HYSTERESIS CIRCUIT

Abstract: A hysteresis circuit is provided with a thyristor circuit (101) which outputs a low voltage when being in an ON state and outputs a high voltage when being in an OFF state. A first switching circuit (102) turns on the thyristor circuit when an input voltage exceeds the threshold. A second switching circuit (103) turns off the thyristor circuit when the input voltage falls below the threshold.

Description: This invention relates to a hysteresis circuit comprising a positive feedback circuit having first and second input terminals and an output terminal; a first switching circuit having its output connected to the first input terminal, the first switching circuit only turning on the feedback circuit when an input voltage at an input of the first switching circuit exceeds a threshold value of the first switching circuit; and a second switching circuit having its output connected to the second input terminal, the second switching circuit only turning off the feedback circuit when the input voltage at an input of the second switching circuit is lower than a threshold value of the second switching circuit, wherein the threshold voltage of the first switching circuit differs from that of the second switching circuit wherein the inputs of the first and second switching circuits, are connected to each other and to an input terminal of the hysteresis circuit, and wherein, and the respective threshold voltages can be set independently of each other. Such a circuit is described in JP-A-51 86954. The Schmitt Trigger, circuit is well known as a switching circuit which operates in a hysteretic manner. Fig. 1 shows an example of the Schmitt Trigger circuit. As is shown in Fig. 1, the base of an npn transistor Q1 is connected to an input terminal Vin. A resistor R1 has one end connected to the collector of the transistor Q1, and the other end connected to a power source terminal V DD . A resistor R2 has one end connected to the emitter of the transistor Q1 and the other end connected to an earth terminal Vss. The base of an npn transistor Q2 is connected to the collector of the transistor Q1, while the collector of the transistor Q2 is connected to an output terminal Vout. A resistor R3 has one end connected to the collector of the transistor Q2 and the other end connected to the power source terminal V DD . The emitter of the transistor Q2 is connected to the one end of the resistor R2. The maximum hysteresis error of the above circuit is determined by the values of the resistors R1 - R3. The output voltage of the circuit varies from a low voltage (hereinafter called "L") to a high voltage (hereinafter called "H") when the transistor Q1 is turned on, while it varies from "H" to "L" when the transistor Q2 is turned on. That is, when one of the transistors Q1 t and Q2 is on, the other is off. The Schmitt Trigger circuit shown in Fig. 1 operates in a hysteretic manner as shown in Fig. 2. Then, this operation will be explained with reference to Figs. 1 and 2. When the transistor Q2 is on, and the transistor Q1 is off, the output voltage is "L". Thereafter, the input voltage Vin increases, and reaches a value which satisfies the following formula (1). At this time, the transistor Q1 is turned on, and accordingly the transistor Q2 is turned off. Thus, the output voltage varies from "L" to "H". Vin > V th(L → H) = V DD × R2/(R2+R3) + V BEQ1 where V BEQ1 represents the threshold voltage of the transistor Q1, and V th(L → H) that threshold voltage of the Schmitt trigger circuit which is assumed when the output voltage varies from "L" to "H". Further, the earth voltage is applied to the earth terminal Vss, and the saturation voltage of the transistor Q2 is ignored. Thus, the output voltage Vout varies in the order of a, b, c, d, and e in Fig. 2. When the input voltage Vin reduces, and reaches a value which satisfies the following formula (2), the transistor Q2 is turned off, and the transistor Q1 is turned on. Thus, the output voltage varies from "H" to "L". Vin < V th(H → L) = V DD × R2/(R1+R2) + V BEQ1 where V BEQ1 represents the threshold voltage of the transistor Q1, and V th(H → L) that threshold voltage of the Schmitt trigger circuit which is assumed when the output voltage varies from "H" to "L". Further, the earth voltage is applied to the earth terminal Vss, and the saturation voltage of the transistor Q2 is ignored. Thus, the output voltage Vout varies in the order of e, d, f, b, and a in Fig. 2. In the above embodiment, the values of the resistors R1 and R3 are determined so as to satisfy the following formula (3): Vth(L → H) > Vth(H → L.) R1 > R3 This means that the hysteresis characteristic is obtained by setting to different values the threshold voltages Vth(L → H) assumed when the output voltage varies from "L" to "H", and V th(H → L) assumed when the output voltage varies from "H" to "L". However, as is evident from the formula (1), the threshold voltage V th(L → H) assumed when the output voltage varies from "L" to "H" is determined by the resistance ratio between the resistors R1 and R3. Similarly, as is evident from the formula (2), the threshold voltage V th(H → L) assumed when the output voltage varies from "H" to "L" is determined by the resistance ratio between the resistors R1 and R2. Accordingly, the threshold voltages V th(L → H) and V th(H → L) depend upon each other, and hence must be set to values