JPS5926671Y2 - schmitt circuit - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a Schmitt circuit which has its own detection function and whose threshold value does not change due to fluctuations in the power supply.

Schmitt circuits, which have been commonly used as hysteresis circuits, require a rectifier circuit in front of them to perform hysteresis operation according to the level of an AC signal, and perform AC-DC conversion. However, there were drawbacks such as the increase in man-hours being completely eliminated, and the rise and fall thresholds being affected by power fluctuations.

For example, a circuit as shown in FIG. 1 is used as a general Schmitt circuit.

That is, in FIG. 1, Ql and Q2 are transistors,
R1, R2゜R3 are resistors that constitute the bias circuit, R4
is the collector resistance of transistor Q2, R5 is the collector resistance of transistor Q1. The common emitter resistance of Q2 is shown, and the base of transistor Q1 where transistor Q0 is turned on is shown.
When the emitter voltage is VBEl, and the transistor Q2 is ON, the voltage generated across the common emitter resistor R5 is ■5.
Then, voltages VBEI and V5 both have the same polarity with respect to the ground potential, so transistor Q1 is turned on.
The base-earth voltage (rising threshold) is V B
This results in EI + Vs.

Furthermore, since the voltage 5 is (VBE2 is the voltage between the base and emitter of the transistor Q2, and Vcc is the power supply voltage), it can be seen that the rising threshold is influenced by the power supply voltage Vcc.

Similarly, the falling threshold and the history width (difference between the rising and falling thresholds) are also affected by the power supply voltage.

Further, as a circuit for obtaining a history corresponding to the level of an AC signal, there is a circuit as shown in FIG.

In FIG. 2, the circuit within the range indicated by a dotted line frame I is exactly the same as that in FIG. 1, and a rectifier circuit indicated by a dotted line frame II is placed in front of the base of the transistor Q1, that is, the input of this Schmitt circuit.

This rectifier circuit consists of capacitors C1 and C2, and diode D.
1. D2 and resistors R6 and R7 form a voltage doubler rectifier circuit, the output of which is divided by resistors R6 and R7 and input to the base of transistor Q1.

In the Schmitt circuit configured in this way, the part of the rectifier circuit indicated by the dotted line frame II does not respond to the input DC component due to the capacitance of the capacitor C1, and also does not respond to the AC input with a very long period. Therefore, the capacitor C1 is required to have a capacitance such that its impedance is sufficiently low.

This capacitor C1 has a capacitance value that is necessary when using the voltage doubler rectification method in the rectifier circuit described above, and if the diode D2 is shorted and a half-wave rectification method is used in order to eliminate the capacitor C1, the rising threshold of this Schmitt circuit will become significantly higher. .

This apparently deteriorates the sensitivity of the AC hysteresis circuit.

The reason for this is that half-wave rectification reduces rectification efficiency, and the voltage generated across the common emitter resistor R5 (when transistor Q1 is OFF, transistor Q2 is ON, so resistor R5 is This is because the transistor Q1 is biased in the direction in which the base voltage at which it starts operating becomes higher due to the emitter current flowing.

In order to make it easier to understand what has been said above, the operation of the circuit in FIG. 1 and FIG.
At time T1, the level of input signal ■ is low, and at time T2
For a general AC signal whose level increases at It can be seen that there is no function as a Schmitt circuit.

In addition, in the circuit of Fig. 2, as shown in Fig. 4, the output voltage P is zero during the time T1 when the level of the input signal ■ is low, and the output voltage P becomes high level during the time T2 when the level is high, and the input signal It can be seen that it operates as a Schmitt circuit for the level of .

As is clear from the above, in the conventional Schmitt circuit, in order to keep the ON and OFF thresholds of the Schmitt circuit constant, the power supply voltage must be kept constant. It can be seen that it is necessary to redesign the Schmitt circuit when changing the value or changing the history width.

Moreover, it is clear that it is expensive to operate as a Schmitt circuit for both AC and DC use.

The present invention was devised to eliminate the drawbacks of the conventional Schmitt circuit, and has a simple configuration that enables history operation according to the level of an AC signal, can also operate with a DC signal, and is not affected by fluctuations in power supply voltage. It is an object of the present invention to provide a Schmitt circuit that has a small amount of noise and can set an operation history width.

The present invention will be explained below with reference to FIGS. 5 and 6 with reference to embodiments.

The emitter of the NPN transistor Q3 (which inputs the input (2) to the base) is grounded, and its collector is connected to the + side of the power supply V together with the emitter of the transistor Q4 through a parallel circuit consisting of a capacitor C3 and a series circuit consisting of resistors R8 and R9.

Connected to 0. PNP) The collector of transistor Q4 is grounded by diode D through a series circuit of resistor RIO and diode D.

The voltage between diode D and ground is variable resistor VR and resistor Rn.
The voltage is divided by a voltage dividing circuit consisting of a series circuit of , and is positively fed back to the base of the transistor Q3.

