JPS6130341Y2 - - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a monostable multivibrator whose characteristics are stabilized.

Conventionally, a monostable multivibrator has a gate circuit 2 consisting of a NAND gate element G, etc., a differentiating circuit 3 consisting of a resistor R _T and a capacitor C _T , transistors Q ₁ , Q ₂ and a constant current source S, as shown in Fig. 1. The inverter circuit 4 is composed of a differential amplifier and the like. The operation of this conventional monostable multivibrator will be explained with reference to FIG. Fourth
Figure A shows the input pulse waveform applied to input terminal 1, Figure 4B shows the output pulse waveform of NAND gate element G, Figure 4C shows the base voltage waveform of transistor _Q1 , and Figure 4D shows the transistor FIG. 4E shows the collector voltage waveform of Q ₁ and the emitter voltage waveform of transistor Q ₄ . In a stable state before the input trigger pulse is applied, input terminal 1 is supplied with a high voltage potential (for example, 5V), and the base of transistor Q ₁ is connected to the transistor
V _BE of transistor Q ₃ with respect to the base of Q ₂ (〓
0.6V), transistor Q ₁ is off and transistor Q ₂ is on.
Therefore, a high potential output is generated at the output terminal 5, and the emitter of the transistor _Q4 is also at a high potential (for example, 5V
-0.6V=4.4V). Here, the base voltage of transistor _Q1 is set to -5V or less so that even if the output of NAND gate element G is reversed and becomes a high potential (5V), the base voltage of transistor _Q1 will not be saturated. . Therefore, the capacitor C _T is charged so that the voltage between its electrodes V _CT (0V) - (base voltage of the transistor Q ₁ ). Further, the current of the constant current source S is set so that the voltage drop across the resistor _R2 is 4.4V, so that the emitter potential of the transistor _Q4 does not become a negative potential.

Next, when a trigger pulse is applied to the input terminal ₁ as shown in FIG. state, transistor Q ₂ is in the off state, transistor Q ₃ is in the off state, and the emitter potential of transistor Q ₄ is 〓0V
Therefore, even if input terminal 1 returns to the high potential state,
The output of the NAND gate element G remains at a high potential and in an inverted state. The capacitor C _T starts charging from the moment the output of the NAND gate element G is reversed, and the base voltage of the transistor Q ₁ is changed by the time constant τ=
As C _T ×R _T begins to fall as shown in FIG. 4C. When the base potential of transistor Q ₁ becomes lower than the base potential of transistor Q ₂ due to this drop, transistor Q ₁ turns off, transistor Q ₂ turns on, and the emitter potential of transistor Q ₄ becomes 〓5V, resulting in NAND The output of the gate element G becomes 〓0V, and the output of the NAND gate element G
The output of transistor Q ₁ is passed through capacitor C _T
positive feedback to the base of the current and returns to a stable state.

Therefore, an output pulse having an output width T proportional to the time constant τ is obtained at the output terminal 5.

Generally, in such a monostable multivibrator, the gate circuit 2 and the inverter circuit 4 are
Since it is composed of switch elements, the voltage gain of the circuit is high. Also, the time required to return from an unstable state to a stable state must be short. Therefore,
For example, in a monostable multivibrator that uses a non-saturated inverter circuit consisting of a differential amplifier as shown in Figure 1, a bias circuit consisting of transistor Q ₂ is connected to the base of transistor Q ₁ in order to shorten the recovery time. Connected.

As mentioned above, due to the charge stored in the capacitor C _T during the unstable period, the voltage V _T between the electrodes of the capacitor C _T increases by the amount of the change in the output voltage of the NAND gate element just before returning to the stable state. . When returning to a stable state, when the output of the NAND gate element becomes 0V again, a negative voltage is applied to the emitter of the transistor _Q3 by the amount that the output of the NAND gate element G has changed via the capacitor _CT . will be done. This results in transistor Q ₃
turns on. However, the transistor _Q3 constitutes an emitter follower with the resistor R _T , and can be regarded as almost a constant voltage source, and the NAND gate element G
If TTL is used, it can be regarded as a constant voltage source, so the emitter current of transistor _Q3 discharges the charge in capacitor C _T in a very short time, and the amount of charge in capacitor C _T becomes the amount of charge in a stable state. That is, the transistor _Q3 acts to return the charge amount of the capacitor C _T to the stable state charge amount in a short time, and when the stable state is returned, the capacitive load is being driven.

