GB2062996A

GB2062996A - Comparator circuit

Info

Publication number: GB2062996A
Application number: GB8035731A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1979-11-06
Filing date: 1980-11-06
Publication date: 1981-05-28
Also published as: FR2468915A1; DE2944657A1; DE2944657C2; JPS5675722A

Abstract

A comparator circuit is proposed which has a current comparator connected as a current mirror (1). An additional current flow through the transistors (2, 3) of the current mirror (1) is generated through a switch (5) controlled by the output level for providing a switching hysteresis. A second additional unconnected current flow through the transistor (2, 3) of the current mirror (1) is generated for varying the hysteresis zone. <IMAGE>

Description

SPECIFICATION Comparator Circuit The invention originates from a comparator circuit according to the preamble to the main claim.

Differential amplifiers with a sufficiently high amplification which deliver a digital output value are frequently used as simple comparator circuits for comparing analogue input values, for example electrical voltages or currents. If the input voltage Ujn to the differential amplifier exceeds its reference voltage Ref then the output level switches from the L state to the H state. In a certain transition zone, which is smaller the greater the amplification of the differential amplifier, there exists neutral output conditions which are neither associated with the L state nor with the H state. So that the output values do not switch backwards and forwards between the L state and the H state an hysteresis zone must be provided which covers the neutral zone.The hysteresis zone is achieved by the fact that the comparator circuit is provided with positive feedback in the region of the switching over point (input voltage = reference voltage) and acting in phase with the input voltage.

The hysteresis zone and the switching points are established in the manner of a feedback of the output signal to the input. In addition, the reference input is required. However, that has the disadvantage that the reference input is then established in the form of a potential thus, it can no longerfollow an in-phase voltage.

Advantages of the invention As opposed to this, the comparator circuit in accordance with the invention comprising the char acterising features of the main claim has the advantage that the signal feedback necessary for generat ing the hysteresis lies completely within the compa rator circuit so that the inputs are not affected. Thus, both inputs remain high impedance and can be switched freely. In this case, the comparator switch ing points are determined by single constructively established difference between input and reference signal and follow the reference signal at a distance from the said established constant difference. In that way, operation is possible with a variable reference and the suppression of in-phase voltages.

Advantageous further developments and improvements of the comparator circuit set forth in the main claim are made possible by the measures set forth in the sub-claims.

Of speciai advantage, is the high accuracy which can be achieved with the few required components and the non-dependence on the battery voltages with which both the value of the hysteresis zone and also its position in relationship to the reference signal can be set. As a further advantage it can be seen that the disengagement of the output signal is possible so that it remains without a feedback effect on the position of the switching points. Furthermore, the comparator circuit in accordance with the invention is especially suitable for an integrated circuit.

Drawing Embodiments of the invention are illustrated in the drawing and are described in detail in the following specification. Figures 1 to 3 show circuit arrangements of the comparator circuit in accordance with the invention. In addition, two circuit arrangements of a decoupling circuit are illustrated in Figures 4 and 5. Figure 6 shows one form in which all transistor operations in Figure 5 are replaced by the transistor functions complementary thereto.

Description ofthe embodiments Figure 1 shows the basis principle of the solution in accordance with the invention in a current comparator. The comparator circuit comprises a differential amplifier connected as a current mirror 1. Moreover, the base of one transistor 2 is connected in a diode circuit with its own collector and with the base of a further transistor 3. This junction point is the input connection to the current mirror 1 and the collector of the transistor 3 is the output connection from the current mirror 1. The emitters of the transistors 2,3 are both connected directly to earth 4 but can also be connected to earth 4 through symmetrical resistors for compensating residual current non-symmetry.A switch 5, which is controlled by the output voltage Ua derived from the collector of the transistor 3, is connected to the input to the current mirror 2 by the bases of the transistors 2, 3. Furthermore, the switch is connected to a current generator 6 which generates a current IHS. An input current 1e is supplied to the input to the current mirror 1 and a reference current Iret is supplied to the output from the current mirror 1. Furthermore, a current generator 7 is provided which feeds a current 1H0 into the input to the current mirror 1.

The method of operation of the circuit of Figure 1 is as follows: if the current 1H0 and IHS are first of all not taken into consideration then the output voltage from the current mirror 1 consisting of the similar transistors 2,3 switches from the H-state (high state, high output voltage) into the L-state (low state, low output voltage) when the input current 1e reaches or exceeds the value of the reference current Iref. If, through the switch 5 controlled by the output voltage, the current IHS iS introduced in such a manner that it arrives at the input to the current mirror 1 when the output voltage is in the L-state and the current IHs does not arrive at the input to the current mirror when the output mirror is in the H-state, then the comparator has a switching hysteresis. With the constant current IHO, which arrives directly at the input to the current mirror 1, it is possible to adjust the position of the hysteresis zone produced by the current IHS with respect to the reference current.The output from the comparator switches under the given conditions from the H-state into the L-state when the input current has risen to the value of the difference between the reference current and the current Ic and switches conversely from the L-state into the H-state when the value has fallen to the difference between the reference current and the sum of 1H0 and IHS- Thus, both switching points vary with 1H0 according to value and direction in the same manner whereas the size of the hysteresis zone depends only on IHs.

