Abstract:
According to an embodiment of the inventive concept disclosed in this application, a ramp signal generator may include a ramp signal generation unit suitable for generating a ramp signal, a gain amplification control unit suitable for outputting a gain amplification control signal for controlling a voltage gain in response to a control signal from a control unit; and a programmable gain amplifier (PGA) suitable for controlling the voltage gain by amplifying the ramp signal provided from the ramp signal generation unit in response to the gain amplification control signal from the gain amplification control unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority of Korean Patent Application No. 10-2014-0023196, filed on Feb. 27, 2014, which is incorporated herein by reference in its entirety. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    Exemplary embodiments of the present invention relate to a ramp signal generator used in a complementary metal oxide semiconductor (CMOS) image sensor (CIS) or the like and, more particularly, to a ramp signal generator including a programmable gain amplifier (GPA) as a voltage buffer. 
         [0004]    2. Description of the Related Art 
         [0005]    When a CMOS image sensor (CIS) including a single slope analog-digital converter (ADC) uses a current steering digital-analog converter (DAC) for a ramp signal generator, the CIS may control its gain by adjusting the amount of current flowing therein. 
         [0006]    With an increase in gain of the OS, the slope of the ramp voltage decreases. This is because the ramp voltage is generated by current flowing through a constant resistor and the current decreases to increase the gain. 
         [0007]    Thus, when the CIS has a high gain, very little current may flow and cause serious setting time delays when driving the comparator array. Furthermore, with a reduction in current, the current flowing through each current cell may also decrease and reduce the overdrive voltage of the transistors. In an extreme case, the transistors may operate in a sub-threshold region instead of the saturation region. This may result in the transistors not transmitting an accurate signal current. 
       SUMMARY 
       [0008]    Various embodiments are directed to a ramp signal generator capable of controlling the magnitude of a voltage using a programmable gain amplifier (PGA) as a voltage buffer. 
         [0009]    In an embodiment, a ramp signal generator may include a ramp signal generation unit suitable for generating a ramp signal a gain amplification control unit suitable for outputting a gain amplification control signal for controlling a voltage gain in response to a control signal from a control unit, and a PGA suitable for controlling the voltage gain by amplifying the ramp signal provided from the ramp signal generation unit in response to the gain amplification control signal from the gain amplification control unit. 
         [0010]    The gain amplification control unit may include a reset control block suitable for outputting a reset switch control signal to the PGA in response to a reset control signal from the control unit, a variable common mode voltage (VCM) generation unit suitable for generating a variable VCM in response to a VCM level control signal from the control unit, and applying a generated VCM to the PGA, and a gain control block suitable for outputting a capacitor control signal to the PGA in response to a gain control signal from the control unit. 
         [0011]    The PGA may include a sampling capacitor having a sampling capacitor value and suitable for sampling the ramp signal provided from the ramp signal generation unit, a feedback capacitor having a feedback capacitor value which is controlled in response to the capacitor control signal from the gain control block, a differential amplifier suitable for amplifying the ramp signal sampled through the sampling capacitor at the ratio of the sampling capacitor value and the feedback capacitor value, and a reset switch suitable for resetting the differential amplifier to the VCM applied from the variable VCM generation unit in response to the reset switch control signal from the reset control block. 
         [0012]    In an embodiment, a ramp signal generator may include a ramp signal generation unit suitable for generating a ramp signal through current steering, and a programmable gain amplifier (PGA) suitable for controlling a voltage gain in response to a control signal and amplifying the ramp signal based on a controlled voltage gain. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a circuit diagram of a voltage mode gain control unit to help understand the present invention. 
           [0014]      FIG. 2  is a configuration diagram of a ramp signal generator using a programmable gain amplifier (PGA) in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Various embodiments will be described below in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. Throughout the disclosure, like reference numerals refer to like parts throughout the various figures and embodiments of the present invention. 
         [0016]    Throughout the specification, when an element is referred to as being “coupled” to another element, the element may be “directly coupled” to the other element or “indirectly coupled” to another element, with elements existing therebetween. Similarly if two units or entities are “electrically coupled this may mean they are “directly coupled” or “indirectly coupled”, with further units or entities existing therebetween. Furthermore, when it is stated that when a unit or entity comprises” (or “includes” or “has”) some elements, it should be understood that it may consist of only those elements or it may comprise (or include or have) additional elements as well the listed or stated elements. Additional, in this document, the singular form may include the plural form, and vice versa. 
         [0017]      FIG. 1  is a circuit diagram of a voltage mode gain control unit to help understand the present invention. 
         [0018]    The voltage mode gain control unit for promoting an understanding of the present invention may control the gain of a ramp signal using a non-inverting voltage amplifier  20 . 
         [0019]    As illustrated in  FIG. 1 , the voltage mode gain control unit for promoting an understanding of the present invention may include a buffer amplifier  10 , a plurality of resistors  24 , and a differential amplifier  22 . The buffer amplifier  10  may maintain a constant output voltage (ramp signal) from a capacitive element (not illustrated) at the front stage of the voltage mode gain control unit. Specifically, the buffer amplifier  10  serves to prevent a change in waveform of the ramp signal due to the operation of the voltage mode gain control unit. 
         [0020]    A voltage amplification gain Gv is determined depending on the resistance values of the plurality of resistors  24 , as expressed by Equation 1. 
         [0000]    
       
