[0001] A Comparator is usually applied to differentiate between two different signal levels. Thereby, two voltages or currents are compared and a digital signal indicating which one is larger is output. Comparators are frequently used components or circuit elements, e.g., for analog-to-digital converters (ADCs) , two-point or three-point controllers, or switching power supplies.

[0002] In greater detail, common comparator circuits comprise two analog input terminals for receiving input signals e.g., V_ ,V ₊ , and one binary digital output terminal for outputting an digital output signal e.g., V _out • If V ₊ is larger than V_ _f then V _out is 1 (logic high) . If V ₊ is smaller than V_ _f then V _out is 0 (logic low) . However, one drawback of such a configuration is that small voltage fluctuations due to noise, which are always present on the input signals V ₊ , V_, may cause undesirable rapid changes between the two output states V _out •

[0003] To prevent this output oscillation, a small hysteresis of a few millivolts is integrated into many common comparator circuits. For example, such a comparator may be operated by applying a positive feedback to the comparator. The potential difference between the high and low output signals V _out and a feedback resistor may be adjusted to change the voltage that is taken as a comparison reference to the input signal V_ .

[0004] For example, hysteresis may be using two different threshold voltages, e.g., V _t hL, V _t hH, to avoid multiple transitions. In one exemplary configuration, the input signal V_ may exceed an upper threshold V _t hH for V _out to transition to low or below a lower threshold V _t hL for V _out to transition to high. Since a margin is provided between the High-to-Low V _t hH and Low-to-High thresholds, V _out is not affected (i.e. , no chattering occurs) even when the input signal V_ has a voltage near the threshold voltages V _t hL, V _t hH-

[0005] However, for applications requiring fast signals an adaption is needed to provide sufficient robustness against noise in the input signals.

[0006] It is therefore an object of the present invention to provide an improved comparator.

SUMMARY

[0007] According to embodiments, a comparator comprises an input stage, configured to receive a pair of input signals to generate at least one differential current signal, an output stage configured to generate an output signal depending on the differential current signal, the output stage comprising a current mirror and a node electrically connected to the current mirror, the current mirror comprising a first transistor, a second and a third transistor, the second and the third transistors being connected in parallel, a gain stage comprising a first and a second inverter, wherein an output of the first inverter is input to the second inverter. In greater detail, an output of the second inverter is configured to control the third transistor to be in a floating or conducting state .

[0008] In other words, the third transistor serves as a switch that is switched on and off depending on the output of second converter. This switching configuration implements hysteresis by adapting the current mirror load. [0009] For example, if the output of the second inverter is high, the third transistor may be controlled to be conductive. In this case, a higher current may flow through the output stage raising an upper threshold voltage.

[0010] On the other hand, if the output of the second inverter is low, the third transistor may be controlled to be floating. In this case, a smaller current may flow through the output stage reducing a lower threshold voltage.

[0011] By the above, hysteresis may be implemented. This configuration may allow adapting the speed of the switching phases such that the comparator may be used for fast signals.

[0012] The input stage may comprise a first input transistor and a second input transistor, the first input transistor being different from the second input transistor. In other words, in one configuration of the comparator it may be possible to realize an unsymmetrical input transistor pair. For example, the second input transistor may be smaller than the first input transistor and may be connected to a switching input. The first transistor may be connected to reference. This may allow a faster switching and may provide more headroom for a current source (such as a bias transistor) resulting in a constant and stable current source in saturation.

[0013] The comparator may further comprise an inverter and a control transistor. The output of the second inverter may be fed to a gate terminal of the control transistor via the inverter. The control transistor may, depending on the output of the second inverter, control switching the third transistor on or off. [0014] The control transistor may be implemented as an NMOS transistor .

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The accompanying drawings are included to provide a further understanding of embodiments of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate the embodiments of the present invention and together with the description serve to explain the principles. Other embodiments of the invention and many of the intended advantages will be readily appreciated, as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference numbers designate corresponding similar parts.

Fig. 1 is a schematic diagram illustrating components of a comparator according to embodiments.

Fig. 2 shows an example of input and output signals of a comparator according to embodiments.

Fig. 3 shows an example of an electronic device according to embodiments .

DETAILED DESCRIPTION

[0016] In the following detailed description reference is made to the accompanying drawings, which form a part hereof and in which are illustrated by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology such as "top", "bottom", "front", "back", "over", "on", "above", "leading", "trailing" etc. is used with reference to the orientation of the Figures being described. Since components of embodiments of the invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope defined by the claims .

[0017] The description of the embodiments is not limiting. In particular, elements of the embodiments described hereinafter may be combined with elements of different embodiments.

[0018] As employed in this specification, the terms "coupled" and/or "electrically coupled" are not meant to mean that the elements must be directly coupled together - intervening elements may be provided between the "coupled" or "electrically coupled" elements. The term "electrically connected" intends to describe a low-ohmic electric connection between the elements electrically connected together.

[0019] As used herein, the terms "having", "containing", "including", "comprising" and the like are open ended terms that indicate the presence of stated elements or features, but do not preclude additional elements or features. The articles "a", "an" and "the" are intended to include the plural as well as the singular, unless the context clearly indicates otherwise.

[0020] Fig. 1 is a schematic diagram illustrating components of a comparator 10 according to embodiments. The comparator 10 comprises an input stage IS, an output stage OS, and a gain stage GS, as indicated by corresponding arrows in Fig. 1. [0021] The input stage IS may include a bias transistor 34 and input transistors 26, 28. The bias transistor 34 may be configured to operate as a tail current source for a differential input circuit formed by the input transistors 26, 28. A gate terminal of the bias transistor 34 may receive a bias voltage. A source terminal of the bias transistor 34 may be electrically connected to a reference potential, typically Vss or GND.

