JPH01109916A - Schmitt comparator circuit - Google Patents

Abstract

PURPOSE:To obtain a Schmitt comparator while keeping a reference voltage to be constant by varying the amplifier gain in response to the output level so as to vary the input threshold value. CONSTITUTION:A current I0 flowing through input differential pair transistors(TRs) Q1, Q2 is expressed as I0=V3/R4 when the output is at a low level and as I0=V3/R4//R7 when the output is at a high level, where V3 is an emitter voltage of a TR Q3. Let the base-emitter voltage when the TR Q4 starts its conduction be VBE4, then the threshold value is lower when the output is at a high level in comparison with that when the output is at a low level in selecting R1 to be VBE4/R1<V3/(2XR4) to constitute a Schmitt comparator. In varying the amplifier gain in response to high/low output, since the input threshold value is changed, the Schmitt comparator is obtained while the reference voltage is kept constant.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a Schmitt comparator, that is, a Schmitt comparator, which operates when the input voltage changes from a low voltage side to a high voltage side and when the input voltage changes from a high voltage side to a low voltage side. The present invention relates to voltage comparator circuits with different threshold values.

Prior Art Conventional Schmitt comparators vary the input threshold by varying the voltage on the reference side of the comparator depending on whether the output is at a high or low level.

FIG. 3 shows a circuit example of a conventional Schmitt comparator. In the figure, 1 is a power supply terminal, 2 is a ground terminal, 3 is an input terminal, 4 is an output terminal, 5 is a bias source, R1-R6,
R8 is a resistor, and Q+-Qs are transistors. The comparator of the circuit shown in FIG. (However, Rs//Rs indicates a parallel combined resistance.) Problems to be Solved by the Invention In conventional Schmitt comparators, the input threshold is changed by changing the voltage on the reference side, so multiple comparators When the reference voltage is shared between the two, there is a problem in that the threshold value of the comparator changes every time the input of one comparator changes.

Means for Solving the Problems The present invention provides an emitter of a first NPN transistor whose base is an input terminal and whose collector is connected to a power supply terminal through a first resistor; the emitters of the second NPN transistors connected to the power supply terminal through the resistor and the ground through the third resistor are commonly connected, and the emitters are connected to the ground through the fourth resistor;
A third NPN whose base is connected to a constant voltage source.) A third NPN transistor whose base is connected to a constant voltage source.
N) A first PNP that is connected to the collector of the transistor and whose emitter is connected to the power supply terminal.) A first PNP that is connected to the collector of the transistor and whose emitter is connected to the power supply terminal.) The collector of the transistor is grounded through the fifth and sixth resistors connected in series. A fourth transistor is provided with a comparator whose output terminal is the midpoint of the transistor, whose base is connected to the output terminal, whose emitter is grounded, and whose collector is connected to the emitter of the third NPN transistor through a seventh resistor. This is a Schmitt comparator circuit with an added configuration.

According to the present invention, the offset voltage changes depending on the amplifier gain, so by adding an offset between the input threshold of the comparator and the reference voltage, the amplifier gain can be changed depending on whether the output voltage is high or low. , the input threshold can be changed without changing the reference voltage.

Embodiment FIG. 1 is a circuit diagram of a Schmidt comparator according to an embodiment of the present invention. In the figure, circuit elements indicated by respective symbols correspond to those in FIG. 3. If the emitter voltage of transistor Q3 is v3, the current flowing through the input differential pair Ql and Q2 is 10
When the output is low level, Io=V3/R+, and when the output is high level, Io=Vs/R4//R7. Now, assuming that the base-emitter voltage when transistor Q4 starts conducting is VB+14, RI is VBE4/R1<
By selecting V3/(2X R4), the threshold value will be lower when the output is at a high level than when the output is at a low level, and a Schmitt comparator can be constructed.

Figure 2 shows the input voltage of the circuit of Figure 1 and transistor Q4.
The input/output characteristics of the base voltage are shown. In the figure, 6 is the input/output characteristic when the output is at a high level, and 7 is the input/output characteristic when the output is at a low level. Also, V ref is the reference voltage, VT
HI, VTH2c Reso tL, input threshold at output high level and output low level. As shown in the figure, the input threshold value can be changed by changing the gain of the amplifier depending on whether the output is high or low.

Effects of the Invention In the present invention, since the input threshold value can be changed by changing the amplifier gain depending on whether the output is high or low, a Schmitt comparator can be configured while keeping the reference voltage constant.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a Schmidt cota according to an embodiment of the present invention. 1...Power terminal, 2...Ground terminal, 3
・Old...Input terminal, 4...Output terminal, Ql,
Q2. Q5. Q6, old...NPN transistor,
Q4...PNP transistor, Rl + R2
・R3・R4・R5・R6・R7, R8°゛・・・Resistance.

Claims (1)

[Claims] The common emitters of the first and second transistors in the differential pair configuration are provided with a current source having a circuit connection that allows a third transistor to connect and disconnect one resistor circuit of the parallel resistors;
A Schmitt comparator circuit configured to drive a transistor with an output potential.