JPH0321074Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a Schmitt circuit, and particularly to one suitable for a system in which the power supply voltage fluctuates.

[Conventional technology]

A Schmitt circuit is a circuit that converts a continuously changing sinusoidal input into a square wave having a constant relationship with the input waveform, and one example of the known circuit is the one shown in FIG.

In the circuit shown in FIG. 3, it is assumed that when the input signal V _IN applied to the input terminal is 0V, the transistor Tr ₁ is off and the transistor Tr ₂ is on. When the input signal V _IN changes as shown in Figure 4 and reaches the upper trigger voltage threshold voltage V _SH , the feedback action of the resistor R ₅ quickly turns on the transistor Tr ₁ , thereby turning on the transistor Tr 1. ₂ is turned off. Then, when the input signal V _IN drops to the lower trigger voltage V _SL , transistor Tr ₁ is turned off and at the same time transistor Tr ₂ is turned off.
is turned on. In this way, as shown in Fig. 4, the output terminal outputs a high level signal synchronously when the input signal V _IN reaches the threshold voltage V _SH , and when it drops to the lower trigger voltage V _SL . A square wave that is inverted and becomes an "L" level is output. Each of the above trigger voltages V _SH , V _SL
is determined by the power supply voltage Vcc and the resistance division ratio of the resistors R ₂ , R ₃ , R ₄ , R ₅ , and R ₆ and is expressed by the following equations (1) and (2).

V _SH = R ₅ / R ₅ + R ₆ Vcc … (1) V _SL = 1/1 + R ₃ + R ₄ + R ₅ / (R ₃ + R ₄ ) R ₅ R ₂ Vcc … (2) [Problem that the invention aims to solve point] However, as is clear from the above equations (1) and (2),
If a battery B or the like that is likely to fluctuate as the power supply voltage Vcc is used, the trigger voltages V _SH and V _SL will change as the voltage fluctuates, so the same input signal V _IN will There was a problem in that the waveform of the output square wave was not uniquely determined even when it was input.

In addition, since the resistance R ₆ in the output circuit section has a value related to the threshold voltage V _SH , there are restrictions on the loads that can be connected to the output terminal, and it is not recommended to directly connect loads such as light emitting diodes and relay circuits. I can't. Therefore, in order to use a relay circuit or the like as a load, a transistor Tr ₄ for driving the relay circuit Ry is required as shown in FIG. 5, which is a problem.

Furthermore, as shown in FIG. 5, it is possible to provide a Zener diode ZD in order to stabilize the threshold voltage V _SH mentioned above, but in that case, a transistor Tr ₃ and resistors R ₈ and R ₉ are required. There is a problem that the circuit becomes complicated.

In other words, when the voltage is made constant by resistor R ₁₂ and Zener diode ZD, the next stage transistor Tr ₃
In order to completely turn off the transistor Tr ₂
When the transistor Tr ₃ is off, it is necessary to connect the transistor Tr 3 and the resistor R ₈ to the same potential so that the base current of the transistor Tr ₃ becomes zero. Therefore, the emitter of the transistor _Tr3 must be connected to the connection point between the Zener diode ZD and the resistor _R12 . Further, when the transistor Tr ₂ is on, the current passing through the emitter-base of the transistor Tr ₃ and the resistor R ₃ also affects the threshold level, so it is necessary to keep the voltage constant. However, in any of the above cases, the emitter of transistor Tr ₃ cannot be directly connected to the power supply, and in order to drive the relay circuit Ry,
Furthermore, the transistor Tr ₄ and the resistors R ₁₀ and R ₁₁ are required, which complicates the circuit described above.

An object of the present invention is to provide a Schmitt circuit that operates stably even when the power supply voltage fluctuates.

[Means for solving problems]

In order to achieve the above object, the present invention provides a circuit whose base is connected to an input terminal, whose emitter is grounded through a first resistor, and whose collector is connected to the output of a constant voltage circuit through a second resistor. a second transistor whose base is connected to the collector of the first transistor and grounded via a third resistor, whose emitter is connected to the emitter of the first transistor and whose collector is connected to the output terminal; A load is connected between a power supply and the output terminal, and the second transistor is set to be used in an active range for the load current.

[Effect]

By configuring it in this way, the first stage transistor (first transistor) of the Schmitt circuit is made to have a constant voltage, and the second stage transistor (second transistor) is in the active region (unsaturated region) when turned on. Since I am trying to make it work with
The threshold voltage becomes stable regardless of fluctuations in the power supply voltage, and thereby the circuit operation becomes stable.

In addition, only the first transistor has a constant voltage to stabilize the circuit operation, and the load can be driven by the power supply voltage, so the operating voltage of the first and second transistors can be lowered and the load can be sufficiently high. Can be driven at constant current using power supply voltage. Furthermore, since the constant voltage circuit only needs to have a small capacity, loss can be reduced.

〔Example〕

The present invention will be explained below based on examples.

FIG. 1 shows a circuit diagram of an embodiment in which the present invention is used in a circuit for driving a light emitting diode (LED). As shown in the figure, the base of the first stage transistor Tr ₁ is connected to the input terminal T ₁ via the resistor R ₁ , the emitter is grounded via the resistor R ₅ , and the collector is connected via the resistor R _2. It is connected to the connection point of Zener diode ZD and resistor _R7 , which form a constant voltage circuit. The other end of the Zener diode ZD is grounded,
The other end of resistor _R7 is connected to battery B via power supply terminal T _B.
connected to the positive terminal of the

The base of the second stage transistor Tr ₂ is connected to the collector of the transistor Tr ₁ via a resistor R ₃ and grounded via a resistor R ₄ . Also,
The emitter is commonly connected to the emitter of the transistor _Tr1 , and the collector is connected to the output terminal _T0 .