         
           
             
               
                 
                   Gv 
                   = 
                   
                     
                       ( 
                       
                         1 
                         + 
                         
                           Rb 
                           Ra 
                         
                       
                       ) 
                     
                     × 
                     
                       Rd 
                       
                         Rc 
                         + 
                         Rd 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
         [0021]    Thus, as the resistance values Ra, Rb, Rc, and Rd of the resistors  24  are set using variable resistors, the gain of the ramp signal may be controlled. 
         [0022]    The voltage mode gain control unit may further include a filtering unit  30  formed at an output terminal thereof, in order to remove various forms of noise. 
         [0023]    The filtering unit  30  may include a transconductance amplifier  2  and a compensation capacitive element  34 . The transconductance amplifier  32  may be coupled in series to the output terminal of the voltage mode gain control unit, and the compensation capacitive element  34  may have one end coupled to an output terminal of the transconductance amplifier  32 . The filtering unit  30  may serve to remove noise included in the ramp signal and then output the ramp signal. 
         [0024]      FIG. 2  is a configuration diagram of a ramp signal generator using a programmable gain amplifier (PGA) in accordance with an embodiment of the present invention. 
         [0025]    A CMOS image sensor (CIS) using a single-slope ADC changes the slope of a ramp voltage by adjusting a current flowing in a ramp signal generation unit  210 , when performing gain control in an analog domain. The ramp signal generation unit  210  may be implemented with a current steering DAC, for example. 
         [0026]    The flowing current may differ in the range of a one-time gain to a 16-times gain. The maximum current may where there is a one-time gain. 
         [0027]    For example, when the fun swing of the ramp voltage is 800 mV and a resistance of 200Ω is used, the current flow is 4 mA. In this case, the one-time gain is applied. 
         [0028]    Since the flowing current becomes 4 mA/16 with the 16-times gain, a very small current may flow to cause serious setting time delays when driving a comparator array. Furthermore, with a decrease in current flow, the current flowing in each current cell may also decrease to reduce the overdrive voltage of the transistors. In an extreme case, a transistor may operate in the sub-threshold region instead of the saturation region. Then, the transistor may not transmit accurate current. 
         [0029]    In order to remove the above-described concerns, the flowing current may be set to have a large value. However, the current may be significantly increased in the case of the one-time gain. That is, when the current is set to sufficiently drive a load based on the 16-times gain, the current may be significantly increased in the case of the one-time gain. Then, the magnitude of the resistance may be reduced to less than 200Ω in order to maintain the full swing at 800 mV. 
         [0030]    In order to remove these concerns, a voltage buffer for driving the load may be additionally provided. When the voltage buffer is implemented with a programmable gain amplifier (PGA)  230  to control the magnitude of the voltage outputted from the voltage buffer, the ramp signal generation unit  210  may be configured in such a form to reduce current consumption and increase resistance. 
         [0031]    Although current consumption is increased by the addition of the voltage buffer, the total current consumption may be significantly reduced because the reduction in current consumption of the ramp signal generation unit  210  is much greater than the increase of the current consumption. 
         [0032]    As the PGA  230  performs the gain control function of the ramp signal generation unit  210  at the output terminal of the ramp signal generation unit  210 , the current consumed by the ramp signal generation unit  210  may be significantly reduced. 
         [0033]    The ramp signal generator using a PGA in accordance with the embodiment of the present invention will be described with reference to  FIG. 2 . 
         [0034]    As illustrated in  FIG. 2 , the ramp signal generator using a PGA in accordance with the embodiment of the present invention includes a ramp signal generation unit  210 , a gain amplification control unit  220 , and a PGA  230 . The ramp signal generation unit  210  may generate a ramp signal (ramp voltage). The gain amplification control unit  220  may output a gain amplification control signal for controlling a voltage gain in response to a control signal from a control unit (not illustrated). The PGA  230  may control the voltage gain by amplifying the ramp signal provided from the ramp signal generation unit  210  in response to the gain amplification control signal from the gain amplification control unit  220 . 
         [0035]    The above-described components will now be described in more detail. 
         [0036]    First, the ramp signal generation unit  210  generates a ramp voltage (ramp signal) used as a reference voltage and applies the generated ramp voltage to one terminal (negative terminal) of a differential amplifier  234  of the PGA  230 , and it may be implemented with a common current steering DAC. Since the PGA  230  performs the gain control function of the current steering DAC at the output terminal of the ramp signal generation unit  210 , the current steering DAC used in the embodiment of the present invention does not perform the gain control function. 
         [0037]    The gain amplification control unit  220  includes a reset control block  221 , a variable common mode voltage (VCM) generation unit  222 , and a gain control block  223 . The reset control block  221  may output a reset switch control signal to the PGA  230  in response to a reset control signal from the control unit (not illustrated). The variable VCM generation unit  222  may generate a variable common mode voltage in response to a VCM level control signal from the control unit, and apply the generated common mode voltage VCM to the PGA  230 . The gain control block  223  may output a capacitor control signal to the PGA  230  in response to a gain control signal from the control unit. 
         [0038]    The reset control block  221  then generates the reset switch control signal for turning on or off a reset switch  231  of the PGA  230  in response to the reset control signal from the control unit, and outputs the generated reset switch control signal to the reset switch  231  of the PGA  230 . The reset control signal provided from the control unit is a reset control timing signal which is generated once as a pulse before each row of the CIS starts a read-out operation and is received from a control unit within a CIS chip. As an example the control unit may include a digital core block. 
