Abstract:
The present invention, belonging to a field of fingerprint detecting technology, relates to a fingerprint detecting circuit including: a first conductive layer; a second conductive layer; an inverting amplifier, comprising a single input terminal coupled to the first conductive layer; and an output terminal coupled to the second conductive layer; and a switching unit, configured for establishing a DC operating point of the inverting amplifier, wherein a terminal of the switching unit is coupled to the input terminal, and another terminal of the switching unit is coupled to the output terminal. The present invention utilizes the inverting amplifier with single input single output and the switching unit to quickly and correctly establish the DC biasing point of the inverting amplifier, and convert the sensing capacitance into the output signal. No need for another biasing circuit. The circuit structure thereof is simple and the accuracy is high.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of international application No. PCT/CN2015/094638, filed on Nov. 16, 2015, which claims the priority to Chinese Patent Application No. 201510448095.8, filed with the Chinese Patent Office on Jul. 27, 2015, both of which are incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
       [0002]    The present invention, belonging to a field of fingerprint detecting technology, relates to a fingerprint detecting circuit and a fingerprint identification system, and more particularly, to a fingerprint detecting circuit and a fingerprint identification system with high accuracy and simple circuit structure. 
       2. Description of the Prior Art 
       [0003]    With the development of technology, mobile phones, digital cameras, tablet PCs, notebook computers and other portable electronic devices become more and more popular. The portable electronic devices are intended for personal use, with certain privacy. The information stored in the portable device such as phonebook, photos, personal information, etc., are privately owned. If the electronic device is lost, these data may be accessed by other people, causing unnecessary loss. Even though there are several ways using password to protect the electronic device from being used by the other people, the password may be easily to be spread or cracked, which lowers the security. Moreover, the user needs to remember the password so as to use electronic devices. If the user forgets the password, troubles would be brought to the user. Hence, the personal fingerprint identification is utilized to achieve the purpose of personal identification, for enhancing the data security. 
         [0004]    Capacitive fingerprint identification system is a popular fingerprint identification method, which is to sense a capacitance variation to determine a finger ridge or a finger valley of a user&#39;s finger. Specifically, the capacitive fingerprint identification system utilizes a metal electrode to receive a touch from a user, and the fingerprint detecting circuit therein may transform a capacitance between the metal electrode and the finger into a voltage signal and output the voltage signal to a backend fingerprint determining module, such that fingerprint identification is performed. However, the fingerprint detecting circuits within the capacitive fingerprint identification system usually requires complicated biasing circuits, to make sure that the fingerprint detecting circuits operate correctly (i.e., the direct current (DC) biasing point of the fingerprint detecting circuit lies in an appropriated region). A biasing circuit that is excessively simple would affect an accuracy of the fingerprint detecting circuit. Therefore, how to provide a fingerprint detecting circuit with simple circuit structure and high accuracy is a significant objective in the field. 
       SUMMARY OF THE INVENTION 
       [0005]    It is therefore a primary objective of the present invention to provide a fingerprint detecting circuit and a fingerprint identification system, to enhance an accuracy thereof with simple circuit structure. 
         [0006]    The present invention is implemented as follows: fingerprint detecting circuit comprises: 
         [0007]    a first conductive layer; 
         [0008]    a second conductive layer; 
         [0009]    an inverting amplifier, comprising a single input terminal coupled to the first conductive layer; and an output terminal coupled to the second conductive layer; and 
         [0010]    a switching unit, configured for establishing a direct current (DC) operating point of the inverting amplifier, wherein a terminal of the switching unit is coupled to the input terminal of the inverting amplifier, and another terminal of the switching unit is coupled to the output terminal of the inverting amplifier. 
         [0011]    The object of the present invention is also to provide a fingerprint identification system. The fingerprint identification system comprises: 
         [0012]    a plurality of fingerprint detecting circuits, each fingerprint detecting circuit comprising: 
         [0013]    a first conductive layer; 
         [0014]    a second conductive layer; 
         [0015]    an inverting amplifier, comprising a single one input terminal coupled to the first conductive layer; and an output terminal coupled to the second conductive layer; and 
         [0016]    a switching unit, configured for establishing a direct current (DC) operating point of the inverting amplifier, wherein a terminal of the switching unit is coupled to the input terminal of the inverting amplifier, and another terminal of the switching unit is coupled to the output terminal of the inverting amplifier; 
         [0017]    a fingerprint determining module, coupled to the plurality of fingerprint detecting circuit, configured to determine whether each fingerprint detecting circuit corresponds to a finger ridge or a finger valley. 
         [0018]    The embodiments in the present invention utilizes the inverting amplifier with single input single output and the switching unit to quickly and correctly establish the DC biasing point of the inverting amplifier, and convert the sensing capacitance into the output signal. No need for another biasing circuit. The circuit structure thereof is simple and the accuracy is high. 