falling within a very narrow range. In addition, in a case where the Schmitt Trigger circuit comprises an integrated circuit, great variations may exist among Schmitt Trigger circuits if the ratio of the largest resistance of the resistors R1 - R3 to the smallest resistance of them is set to a large value. Further, as can be understood from the formulas (1) and (2), the threshold voltages V th(L → H) and V th(H → L) vary in accordance with a change in the power source voltage V DD . Hence, a constant voltage source dedicated for supplying the threshold voltages is required in the integrated circuit. It is the object of the invention to provide a hysteresis circuit in which the threshold voltage V th(L → H) assumed when the output voltage varies from "L" to "H", and the threshold voltage V th(H → L) assumed when the output voltage varies "H" to "L" are set independently of each other, and do not vary in accordance with a change in the power source voltage of the circuit. According to the invention, the hysteresis circuit initially defined is characterised in that the positive feedback circuit is a thyristor circuit operable to output a low voltage from the output terminal when in an on-state and to output a high voltage from the output terminal when in a off-state, wherein the thyristor circuit comprises a first transistor having an emitter connected to the output terminal, a collector connected to the second input terminal and a base connected to the first input terminal, a second transistor having a collector connected to the base of the first transistor, an emitter connected to an earth terminal, and a base connected to the second input terminal, a first resistor connected between the base and emitter of the first transistor, a second resistor having one end connected to a power source terminal and the other end connected to the output terminal, and a third resistor connected between the base and emitter of the second transistor, the respective threshold voltages being defined independently of the voltage on said power source terminal by a respective pair of resistors constituting an input voltage divider in the first and second switching circuits, respectively. As described above, the threshold voltages of the first and second switch circuits can be set independently of each other, and do not vary in accordance with a change in the voltage source, so that the conditions of hysteresis can be determined in a simple manner. For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Fig. 1 is a circuit diagram, showing a conventional hysteresis circuit, Fig. 2 is a view, showing a hysteresis characteristic of the hysteresis circuit of Fig. 1, Fig. 3 is a circuit diagram, showing a hysteresis circuit according to an embodiment of the invention, Fig. 4 is a circuit diagram, showing a switching circuit in detail, and Fig. 5 is a view, showing a hysteresis characteristic of the hysteresis circuit of Fig. 3. The embodiment of the invention will be explained in detail with reference to the drawings. Fig. 3 shows a hysteresis circuit according to the embodiment. A pnp transistor Q1 has an emitter connected to an output terminal Vout. An npn transistor Q2 has a collector connected to the base of the transistor Q1, and an emitter connected to an earth terminal Vss. A resistor R1 has one end connected to the base of the transistor Q1, and the other end connected to the emitter of the same. A resistor R2 has one end connected to a power source terminal V DD , and the other end connected to the output terminal Vout. A resistor R3 has one end connected to the base of the transistor Q2, and the other end connected to the emitter of the same. The transistors Q1 and Q2, and the resistors R1 - R3 form a thyristor circuit 101. The resistors R1 and R3 are provided for preventing malfunction of the circuit due to leak current, while the resistor R2 is the load resistor of the circuit. The thyristor circuit 101 is a positive feedback circuit, so that it cannot shift to an on-stage independently. To turn on the thyristor circuit, a switching circuit 102 is provided. The input of the switching circuit 102 is connected to the input terminal Vin, and the output of the same is connected to the base of the transistor Q1 of the thyristor circuit 101. Further, a switching circuit 103 is provided for turning off the thyristor circuit 101. The input of the switching circuit 103 is connected to the input terminal Vin, while the output of the same is connected to the base of the transistor Q2 of the thyristor circuit 101. The threshold voltages of the switching circuits 102 and 103 can be determined independently of each other. If these threshold voltages are set to different values, hysteretic operation can be performed. Fig. 4 shows in detail the switching circuits 102 and 103 incorporated in the hysteresis circuit shown in Fig. 3. The switching circuit 102 comprises an npn transistor Q3 and resistors R4 and R5. The transistor Q3 has a collector connected to the base of the transistor Q1, and an emitter connected to the earth terminal Vss. The resistor R4 has an end connected to the base of the transistor Q3, and the other end connected to the earth terminal Vss. The resistor R5 has an end connected to the base of the transistor Q3, and the other end connected to the input terminal Vin. The transistor Q3 is provided for turning on the thyristor 101, i.e., for