         [0039]    The variable VCM generation unit  222  may generate a variable common mode voltage in response to a VCM level control bit from the control unit, and apply the generated common mode voltage VCM to the differential amplifier  234  of the PGA  230 . The VCM level control bit may include 0 or 1, and is received from the control unit within the CIS chip. The variable VCM generation unit  222  may increase or decrease the common mode voltage VCM based on the value of the VCM level control bit having a 0 or 1, and apply the generated common mode voltage VCM to the other terminal (positive terminal) of the differential amplifier  234 . The generated common mode voltage VCM may linearly increase or decrease based on the value of the VCM level control bit having a 0 or 1. The control unit may include a digital core block, for example. 
         [0040]    The gain control block  223  may generate a capacitor control signal for controlling feedback capacitor value and sampling capacitor value or the feedback capacitor value of the PCA in response to the gain control signal from the control unit, and output the generated capacitor control signal to the sampling capacitors  233  and the feedback capacitor  232  of the PGA  230 . At this time, the gain control signal provided from the control unit is a gain control bit including a 0 or 1, and may be received from the control unit within the CIS chip. Then, the gain control block  223  may generate the capacitor control signal for increasing or decreasing the feedback and sampling capacitor values or the feedback capacitor value in response to the value of the gain control bit including a  0  or  1 , and output the generated capacitor control signal to the feedback and sampling capacitors  232  and  233  or the feedback capacitor  232 . Thus, as expressed by Equation 2 below, the voltage amplification gain linearly increases or decreases. The control unit may include a digital core block, for example. 
         [0041]    The voltage amplification gain of the PGA  230  is not changed while the PGA  230  is operated, but previously set before the CIS performs an operation for one frame. That is, the digital core block of the CIS may determine the voltage amplification gain based on the brightness of the surrounding environment. 
         [0042]    Then, the PGA  230  may control a voltage gain by amplifying the ramp signal provided from the ramp signal generation unit  210  at the ratio of the capacitor value controlled in response to the gain amplification control signal from the gain amplification control unit  220 . 
         [0043]    The PGA  230  may include a reset switch  231 , a feedback capacitor  232 , a sampling capacitor  233 , and a differential amplifier  234 . The reset switch  231  may reset the differential amplifier  234  to the common mode voltage VCM provided from the variable VCM generation unit  222  in response to the reset switch control signal from the reset control block  221 . The feedback capacitor  232  may have a feedback capacitor value CF which is controlled in response to the capacitor control signal from the gain control block  223 . The sampling capacitor  233  may sample the ramp signal provided from the ramp signal generation unit  210 . The differential amplifier  234  may amplify the ramp signal sampled by the sampling capacitor  233  based on the ratio of the value of the sampling capacitor  233  and the value of the feedback capacitor  232  (sampling capacitor value/feedback capacitor value), and output the amplified ramp signal as a signal VOUT. 
         [0044]    The reset switch  231  may be formed between one input terminal and an output terminal of the differential amplifier  234 . When the reset switch control signal provided from the reset control block  221  is turned to an on-state, the reset switch  231  may be closed. Then, a closed loop feedback network may be formed in the PGA  230 , and one input node VINN and an output node VOUT of the differential amplifier  234  may have the same level as the common mode voltage VCM inputted to the other input terminal of the differential amplifier  234  from the variable VCM generation unit  222 . This means that the operating (potential) points of the input node and the output node of the differential amplifier  234  are reset to the level of the common mode voltage VCM in order for the differential amplifier  234  to operate at a proper operating point. The variable VCM generation unit  222  is used to apply a variable function in order to prevent a malfunction of the differential amplifier  234 , which may occur when the differential amplifier  234  has no proper operating point. 
         [0045]    The feedback capacitor  232  may be provided between the input terminal and the output terminal of the differential amplifier  234 , and has a value (amount) which is controlled to increase or decrease in response to the capacitor control signal from the gain control block  223 . The feedback capacitor  232  may then be implemented with a combination of a plurality of capacitors and switches (not illustrated). Each of the switches may be opened or closed to determine whether to use the corresponding capacitor. 
         [0046]    The sampling capacitor  233  may be provided between the output terminal of the ramp signal generation unit  210  and the input terminal of the differential amplifier  234 , and may have a value (amount) which is controlled to increase or decrease in response to the capacitor control signal from the gain control block  233 . The sampling capacitor  233  may be implemented to have a fixed capacitor value. 
         [0047]    The differential amplifier  234  is an inverting differential amplifier which inverts an input of the input terminal (negative terminal). As the reset switch  231  is turned on, the differential amplifier  234  may be reset to the level of the common mode voltage VCM inputted to the other input terminal thereof. Then, when the ramp signal sampled by the sampling capacitor  233  is inputted to the input terminal thereof, the differential amplifier  234  may amplify the ramp signal based on the ratio of the value of the sampling capacitor  233  and the value of the feedback capacitor  232  (sampling capacitor value/feedback capacitor value), and output the amplified ramp signal as a signal VOUT. 
         [0048]    Thus, the voltage amplification gain Gv may be determined based on the ratio of the sampling capacitor value CS and the feedback capacitor value CF, as expressed by Equation 2 below. 
         [0000]        Gv =CS(sampling capacitor value)/CF(feedback capacitor value)  [Equation 2]
 