Further, a connection point between resistors R8 and R9 is connected to the base of transistor Q4.

According to such a configuration, when the input (2) is zero (more precisely, below the voltage at which the transistor Q3 is turned on), the transistor Q4 is turned off, and the collector output of the transistor Q4 becomes zero.

Next, if input I is an AC signal, transistor Q3
The wave is detected by

That is, it can be considered that the human input signal (2) is rectified between the base and emitter of the transistor Q3, and the rectified current is multiplied by the current amplification factor hFE of the transistor Q3 and output to the collector.

In this case, the current amplification factor hF of a normal small signal transistor is
Since E has a value of about 100 to several 100, the rectifier circuit has higher efficiency and operates at higher speed than a normal rectifier circuit using diodes.

When the above AC signal input ■ is input, the transistor Q
The detected output appearing at the collector of the transistor Q3 becomes a smooth DC output by a smoothing circuit composed of a capacitor C3 and resistors R8 and R9, thereby lowering the base voltage of the transistor Q4 and turning on the transistor Q4.

When transistor Q4 is turned on, its collector current flows through diode D via resistor RIO.

When current flows through diode D, diode D
Depending on the polarity of the zener voltage or forward characteristic, an approximately constant voltage is produced across the diode D, which voltage is almost independent of the current flowing through the diode D and is therefore unaffected by fluctuations in the power supply voltage.

Let the voltage generated across the diode D be Vl.

Voltage ■1 is generated when transistor Q4 is ON, so
This voltage is divided by a resistor R01 and a variable resistor VR as a positive feedback voltage source, and is applied as positive feedback to the base of a transistor Q3.

This positive feedback voltage becomes a forward bias for the transistor Q3 (bias in the direction from the insulating region to the active region), increasing the efficiency of transistor detection in the transistor Q3.

Therefore, even if the input drops somewhat (by the history width), the transistor Q4 can be kept turned on.

In other words, this kind of action allows you to wander through history operations.

Furthermore, as mentioned above, when the transistor Q4 is ON, a voltage is generated across the diode D that provides positive feedback to the transistor Q3, but by adjusting the variable resistor VR, the amount of positive feedback to the transistor Q3 can be changed. The history width of the Schmitt circuit of the lever can be freely varied.

Therefore, since the forward bias of transistor Q3 can be increased or decreased, the detection efficiency of transistor Q3 can be increased or decreased.

By changing the history width, for example, in the case of a noisy input signal I as shown in FIG. 6a, it is possible to demodulate the waveform as shown in FIG. 6 without including noise.

Further, in the above, the diode D may be a normal diode used in the forward direction, but it may also be used for positive feedback using the Zener voltage of the Zener diode.

Alternatively, a light emitting diode may be used in the forward direction so that when there is an input, it lights up to indicate manual effort.

As described above, the Schmitt circuit of the present invention is configured to detect an alternating current signal using the transistor in the first stage, and its output is smoothed and applied to the input of the transistor in the subsequent stage. There is no need to install it in front of the circuit, and it has a simple configuration, so it is possible to reduce costs.

It can also operate with DC signals and can respond to signals with slow periods.

Furthermore, it is stable against power supply fluctuations, and the threshold width can be set independently, so it can cope with noisy input signals.

Therefore, it is useful for amplitude modulation beacon detection circuits, squelch circuits such as tone squelch and noise rectification squelch, applications of these to in-vehicle equipment, signal processing of noisy communication channels, and combinations of data tape records in microcomputers, etc. It has a wide range of applications, including circuits for preventing signal damage caused by noise, and audio transmission/reception switching circuits for transceivers and the like.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing a conventional Schmitt circuit, Figure 2 is a circuit diagram that also obtains a history according to the level of an AC signal, Figure 3 is a diagram showing the waveform of a general input signal, and Figure 4 a. is a diagram showing the response characteristics of the circuit in Figure 3 when the input signal shown in Figure 3 is applied manually to the circuit in Figure 1, and Figure 4 is a diagram showing the response characteristics of the circuit in Figure 2 when the input signal shown in Figure 3 is applied manually. FIG. 5 is a circuit diagram showing an embodiment of the Schmitt circuit according to the present invention; FIG. 6a is a diagram showing the waveform of a noisy input signal; 6 is a diagram showing the waveform of an output signal obtained by processing the input signal shown in FIG. 6a by the Schmitt circuit according to the present invention. Q3... First stage transistor, Q4...
・Late stage transistor, D...Diode, C
3... Capacitor, R8, R9... Resistor.

Claims (1)

[Scope of utility model registration request] The first stage transistor has a first stage transistor whose load is a smoothing circuit consisting of a capacitor and a resistor, and a second stage transistor whose load is a diode. In addition to providing the transistor with a detection function,
The output of the smoothing circuit is input to the input terminal of the transistor in the subsequent stage, and the voltage proportional to the voltage generated between the terminals of the diode is connected to the input terminal of the transistor in the first stage so as to provide positive feedback. Schmitt circuit.