A circuit arrangement for the switch 5 with the above-described switching functions in the form of an emitter coupled transistor pair 8,9 is illustrated in Figure 2. If the output voltage is in the L-state then the transistor 8 receives the current IHS and conducts it to the input to the current mirror 1. If the output voltage is in the H-state, then the transistor 9 receives the current- IHS and conducts it to earth 4.

Figure 3 shows a circuit arrangement of a voltage comparator in which an arrangement for the various necessary current generators advantageous for integrated circuits is also shown. In addition, a lateral transistor 12 is provided which, from each of its part collectors, provides a current of structurally adjustable value proportional to the current in the reference part collector 17. The emitter of the transistor 12 is connected to the conductor 13 which is at battery voltage. The part collector 14 of the transistor 12 supplies the total current 1q for an emitter coupled input transistor pair 10, 11 and is connected to the emitters of the said transistor pair 10, 11. The collector of the transistor 10 is connected to the output from the current mirror 1 and the collector of the transistor 11 is connected to the input to the current mirror 1.The input voltage is applied to the base of the transistor 11 whereas the reference voltage for the comparator is applied to the base of the transistor 11. The part collector 15 of the lateral transistor 12 is connected to the emitters of the emitter coupled transistor pair 8, 9. The collector currents of the emitter coupled input transistor pair 10, 11 correspond to the reference current lref and to the input current le illustrated in Figure 2. The fact that the output voltage from the comparator switches for the H-state into the L-state when the input voltage has dropped to the value of the reference voltage and switches back from the L-state into the H-state when the input voltage has risen once again to the value of the sum of the reference voltage and the hysteresis voltage, applies to both switching points.

The hysteresis voltage is a logarithmic function of the current quotient IqllHs which is made invariable by the ratio of the marginal lengths of the part collectors 14 and 15 of the lateral transistor 12. Thus, the hysteresis voltage is not dependent on the batter voltage. In the circuit example according to Figure 3, the hysteresis zone from the values of the hysteresis voltage lies to one side above the reference voltage but can be displaced according to Figure 1 by introducing a further current 1H0 which can be derived from a further part collector (not shown) of the lateral transistor 12. In so doing, the part collector would need to be connected directly to the input to the current mirror. Likewise, the variation in the hysteresis zone would not be dependent on the battery voltage.

A further possibility for influencing the hysteresis zone and its position with respect to the reference value exists in influencing the current density in the emitters of the transistors 2 and 3 of the current mirror 1. That takes place by the transistors 2, 3 being provided with different emitter areas.

So that, in the interests of the switching point accuracy, the marginal lengths of the part collector 15 of the lateral transistor 12 need not be made too small for very small hysteresis voltages, the collectors of the emitter coupled transistor pair are subdivided into part collectors and only one part collector of the transistor 8 is connected to the input to the current mirror 1 so that only the partial current of IHS flowing through the said part collector acts to generate hysteresis. All other part collectors of the emitter coupled transistor pair 8,9 are connected to earth 4. With the previously described embodiment, it is possible to accurately set the switching points in particular with small hysteresis zones.Larger hysteresis zones can be generated by corresponding voltage drops across emitter resistors which can be connected in the emitter branches of the emitter coupled input transistor pair 10,11.

Since, in the described comparator, a current comparison is always effected, such a comparator reacts with a displacement of the switching points when, during uncoupling of the output signal at the collector of the transistor 3 of the current mirror 1, a current is removed. Thus, with high requirements as to accuracy of the switching points, the uncoupled current at the collector of the transistor 3 should be as low as possible. This is guaranteed by an uncoupling circuit as is illustrated in Figure 4. An additionally provided uncoupling current mirror 23 provided with the transistors 19, 20 is controlled through an emitter follower 21. The collector of the emitter follower 21 is connected in the positive conductor 13. The collector of the transistor 20 of the uncoupling current mirror 23 is connected to a further part collector of the lateral transistor 12.In this described embodiment, the output voltage swing is no longer limited to substantially the reference voltage but includes the maximal possible zone within the limits of the battery voltage. Moreover, the circuit prevents the transistor 10 of the emitter coupled input transistor pair from going into saturation, thus, at the same time, the circuit improves the input characteristics of the comparator at the base of the transistor 10. In a simplified embodiment, the transistor 19 of the uncoupling current mirror 23 connected as an input diode can be omitted so that the uncoupling circuit has a Darlington input.