         [0049]    As described above, when the ramp signal is inputted through the sampling capacitor  233  from the ramp signal generation unit  210 , the PGA  230  may amplify the ramp signal based on the ratio of the sampling capacitor value CS and the feedback capacitor value CF, which is controlled in response to the gain amplification control signal from the gain amplification control unit  220 . 
         [0050]    The ramp signal generator in accordance with the embodiment of the present invention may control the slope of the ramp signal (ramp voltage) generated from the ramp signal generation unit  210  (current steering DAC) using the capacitor-based PGA  230  thereby controlling the entire gain of the CIS. 
         [0051]    The ramp signal generator in accordance with the embodiment of the present invention may be applied to various structures using a single-slope ADC. 
         [0052]    Now, the PGA  230  in accordance with the embodiment of the present invention and the voltage mode gain control unit  20  of  FIG. 1  will be comparatively described. 
         [0053]    In the voltage mode gain control unit  20  of  FIG. 1 , the passive element used in the feedback network is implemented with a resistor. On the other hand, in the embodiment of the present invention, the capacitor may be used to prevent current consumption in the feedback network. 
         [0054]    Furthermore, the voltage mode gain control unit  20  of  FIG. 1  receives the ramp voltage through the resistor. In this case, when the current steering DAC is used, an accurate voltage may not be obtained because the resistor for generating the voltage of the current steering DAC and the resistor in the feedback network of the voltage mode gain control unit  20  are coupled in parallel to each other. Thus, in the voltage mode gain control unit  20  of  FIG. 1 , the buffer  10  may be inserted between the part for generating the ramp voltage and the voltage mode gain control unit  20  to isolate the two circuits from each other. In this case, an additional current for the buffer  10  is generated. In the embodiment of the preset invention, however, since the capacitor is used in the feedback network, the above-described situation does not occur. 
         [0055]    Furthermore, a capacitor has more accurate matching characteristics than a resistor. Thus, the PGA using a capacitor in accordance with the embodiment of the present invention may perform gain control more precisely than the voltage mode gain controller  20  of  FIG. 1 , which controls the gain using the resistor. 
         [0056]    Furthermore, in the voltage mode gain control unit  20  of  FIG. 1 , the starting level of the ramp voltage is determined depending on the current flowing in the feedback network. However, in accordance with the embodiment of the present invention, the starting level of the ramp voltage may be controlled by changing the common mode voltage inputted to the PGA. 
         [0057]    In accordance with the embodiment of the present invention, the voltage buffer may be implemented with the PGA so as to control the magnitude of the voltage. 
         [0058]    Furthermore, the amount of current flowing in the ramp signal generation unit may be reduced. 
         [0059]    Furthermore, although current consumption is increased by the addition of the voltage buffer, the total current consumption may be significantly reduced because the reduction in current consumption of the ramp signal generation unit is much larger than the corresponding increase in current consumption. 
         [0060]    Although various embodiments have been described for illustrative purposes, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.