And a light emitting diode as a driving load
The LED is connected between the power supply terminal T _B and the output terminal T ₀ .

The operation of the embodiment configured in this way will be described next. When the input signal V _IN of the voltage applied to the input terminal T ₁ is 0V, the transistor Tr ₁
has been turned off. At this time, transistor Tr ₂ is in the active region and operates as an emitter follower.
If the voltage drop between the base and emitter of Tr ₂ is ignored, the voltage V _R5 between the terminals of resistor R ₅ is expressed by the following equation (3). Note that in the same equation, Vz is the output voltage of the constant voltage circuit, that is, the voltage between the terminals of the Zener diode ZD.

V _R5 = R ₄ / R ₂ + R ₃ + R ₄ Vz...(3) As is clear from equation (3), while Tr ₁ is off, the voltage
Since V _R5 is a constant value regardless of the power supply voltage Vcc, the emitter current I _E of transistor Tr ₂ is calculated by the following formula:
It becomes a constant value as shown in (4).

I _E = V _R5 / R ₅ = constant value...(4) Therefore, the collector current Ic of transistor Tr ₂
is a constant value due to the relationship Ic = I _E , and the light emitting diode LED of the load connected to the collector has the following value:
A constant current is caused to flow, and the brightness of the LED becomes stable regardless of fluctuations in the power supply voltage Vcc.

Next, when the input signal V _IN exceeds the voltage V _R5 ,
Transistor Tr ₁ starts to turn on, and transistor Tr ₂ is rapidly turned off by Schmitt operation.
This causes the voltage V _R5 to drop, so that the transistor Tr ₁ is completely turned on. The voltage V _R5 at this time has a value expressed by the following equation (5), and the transistor Tr ₂ is kept off until the input signal V _IN falls below the voltage V _R5 .

V _R5 = 1/1 + R ₃ − R ₄ + R ₅ / (R ₃ + R ₄ ) R ₅ R ₂ Vz (5) As described above, according to the embodiment in FIG. By driving Tr ₁ with a constant voltage and operating the second stage transistor Tr ₂ in the unsaturated region (active region) when it is on, the collector current (load current) is made to be a constant current. The following effects can be obtained.

(1) The threshold voltage V _R5 for switching the output state is stabilized without being affected by fluctuations in the power supply voltage Vcc, and the operation of the Schmitt circuit becomes stable.

(2) Since the collector resistance of the transistor Tr ₂ (LED in this embodiment) can be set independently of the threshold voltage, it can be connected even to a load whose resistance value changes.

(3) Since the collector current of the transistor Tr ₂ is a constant current, it is possible to stably drive a load such as a light emitting diode LED even if it is directly connected to a power supply whose voltage fluctuates.

FIG. 2 shows a circuit diagram of an embodiment in which the present invention is used in a circuit for driving a relay circuit. It should be noted that parts having the same functions and configurations as those in FIG.

As shown in Figure 2, power terminal T _B and output terminal
Connect voltage dividing resistors R ₈ and R ₉ in series between T ₀ and
The voltage at this connection point turns on and off the transistor Tr ₃ to drive the relay circuit Ry. That is, in order to turn on and off the power supply voltage applied to the relay circuit Ry, the emitter of the transistor _Tr3 is necessarily directly connected to the power supply terminal TB. In addition, resistor R ₈ is connected to transistor Tr ₃ when transistor Tr ₂ is off.
Since the base current of the transistor Tr3 is reduced to zero and the transistor is completely turned off, it needs to be connected to the same potential as the emitter of the transistor _Tr3 .

In this embodiment, the output terminal _T0 is a constant current output, so it can be connected to a power supply directly or via a resistor, and this does not affect the Schmitt characteristics.

Therefore, according to this embodiment, in addition to the effects (1) and (2) of the embodiment shown in FIG. It has the effect of being able to

[Effect of idea]

As explained above, according to the present invention, the first stage transistor of the Schmitt circuit is driven at a constant voltage, and the second stage transistor is operated in the active region when it is on, so that the threshold voltage is This has the effect of being independent of voltage and achieving stable operation even if the power supply voltage fluctuates.

In addition, only the first stage transistor has a constant voltage to stabilize circuit operation, and the load can be driven by the power supply voltage, so the operating voltage of the first and second stage transistors can be lowered, and the load can be driven at a constant current with a sufficiently high power supply voltage. Furthermore, since the constant voltage circuit only needs to have a small capacity, loss can be reduced.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of one embodiment of the present invention, Figure 2 is a circuit diagram of another embodiment of the present invention, Figure 3 is a circuit diagram of a conventional example, and Figure 4 is Figure 3 of a conventional example. The operation explanatory diagram, FIG. 5, is a circuit diagram of a comparative example. Tr ₁ ...First transistor, Tr2 _... Second transistor, _R2 , _R4 , _R5 , _R6 ...Resistor, ZD...
...Zena diode, B...Battery.

Claims (1)

[Scope of claim for utility model registration] The base is connected to the input terminal, and the emitter is connected to the first
a first transistor which is grounded through a resistor and whose collector is connected to the output of the constant voltage circuit through a second resistor; a third resistor whose base is connected to the collector of the first transistor; and a second transistor whose emitter is connected to the emitter of the first transistor and whose collector is connected to the output terminal, and the constant voltage circuit has a resistor connected between the power supply and ground. It consists of a series circuit of a Zener diode and a Zener diode.
The connection point voltage between the resistor and the Zener diode is set as the output voltage, a load is connected between the power supply and the output terminal, and the second transistor is set to be used in the active range for this load current. Characteristic Schmitt circuit.