shifting the output voltage from "H" to "L". The resistors R4 and R5 are provided for determining the threshold voltage of the switching circuit 102. The switching circuit 103 comprises transistors Q4 and Q5 and resistors R6 - R8. The transistor Q4 is an npn type, and has a collector connected to the base of the transistor Q2, and an emitter connected to the earth terminal Vss. The resistor R6 has an end connected to the base of the transistor Q4, and the other end connected to the power source terminal V DD . The transistor Q5 is an npn type, and has a collector connected to the base of the transistor Q4, and an emitter connected to the earth terminal Vss. The transistor Q5 serves as an inverter. The resistor R7 has an end connected to the base of the transistor Q5, and the other end connected to the earth terminal Vss. The resistor R8 has an end connected to the base of the transistor Q5, and the other end connected to the input terminal Vin. The transistor Q5 inverts the logical values of the switching circuits 102 and 103. The transistors Q4 and Q5 turn off the thyristor circuit 101, i.e., shift the output voltage from "L" to "H". The resistor R6 is a bias resistor for the transistor Q4, while the resistors R7 and R8 determine the threshold voltage of the switching circuit 103. With the above structure, the threshold voltages of the switching circuits 102 and 103 can be determined independently of each other in accordance with a change in the input signal. Accordingly, the hysteresis width can be set easily and freely. The hysteresis circuit shown in Fig. 4 performs a hysteretic operation as shown in Fig. 5. This operation will be explained with reference to Figs. 4 and 5. When the thyristor circuit 101 is off, the output voltage is "H". Then, if the input voltage Vin is increased, and reaches a value satisfying the following formula (4)', the transistor Q3 will be turned on. V BEQ3 < Vin × R4/(R4 + R5) where reference code V BEQ3 represents the threshold voltage of the transistor Q3. In other words, when the input voltage Vin reaches the threshold voltage V th(H → L) of the switching circuit 102, i.e., when it satisfies formula (4) given below, the transistor Q3 is turned on, and the thyristor circuit 101 is also turned on (i.e., the transistors Q1 and Q2 are turned on). Accordingly, when the input voltage Vin exceeds the threshold voltage V th(H → L) , the output voltage is shifted from "H" to "L". Vin > V th(H → L) = V BEQ3 × (R4 + R5)/R4 Thus, the output voltage Vout varies in the order of a, b, c, d, and e in Fig. 5. When the thyristor circuit 101 is once turned on, it is kept on by virtue of its positive feedback function, irrespective of the on/off state of the transistor Q3 (i.e., even though the input voltage Vin is lower than V th(H → L) ). On the other hand, when the thyristor circuit 101 is on, the output voltage is "L". Then, if the input voltage Vin is decreased, and reaches a value satisfying the following formula (5)', the transistor Q5 will be turned off. V BEQ5 > Vin × R7/(R7 + R8) where reference code V BEQ5 represents the threshold voltage of the transistor Q5. In other words, when the input voltage Vin reaches the threshold voltage V th(L → H) of the switching circuit 103, i.e., when it satisfies formula (5) given below, the transistor Q5 is turned off, and the transistor Q4 is turned on, whereby the thyristor circuit 101 is turned off (i.e., the transistors Q1 and Q2 are turned off). Accordingly, when the input voltage Vin is lower than the threshold voltage V th(L → H) , the output voltage is shifted from "L" to "H". Vin < V th(L → H) = V BEQ5 × (R7 + R8)/R7 Thus, the output voltage Vout varies in the order of e, d, f, b and a in Fig. 5. When the thyristor circuit 101 is once turned off, it is kept off by virtue of its positive feedback function, irrespective of the on/off state of the transistor Q4 (i.e., even though the input voltage Vin exceeds V th(L → H) ). The threshold voltage V th(H → L) assumed when the output voltage varies from "H" to "L" is determined by the resistance ratio of the resistor R4 to the resistor R5, while the threshold voltage Vth(L → H) assumed when the output voltage varies from "L" to "H" is determined by the resistance ratio of the resistor R7 to the resistor R8. Thus, such a hysteresis operation (a -> b -> c -> d -> e -> d -> f -> b -> a) as shown in Fig. 5 is performed by setting the resistance ratios to satisfy the following formula (6). In this case, VBEQ3 ≃ VBEQ5. R8/R7 < R5/R4 1+R8/R7=(R7 + R8)/R7 < 1+R5/R4=(R4 + R5)/R4 If the resistance ratios are set to satisfy the formula (6), the threshold voltage V th(L → H) assumed when the output voltage varies from "L" to "H" and the threshold voltage Vth(H → L) assumed when the output voltage varies from "H" to "L" will be determined independently of each other, as is evident from the formulas (4) and (5). In other words, the resistance ratio of the resistor R4 to the resistor R5 and that of the resistor R7 to the resistor R8 can be determined at a high degree of freedom, to determine the threshold voltages. Further, since the threshold voltages V th(H → L) and V th(L → H) vary independently from a change in the power source voltage V DD , no constant voltage sources are required. It is needless to say that the threshold voltages V th(H → L) and V th(L → H) may be set to vary in accordance with the power source voltage V DD .

