a transistor is used as the detection section Q ₂ and an FET is used as the control section. The emitter of the transistor Q ₂ is connected to the input terminal of the inverting amplifier 1, and the collector and base are connected to a constant voltage via a resistor Rt. Power supply (not shown)
and the gate of FET (Q ₃ ), respectively.
The drain of the FET (Q ₃ ) is connected to a constant voltage power supply (not shown), and the source is connected to another constant voltage power supply (not shown) through a resistor R ₂ and also through a diode D ₂ . transistor
Connected to the base of _Q1 . Here, assuming that the transistor Q ₁ and the transistor Q ₂ are of the same type, the change in the base-emitter voltage V _BE1 due to the change in the current flowing through the resistor Rt, that is, the change in the collector current of the transistor Q ₁ is , is the same amount as the change in the base-emitter voltage V _BE2 of the transistor Q ₂ through which the same amount of current flows, and this is detected by the FET Q ₃ of the source filter 7 and the resistor R ₂ , and this detection signal is used for level shifting. By feeding the base of transistor Q ₁ through the diode D ₂ used, the emitter voltage of transistor Q ₁ can be stabilized. Therefore, the change in sweep time (change in resistance value of resistor Rt)
It is possible to appropriately suppress fluctuations in the baseline (resting voltage) of the sawtooth wave caused by this. The operating principle will be explained in more detail below. First, the basic circuit of the mirror sweep circuit is constructed as shown in FIG. In this circuit, when the sweep is paused, the gate G becomes a high level H, and both the diode 21 and the diode 22 are turned on, and the current of the diode 21 and the current of the diode 22 (determined by the resistance value of the resistor Rt) are changed. When the voltage is balanced, the output terminal is set to a certain DC level. Since the voltage at the input end of the inverting amplifier 20 is constant, the output voltage Vo at the output end OUT
is given by Vo=V ₂₀ +V ₂₂ −V ₂₁ (where Vo is the output voltage, V ₂₂ is the voltage across the diode 22, and V ₂₁ is the voltage across the diode 21). Here resistance Rt is generally 50 times ~ 1000
Since the voltage changes by about twice as much, the voltage across the diode 22 changes according to its forward voltage characteristics, and the output voltage Vo also changes accordingly. The circuit shown in Fig. 1 is a transistor in place of the diode _D22 in the circuit configuration shown in Fig. 5.
Q ₁₀ is used, and the output voltage in this case is
Vo becomes Vo=V _B +V _BE -V ₂₁ (where V _B is the base voltage of transistor _Q10 ), which is essentially the same as shown in FIG. In the present invention, the above-mentioned transistor _Q10 is
This is to correct the V _BE variation, and by using the configuration shown in Figure 4, the output voltage Vo is Vo = V _B1 + V _BE1 - V _D1 (where, V _B1 is the base voltage of transistor Q ₁ , V _BE1 is the voltage between the base and emitter of transistor Q ₁ , V _D1 is the voltage between the terminals of diode D ₁ , and the same applies hereafter), but V _B1 is V _B1 = V _A − V _BE2 + V _S (however, V _A is the input terminal voltage of the inverting amplifier, V _S
is the shift amount (constant) due to FETQ ₃ and diode D ₂ ), so in the end Vo = V _A − V _BE2 + V _S + V _BE1 − V _D1 . From the above, conventionally, due to changes in resistance Rt,
The amount of change in Vo (output voltage) _is
This was caused by a change in V _BE , but in the present invention, since _the same amount of current flows through transistor Q ₁ and transistor Q ₂ ,
The change in V _BE ΔV _BE1 is approximately equal to that of transistor Q ₂ and can be set as ΔV _BE1 ΔV _BE2 . Therefore, the output Vo becomes a constant value by canceling out the changes in V _BE1 and V _BE2 . Next, a comparison will be made between the conventional example explained in FIG. 2 and the sweep circuit according to the present invention shown in FIG. The purpose is to reduce fluctuations in the output Vo by connecting the resistor Rt and supplying a nearly constant current regardless of changes in the resistance value of the resistor Rt, Vo=V _A +V _D2 −V _D1 ……(1 ) However, in the case of the present invention, as mentioned above, Vo=V _A +V _S −V _D1 +V _BE1 −V _BE2 (2). Here, if V _D1 is constant, the above equation (1) and (2)
The equations are as shown below (1)' and (2)', respectively. Vo=V _D2 +V _cpost1 ……(1)′ (V _A −V _D1 = _cpost1 ) Vo=V _BE1 −V _BE2 +V _cpost2 ……(2)′ (V _A +V _S −V _D1 = V _cpost2 ). If this is shown as a characteristic diagram, the relationship will be as shown in FIG. 6B, and the variation will be ΔVo=ΔV _BE1 −ΔV _BE2 . Therefore, the variation amount is limited to variations in the characteristics of the elements, but in the conventional example shown in FIG. 2, the variation is as shown in FIG. 6A, and ΔVo=ΔV _D22 . For this reason, even if there is no variation in the characteristics of the elements, the output Vo will fluctuate. When the resistance Rt of the circuit according to the present invention and the circuit according to the conventional method was varied from 1 to 1000 times and the fluctuations in the output Vo were measured, the results shown in the table below were obtained.

[Table] As is clear from this table, the product of the present invention has excellent characteristics. (Effects of the invention) According to the sweep circuit in an oscilloscope, etc. according to the invention, the baseline of the output signal (sawtooth wave) does not fluctuate even if the sweep time varies widely, and the output is always stable. I can get a signal. Therefore, when used in an oscilloscope, the sweep starting point on the cathode ray tube surface can always be stabilized, and highly accurate measurements can be performed.

[Brief explanation of drawings]

1 to 3 are circuit diagrams showing conventional sweep circuits, FIGS. 4 and 6 show embodiments of the sweep circuit in an oscilloscope, etc. according to the present invention, FIG. 4 is a circuit diagram, and FIG. 6 is a basic circuit diagram for explaining the operating principle, and FIG. 6 is a characteristic comparison diagram. Q ₁ ... active element, Q ₂ ... detection section, Q ₃ ... control section.

Claims (1)

[Claims for Utility Model Registration] A sweep circuit for an oscilloscope, etc., in which a transistor capable of changing the bias voltage is inserted between a Miller integrating circuit and a square wave oscillator, a detection transistor of the same type as the transistor arranged to detect the current passing through the transistor; and an FET for controlling a bias voltage of the transistor in accordance with a detection signal from the detection transistor; The transistor has its emitter connected to the input terminal of the inverting amplifier.
Its collector and base are connected to a constant voltage power source and the gate of the FET through a resistor, the drain of the FET is connected to the constant voltage power source, and the source is connected to another constant voltage power source through a resistor, A sweep circuit in an oscilloscope, etc., characterized in that the sweep circuit is connected to the base of the transistor via a diode.