A gate terminal of the input transistor 26 may form the noninverting input of the comparator 10 and a gate terminal of transistor 28 may form the inverting input of the comparator circuit 10.

[0022] The input stage IS may further include a load circuit formed by a plurality of transistors as shown in the upper part of Fig. 1. In this circuit, a pair of cross-coupled transistors may be coupled between the differential input circuit and a positive supply node Vdd. In this context, various configurations are possible. The load circuit shown in Fig. 1 is only by way of example, and there may be many alternative implementations and modifications.

[0023] The input stage IS receives a pair of input signals SI, S2 via the input transistors 26, 28, which will be described in detail later on. The input stage IS then generates a differential current signal S3 that is passed to the output stage OS .

[0024] The output stage OS generates an output signal S4 depending on the differential current signal S3. The output stage OS comprises a current mirror 12 and a node 14 electrically connected to the current mirror 12. In greater detail, the current mirror 12 may comprise a first transistor 16, a second and a third transistor 18, 20. The second and the third transistors 18, 20 are connected in parallel as shown in Fig. 1.

[0025] Furthermore, the gain stage GS comprises a first and a second inverter 22, 24, wherein an output of the first inverter 22 is input to the second inverter 24. The gain stage GS may comprise further inverters as shown in Fig. 1. An output of the second converter 24 is then fed back to the current mirror 12.

[0026] In detail, the output of the second inverter 24 controls the third transistor 20 to be in a floating or conducting state.

[0027] That is, the third transistor 20 is operated as a switch that is turned on and off depending on the output of the second inverter 24. With this configuration hysteresis is implemented .

[0028] For example, if the output of the second inverter 24 is high, the third transistor 20 may be controlled to be conductive or turned on. In this first phase, a higher current may pass or flow through the output stage OS. The current mirror 12 may then have a factor of 2 that may be switched on in the output stage OS.

[0029] If the output of the second inverter 24 is low, the third transistor 20 may be controlled to be floating or turned off. In this second phase, a smaller current may pass or flow through the output stage OS. The current mirror 12 may then have a factor of 1 that may be switched on in the output stage OS . [0030] Stated differently, during the first phase the current mirror 12 of factor 2 is switched on, while in the second phase the current mirror 12 of factor 1 is switched on. Via this switching hysteresis is achieved. In one example, the circuit may have an offset value of about 22 mV.

[0031] This configuration may make the comparator 10 robust against noise and unwanted switching. In addition, the above configuration may enable to increase the switching speed between the first and the second phase.

[0032] Furthermore, according to embodiments the input stage IS may comprise the first input transistor 26 and the second input transistor 28 as described above. The first input transistor 26 may be different from the second input transistor 28. For example, the term "the first input transistor may be different from the second input transistor" may mean that a channel width of the first input transistor 26 is different, e.g. larger than the channel width of the second input transistor. The conductivity type of the first input transistor 26 may be equal to the conductivity type of the second input transistor 28, e.g. NMOS .

[0033] For example, the second input transistor 28 may be smaller than the first transistor 26. The second input transistor 28 may be connected to the switching input. This may increase the switching speed. The first transistor 26 may be connected to reference, because the threshold voltage may be smaller, and thus, more headroom for the bias transistor 34 may be available. This may allow a constant and stable current source in saturation. Both effects may enable a fast switching . [0034] The comparator 10 may comprise a control inverter 30 and a control transistor 32 as shown in Fig. 1. These elements may be included in the output stage OS. The output of the second inverter 24 may be fed to a gate terminal of the control transistor 32 via the control inverter 30. The control transistor 32 may control the switching of the third transistor 30 depending on the output of the second converter 24.

[0035] The control transistor 32 may be implemented as an NMOS transistor. A source terminal of the control transistor 32 may be connected to Vss, e.g. GND.

Further, also the bias transistor 34, the first and second input transistors 26, 28, the first, second, and third transistors 16, 18, 20 as well as the control transistor 32 may be implemented as NMOS transistors. According to further examples, the transistors may be implemented as PMOS transistors.

Fig. 2 illustrates an example of voltage characteristics of the comparator. Fig. 2 shows a reference signal SI, a switching input signal S2 and the output signal S4. Approximately at a timing, at which S2 crosses SI, the output S4 is switched from HIGH to LOW or vice versa. Due to the specific configuration discussed herein, a hysteresis is imposed which means that switching to HIGH is performed with a delay after S2 is larger than SI. On the other side, S4 is switched to LOW before S2 is smaller than SI. The voltage difference between S2 and SI which causes the switching is referred to as the offset voltage AV. For example, AV may be smaller than 50 mV and larger than 5 mV. For example, AV may be approximately 15 to 25 mV, e.g. 22 mV.

As has been described above, due to the hysteresis of the comparator described, noise may be suppressed. The comparator may be operated at high speed . The comparator may be implemented with low supply voltages so that the energy consumption may be reduced . The comparator may have a small si ze . Accordingly, the comparator may be used in mobi le devices . Fig . 3 shows an example of an electronic device 11 comprising the comparator 10 . The electronic device may be implemented as a laser driver, as smart glasses or as a mobile device .

[ 0036 ] While embodiments of the invention have been described above , it is obvious that further embodiments may be implemented . For example , further embodiments may comprise any subcombination of features recited in the claims or any subcombination of elements described in the examples given above . Accordingly, this spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein .

LIST OF REFERENCES comparator electronic device current mirror node first transistor second transistor third transistor first inverter second inverter first input transistor second input transistor control inverter control transistor bias transistor input stage output stage gain stage input signal input signal di f ferential current signal output signal