         [0019]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a schematic diagram of a fingerprint detecting circuit provided by a first embodiment of the present invention; 
           [0021]      FIG. 2  is a schematic diagram of an equivalent circuit of the fingerprint detecting circuit in  FIG. 1 ; 
           [0022]      FIG. 3  is a schematic diagram of an inverting amplifier in  FIG. 1 ; 
           [0023]      FIG. 4  is a schematic diagram of a voltage conversion characteristic of the inverting amplifier in  FIG. 3 ; 
           [0024]      FIG. 5  is a schematic diagram of a fingerprint identification system provided by a second embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    In order to make the objects, technical solutions and advantages of the present invention become more apparent, the following relies on the accompanying drawings and embodiments to describe the present invention in further detail. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention. 
         [0026]    As shown in  FIG. 1 , which is a schematic diagram of a fingerprint detecting circuit  10  provided by a first embodiment of the present invention. The fingerprint detecting circuit  10  comprises an inverting amplifier  104 , a switching unit SW and conductive layers  100  and  102 . The conductive layer  100  is a top conductive layer, which maybe a metal electrode or a metal layer within an integrated circuit (IC) layout. The conductive layer  100  is configured to couple a touch from a finger FG. The conductive layer  102  is a non-top conductive layer, which may be a metal electrode, a metal layer or a poly layer within the IC layout. The conductive layer  102  is disposed under the conductive layer  100 . A reference capacitance Cf is formed between the conductive layer  100  and the conductive layer  102 , and a sensing capacitance Cs is formed between the conductive layer  100  and the finger FG. The inverting amplifier  104  is a single input single output amplifier, which comprises only one input terminal and an output terminal. The input terminal of the inverting amplifier  104  is coupled to the conductive layer  100 , and the output terminal is coupled to the conductive layer  102 . The switching unit SW is utilized for establishing a direct current (DC) operating point of the inverting amplifier  104 . An terminal of the switching unit SW is coupled to the input terminal of the inverting amplifier  104 , and another terminal of the switching unit SW is coupled to the output terminal of the inverting amplifier  104 . 
         [0027]    Specifically, the value of the reference capacitance Cf depends on an area of the conductive layer  102 . When the area of the conductive layers  100  and  102  are fixed, the reference capacitance Cf has a fixed capacitance value. On the other hand, the value of the sensing capacitance Cs varies with respect to characteristic of the finger FG. In addition, the finger FG may receive a driving signal Vs through the metal electrode, and the driving signal Vs may be coupled to the input terminal of the inverting amplifier  104  through the sensing capacitance Cs. That is, after the driving signal Vs is passed through the sensing capacitance Cs, an input signal Vi is formed at the input terminal of the inverting amplifier  104 , and the inverting amplifier  104  generates an output signal Vo after receiving the input signal Vi. Therefore, the fingerprint detecting circuit  10  may convert the value of the sensing capacitance Cs into the output signal Vo. The output signal Vo may be delivered to a backend fingerprint determining module, so as to determine whether a location of the fingerprint detecting circuit  10  corresponds to a finger ridge or a finger valley. 
         [0028]    Specifically, please refer to  FIG. 2 , which is a schematic diagram of an equivalent circuit of the fingerprint detecting circuit  10 . As can be seen from  FIG. 2 , a relationship between the driving signal Vs and the sensing capacitance Cs may be expressed as CsVs=−CfVo. Therefore, the fingerprint determining module may compute the value of the sensing capacitance Cs as Cs=−(Cf/Vs)*Vo, according to the output signal Vo, and determine whether the location of the fingerprint detecting circuit  10  corresponds to the finger ridge or the finger valley. 
         [0029]    In another perspective, the realization of the inverting amplifier  104  may be altered according to practical situation. For example, as  FIG. 3  shows,  FIG. 3  is a schematic diagram of the inverting amplifier  104 . The inverting amplifier  104  comprises a first transistor Q 1  and a second transistor Q 2 , where the first transistor Q 1  is a PMOS FET (P-type Metal Oxide Semiconductor Field Effect Transistor), and the second transistor Q 2  is an NMOS FET (N-type MOS FET). A gate of the first transistor Q 1  is coupled to a gate of the second transistor Q 2 , forming the input terminal of the inverting amplifier  104  and configured to receive the input signal Vi. A drain of the first transistor Q 1  is coupled to a drain of the second transistor Q 2  forming the output terminal of the inverting amplifier  104  and configured to output an output signal Vo. A source of the first transistor Q 1  is configured to receive a voltage VDD, and a source of the second transistor Q 2  is configured to receive a voltage VSS. 