On the other hand, the lead wire and wiring conductor of the capacitor _CT act as an inductor as a lumped constant, and this inductor and the capacitor _CT constitute a series resonant circuit. This series resonant circuit consists of transistor Q ₃
Since it is terminated with an emitter follower consisting of a constant voltage source, the Q of the so-called resonant circuit becomes high. Since this is driven by the rectangular wave output from the NAND gate element G that contains many harmonics,
Ringing is likely to occur in the current flowing through the capacitor _CT . Transistor Q ₃ constitutes an emitter follower, but since emitter follower is not an ideal constant voltage source, the ringing current flowing through capacitor C _T during the transition period when transistor Q ₃ changes from off to on is the base of transistor Q ₁ . The time it takes to return to a stable state varies.
Therefore, in addition to not being able to obtain an output pulse having a stable pulse width T, ringing appears at the low level of the output pulse. In addition, as shown in Figure 1, when transistors are used in non-saturation for high-speed operation, there is a moment when all elements become active during the transition period when returning to a stable state, causing positive feedback. Therefore, the ringing that occurs when all the elements are in the active state is amplified. As mentioned above, in conventional monostable multivibrators, the voltage gain of each circuit is high, the capacitive load must be driven at the time of recovery, and positive feedback operation is required, resulting in unstable operation and Problems such as ringing may occur in the waveform. Second
Figures A and B respectively show the output waveform of the output terminal 5 and the base voltage waveform of the transistor Q when ringing occurs.

When such ringing occurs, if the repetition frequency of the trigger pulse applied to the input terminal 1 changes, the pulse width of the output pulse at the output terminal 5 will not be constant and will change. The pulse counting method
When applied to an FM detection circuit, distortion will occur in the detection output.

The purpose of this invention is to provide a monostable multivibrator that solves the above problems.

FIG. 2 is a circuit diagram showing an embodiment of this invention. In FIG. 3, the same components as in FIG. 1 are given the same reference numerals, and the difference from FIG. 1 lies in the configuration in which a resistor Rd is connected in series with a capacitor _CT .

In other words, in order to eliminate the instability of the operation shown in Figure 1, it is necessary to avoid driving only the capacitive load at the time of recovery and to lower the high-frequency closed circuit gain, but in this design, it is necessary to insert the resistor Rd. This simple configuration can satisfy these two conditions and prevent ringing from occurring.

In other words, by inserting a resistor Rd between the output terminal of the NAND gate element G and the capacitor C _T , the input capacitance and resistance of the circuit after the capacitor C _T are
Rd forms a low-pass filter, reducing the high-frequency closed circuit gain, and at the same time lowering the Q of the resonant circuit formed by the capacitor C _T and an inductor such as a lead wire, thereby preventing the occurrence of ringing. . In addition, since the inductor component that makes up the resonant circuit is very small, the resistance Rd only needs to have a low resistance value, which is much smaller than the resistance R _T. It has no effect.

Note that the resistance value of the resistor Rd must be selected to a value that does not affect the time constant determined by the resistance value of the resistor R _T and the capacitance value of the capacitor C _T . This is because if this resistance value is chosen to be too large, instantaneous recovery from an unstable state to a stable state becomes difficult. According to experimental considerations, when the monostable multivibrator of the present invention is used in a pulse count type FM detection circuit, the recovery time must be less than a few tenths of the output pulse width, so the resistance value of the resistor Rd is The value should be selected taking this into consideration.

As described above, according to this invention, the characteristics of a monostable multivibrator can be stabilized and problems such as ringing can be prevented with a simple configuration in which a minute resistor is inserted in series with the capacitor of the differentiating circuit.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of an example of a conventional monostable multivibrator, Fig. 2 is a signal waveform diagram of each part of Fig. 1, and Fig. 3 is a circuit diagram showing an embodiment of this invention.
FIG. 4 is a waveform diagram for explaining the operation of FIG. 1. 2... Gate circuit, 3... Differential circuit, 4... Inverter circuit, R _T ... First resistor, _CT ... Capacitor, Rd... Second resistor.

Claims (1)

[Scope of utility model registration request] a gate circuit to which a trigger signal is supplied as one input; a differentiating circuit comprising a first resistor and a capacitor and to which the output of the gate circuit is supplied; In a monostable multivibrator consisting of an inverter circuit as the other input of the circuit,
A monostable characterized in that a second resistor having a resistance value that does not affect the time constant determined by the resistance value of the first resistor and the capacitance value of the capacitor is connected in series with the capacitor. Multi vibrator.