In Figure 4, the lateral transistor 12 has a further collector 18 which is connected to the input to the current mirror 1 and delivers the current IHO In the embodiment illustrated in Figure 5, the collector of the transistor 9 of the emitter coupled transistor pair is connected to the input to the uncoupling current mirror 23. In so doing, the fact is utilised that the information of the output signal is already fully included in the two different paths of the hysteresis current IHs. At the changeover point of the hysteresis current from the transistor 9 to the transistor 8 and vice versa, the output signal also changes from the H-state into the L-state and vice versa. The previously unused collector current from the transistor 9 conducted to earth 4 is fed into the uncoupling current mirror 23. Since the hysteresis current IHS by which the uncoupling current mirror is controlled can be very small, it is advantageous to undertake simultaneously a current conversion with the uncoupling current mirror 23. For this purpose, the transistors 19 and 20 are provided with different emitter areas. Atransistor 22 connected as a diode is so connected on the one hand to the collector of the transistor 10 of the emitter coupled transistor pair 10, 11 and on the other hand to the input to the uncoupling current mirror 23, that the voltage at the collector of the transistor 10 remains restricted to the value of the voltage at two diodes with respect to earth 4. This measure prevents saturation of the transistor 10 of the emitter coupled input transistor pair 10, 11.Since the diode 22 only switches at an instant in which the real comparator switching point has already been exceeded, the accuracy of the switching point is not influenced. In a simplified arrangement, the transistor 19 of the uncoupling current mirror 23 can be omitted.

The described circuit example of the comparator circuit in accordance with the invention may also be produced with dual transistors. The current sources which, in the described examples, have been provided by collector subdivision of the lateral transistor 12, are then provided by an appropriate number of smaller emitter areas of individual npntransistors. On the other hand, that which has been produced in the described embodiments by different emitter areas can then be provided by collector subdivision. An arrangement corresponding to the embodiment of Figure 5 is shown in Figure 6 wherein corresponding parts are given the same reference numerals with a prime.

Claims

1. A comparator circuit comprising a current comparator connected as a current mirror, characterised in that, an additional first current flow is generated through the transistors of the current mirror for varying the switching threshold of the current comparator by a switch controlled by the output level for generating a switching hysteresis.

2. A comparator circuit according to claim 1, characterised in that a second current flow is generated through the transistors of the current mirror for altering the position of the hysteresis zone.

3. A comparator circuit according to claim 1 or 2 characterised in that the switch is formed as an emitter coupled transistor pair wherein the collector of the first transistor and the base of the second transistor of the emitter coupled transistor pair is connected to the input to the current mirror and the base of the first transistor is connected to the output from the current mirror.

4. A comparator circuit according to one of claims 1 to 4 characterised in that the transistors of the current mirror differ geometrically for influencing the hysteresis zone by the current conversion of the current mirror.

5. A comparator circuit according to claim 3 characterised in that at least one transistor of the emitter coupled transistor pair subdivides the current flowing through it by constructional measures and that only partial currents of at least one transistor arrive at the input to the current mirror.

6. A comparator circuit according to one of claims 1 to 5 characterised in that an emitter coupled input transistor pair is provided wherein the collector of one transistor is connected to the input to the current mirror and the collector of the other transistor is connected to the output from the current mirror and that the input voltage is applied to the base of one transistor of the emitter coupled input transistor and the reference voltage is applied to the base of the other transistor.

7. A comparator circuit according to claim 6 characterised in that emitter resistors are provided in the conductors leading to the emitters of the emitter coupled input transistor pair.

8. A comparator circuit according to one of claims 1 to 7 characterised in that an uncoupling circuit is provided for uncoupling the output signal from the current comparator without using feedback.

9. A comparator circuit according to claim 8 characterised in that the uncoupling circuit is formed as an uncoupling current mirror which is controlled by an emitter follower.

10. A comparator circuit according to one of claims 1 to 8 characterised in that the switch is formed as an emitter coupled transistor pair wherein at least one partial flow from the second transistor is fed to the input to the uncoupling current mirror.

11. A comparator circuit according to claim 10 characterised in that the output from the current mirror is connected to the input to the uncoupling current mirror through a diode or a transistor connected as a diode.

12. A comparator circuit according to one of claims 8 to 11 characterised in that the transistor of the uncoupling current mirror connected as an input diode is omitted.

13. A comparator circuit according to claim 11 or 12 characterised in that the transistors of the uncoupling current mirror differ geometrically for the purpose of the current conversion.

14. Any one of the comparator circuits substantially as herein described with reference to the accompanying drawings.