         [0030]    As can be seen from  FIG. 3 , the circuit structure of the inverting amplifier  104  is the same as a CMOS (Complementary Metal Oxide Semiconductor) inverter. Different from the conventional CMOS invertor, the DC operating point of the inverting amplifier  104  is within the linear amplifying region, instead of the saturation region (note that the DC operating point of the conventional CMOS invertor is within the saturation region). In detail, in the fingerprint detecting circuit  10 , the output terminal of the inverting amplifier  104  is coupled to the input terminal via the switching unit SW. When the switching unit SW is conducted, the output signal Vo is equal to the input signal Vi, so as to make sure that the inverting amplifier  104  operates in the linear amplifying region. 
         [0031]    Specifically, as shown in  FIG. 4 ,  FIG. 4  is a schematic diagram of a voltage conversion characteristic of the inverting amplifier  104 . When the input signal Vi is greater than a voltage Vtp or smaller than a voltage Vtn, the inverting amplifier  104  operates within a saturation region Sat. When the input signal Vi is greater than a voltage VL and smaller than a voltage VH, the inverting amplifier  104  operates within a linear amplifying region Ln. In detail, when the switching unit SW is controlled to be conducted, the input signal Vi of the inverting amplifier  104  is equal to the output signal Vo. At this time, the inverting amplifier  104  is at a DC operating point OP, which means that the inverting amplifier  104  operates within the linear amplifying region Ln. Furthermore, the inverting amplifier  104  has a negative gain, which means that the output signal Vo increases (decreases) as the input signal Vi decreases (increases). 
         [0032]    As can be seen, the fingerprint detecting circuit  10  only utilize the inverting amplifier  104  with simple circuit structure along with the switching unit SW, which convert the value of the sensing capacitance Cs as the output signal Vo precisely. Compared to the prior art, the fingerprint detecting circuit  10  does not require an extra biasing circuit, which has simple circuit structure and high accuracy. 
         [0033]    Notably, the embodiments stated in the above are utilized for illustrating the concept of the present invention. Those skilled in the art may make modifications and alterations accordingly, and not limited herein. For example, in the inverting amplifier  104 , the first transistor Q 1  and the second transistor Q 2  are not limited to be MOS FET. The first transistor Q 1  and the second transistor Q 2  may also be HEMT (High Electron Mobility Transistor), which is also within the scope of the present invention. In addition, the inverting amplifier is not limited to comprise the circuit structure which is illustrated in  FIG. 3 . As long as an inverter, which is originally applied in the digital circuit (i.e., the inverting circuit with a single input, a single output and a negative gain), and the switching unit, coupled between the input terminal and the output terminal of the invertor to limit the invertor to operate in the linear amplifying region, are included, it is within the scope of the present invention. 
         [0034]    In addition, the fingerprint detecting circuit  10  may be applied to a fingerprint identification system. As shown in  FIG. 5 , which is a schematic diagram of a fingerprint identification system  50  provided by a second embodiment of the present invention. The fingerprint identification system  50  comprises a fingerprint determining module  500  and fingerprint detecting circuits  10 _ 1  to  10 _M. The circuit structure of the fingerprint detecting circuits  10 _ 1 - 10 _M are the same as which of the fingerprint detecting circuit  10 , which not narrated herein. 
         [0035]    The fingerprint detecting circuits  10 _ 1  to  10 _M are coupled to the fingerprint determining module  500 , and deliver output signals Vo_ 1  to Vo_M thereof to the fingerprint determining module  500 . The fingerprint determining module  500  may determine whether locations of the fingerprint detecting circuits  10 _ 1  to  10 _M correspond to finger ridges or finger valleys. Notably, the area of the conductive layers  100  and  102  within the fingerprint detecting circuits  10 _ 1  to  10 _M are fixed, and the values of the reference capacitance Cf has fixed capacitance values. Therefore, relative voltage values of the output signals Vo_ 1  to Vo_M may be utilized to determine relative capacitance values of the sensing capacitance Cs corresponding to the fingerprint detecting circuits  10 _ 1  to  10 _M, and to determine the locations of the fingerprint detecting circuits  10 _ 1  to  10 _M correspond to finger ridges or finger valleys. 
         [0036]    In summary, the fingerprint detecting circuit of the embodiments in the present invention utilizes the inverting amplifier with single input single output and the switching unit to quickly and correctly establish the DC biasing point of the inverting amplifier, and convert the sensing capacitance into the output signal. Compared to the prior art, the embodiments of the present invention do not require the extra biasing circuit, which has simple circuit structure and high accuracy. 
         [0037]    The foregoing is only preferred embodiments of the present invention, it is not intended to limit the present invention, any modifications within the spirit and principles of the present invention made, equivalent replacement and improvement, etc., should be included in this within the scope of the invention. 
         [0038]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.