Claims: A hysteresis circuit comprising: a positive feedback circuit (101) having first and second input terminals and an output terminal (Vout); a first switching circuit (102) having its output connected to the first input terminal, the first switching circuit only turning on the feedback circuit when an input voltage at an input of the first switching circuit exceeds a threshold value of the first switching circuit; and a second switching circuit (103) having its output connected to the second input terminal, the second switching circuit only turning off the feedback circuit when the input voltage at an input of the second switching circuit is lower than a threshold value of the second switching circuit, wherein the threshold voltage of the first switching circuit differs from that of the second switching circuit, wherein the inputs of the first and second switching circuits are connected to each other and to an input terminal (Vin) of the hyteresis circuit, and wherein the respective threshold voltages can be set independently of each other, characterised in that: the positive feedback circuit is a thyristor circuit operable to output a low voltage from the output terminal when in an on-state and to output a high voltage from the output terminal when in a off-state, wherein the thyristor circuit comprises a first transistor (Q1) having an emitter connected to the output terminal, a collector connected to the second input terminal and a base connected to the first input terminal, a second transistor (Q2) having a collector connected to the base of the first transistor, an emitter connected to an earth terminal (Vss), and a base connected to the second input terminal, a first resistor (R1) connected between the base and emitter of the first transistor, a second resistor (R2) having one end connected to a power source terminal and the other end connected to the output terminal, and a third resistor (R3) connected between the base and emitter of the second transistor the respective threshold voltages being defined independently of the voltage on said power source terminal by a respective pair of resistors (R4,R5;R7,R8) constituting an input voltage divider in the first and second switching circuits, respectively. . A hysteresis circuit according to claim 1, wherein the first switching circuit comprises a transistor (Q3) having a collector connected to the first input terminal and an emitter connected to an earth terminal, a first resistor (R4) having an end connected to the base of the transistor and the other end connected to the earth terminal, and a second resistor (R5) having an end connected to the base of the transistor and the other end connected to an input terminal for receiving an input signal, and wherein the second switching circuit comprises a first transistor (Q4) having a collector connected to the second input terminal and an emitter connected to an earth terminal, a first resistor (R6) having an end connected to the base of the first transistor and the other end connected to the power source terminal, a second transistor (Q5) having a collector connected to the base of the first transistor and an emitter connected to the earth terminal, a second resistor (R7) having an end connected to the base of the second transistor and the other end connected to the earth terminal, and a third resistor (R8) having an end connected to the base of the second transistor and the other end connected to an input terminal for receiving an input signal. A hysteresis circuit according to claim 2, wherein the threshold value of the first switching circuit (102) is expressed by R4 + R5 R4 × V BEQ3 where R4 is the resistance of the first resistor (R4) of the first switching circuit (102), R5 is the resistance of the second resistor (R5) of the first switching circuit (102), and V BEQ3 is a threshold value of a transistor (Q3) of the first switching circuit (102). A hysteresis circuit according to claim 2 or 3, wherein the threshold value of the second switching circuit (103) is expressed by R7 + R8 R7 × V BEQ5 where R7 is the resistance of the second resistor (R7) of the second switching circuit (103), R8 is the resistance of the third resistor (R8) of the second switching circuit (103), and V BEQ5 is a threshold value of the second transistor (Q5) of the second switching circuit (103). A hysteresis circuit according to claim 2, 3 or 4, wherein the threshold value of the transistor (Q3) of the first switching circuit (102) is equal to the threshold value of the second transistor (Q5) of the second switching circuit (103) and wherein the first resistor (R4) and a second resistor (R5) of the first switching circuit (102) and the second resistor (R7) and the third resistor (R8) of the second switching circuit (103) satisfy the following relationship: R7 + R8 R7 < R4 + R5 R4 where R7 is the resistance of the second resistor (R7) of the second switching circuit (103), R8 is the resistance of the third resistor (R8) of the second switching circuit (103), R4 is the resistance of the first resistor (R4) of the first switching circuit (102), and R5 is the resistance of the second resistor (R5) of the first switching circuit (102).

Inventors: KINOSHITA HIROSHI, KINOSHITA, HIROSHI
Applicants: TOSHIBA AVE KK, TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA, TOSHIBA AVE CO., LTD
Application year: 1991
Main classification: H03K3
IPC codes: H03K17, H03K3