Abstract:
This invention is a clamp circuit for a video input. The clamp circuit includes: a coupling capacitor; a differential amplifier comparing a video input to predetermined reference voltage; a clamp transistor having a gate connected to the output terminal of the differential amplifier and a source-drain path connected between a power supply voltage and a second terminal; a resistive element connecting the second terminal of the clamp transistor and the coupling capacitor; a first current sink carrying a first predetermined current from the coupling capacitor to ground; and a second current sink carrying a second predetermined current from the second terminal of the said clamp transistor to ground. The resistive element can be a transistor, a resistor, a diode or a switch.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims priority under 35 U.S.C. 119(e) (1) to U.S. Provisional Application No. 61/146,028 filed Jan. 21, 2009. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The technical field of this invention is a clamp circuit for video applications. 
       BACKGROUND OF THE INVENTION 
       [0003]    Undesired leakage in output of trans-conductor amplifier of analog-video-clamp circuit causes artifacts in video signal. 
       SUMMARY OF THE INVENTION 
       [0004]    This invention introduces reverse leakage current at a video-node. This causes the ordinary leakage current to be ineffective when input is low such as during the blanking period of video signal. During higher signals levels the ordinary leakage current will be modulated by the reverse leakage current. 
         [0005]    This invention may use a higher reference voltage for the trans-conductor amplifier. This higher reference voltage will cause lower dynamic range. The invention preferably uses a process having low leakage which may increase cost. This invention enables use of a low-cost digital CMOS process with leaky transistor without performance trade-off. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    These and other aspects of this invention are illustrated in the drawings, in which: 
           [0007]      FIG. 1  illustrates a prior art bottom-level clamp circuit; 
           [0008]      FIG. 2  illustrates the ideal non-linear transfer function for the trans-conductance amplifier of the prior art circuit of  FIG. 1 ; 
           [0009]      FIG. 3  illustrates a comparison between the ideal clamp signal and a spice simulation of the clamped signal from the prior art video clamp circuit during vertical blanking; 
           [0010]      FIG. 4  illustrates a comparison between the ideal clamp signal and a spice simulation of the clamped signal from the prior art video clamp during active video; 
           [0011]      FIG. 5  illustrates the clamp circuit of this invention; 
           [0012]      FIG. 6  illustrates a comparison between the ideal clamp signal and a spice simulation of the clamped signal from the video clamp circuit of this invention during vertical blanking; 
           [0013]      FIG. 7  illustrates a comparison between the ideal clamp signal and a spice simulation of the clamped signal from the video clamp circuit of this invention during active video; and 
           [0014]      FIG. 8  compares the spice simulations of the transfer function of the trans-conductance amplifier of the prior art with that of the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0015]    Analog video signals are applied to video processors through AC coupling capacitors. A video processing circuit first needs to restore the DC level to a known reference to put the video signal within its dynamic input range before this signal can be further processed. 
         [0016]      FIG. 1  illustrates a prior art bottom-level clamp circuit  100  used for this purpose. The input video signal is coupled to the clamp circuit via AC coupling capacitor  101 . Amplifier  102  compares the video signal V CL , following the AC coupling capacitor  101  with a clamp reference V ref  and generates an output V O  current proportional to the error voltage. This output V O  is coupled to the gate electrode of charge transistor  103 . Charge transistor  103  supplies a corresponding source-drain current I charge  that is fed back to charge AC coupling capacitor  101  as I clamp . 
         [0017]    If the AC coupled input video signal V CL , is below the clamp-reference V ref , trans-conductance amplifier formed by amplifier  102  and charge transistor  103  generates an output V O  current proportional to the error voltage at amplifier  102 . If the signal is above the reference point, it is desirable to shut-off the charge current completely. Accordingly the trans-conductance amplifier formed by amplifier  102  and charge transistor  103  has a non-linear transfer function. Charge sink  104  carries a small intentional leakage current I L  from the output node. This current I L  pulls charge out of AC coupling capacitor  101  occurring due to any spike that may occur because of input noise, power-up transients etc. 
         [0018]      FIG. 2  illustrates the ideal non-linear transfer function for the trans-conductance amplifier formed by amplifier  102  and charge transistor  103 . For an input voltage V CL  below V ref  the output is in linear region  201 . In linear region  201  the output I charge  is a function of the difference between V CL  and V ref . For an input voltage V CL  above V ref  the output is in clamp region  202 . In clamp region  202  the output I charge  is zero. 
         [0019]    Using the ideal trans-conductance amplifier transfer function illustrated in  FIG. 2  V ref  should be zero to give maximum dynamic range. Because charge transistor  103  cannot turn off completely, the transition to zero charge current is soft. Leakage in a P-channel transistor such as charge transistor  103  as well as design challenges with the non-linear trans-conductor amplifiers results in an undesired leakage current. Even very small currents in order of 500 nA will cause artifacts in video signal causing the DC level to shift during blanking period. The current I L  of current sink  104  will not help during the blanking period since there is no voltage headroom. Increasing V ref  may be helpful but would also limit the dynamic range. 
         [0020]      FIGS. 3 and 4  illustrate results of the undesirable leakage current of the prior art.  FIG. 3  illustrates a comparison between the ideal clamp signal from the transfer function of  FIG. 2  and a spice simulation of the clamped signal from the prior art video clamp circuit of  FIG. 1  during the vertical blanking interval. Vertical blanking occurs during interval  310 . Signal  311  illustrates the response to the ideal clamp signal of the transfer function of  FIG. 2 .  FIG. 3  illustrates that ideal clamp signal  311  remains near zero volts during blanking interval  310 . Signal  312  illustrates a simulation of the response to the prior art circuit illustrated in  FIG. 1 .  FIG. 3  illustrates that the simulated clamp signal  312  drifts upward in voltage during blanking interval  310 . The leakage current of transistor  103  changes the voltage across AC coupling capacitor  101  causing this voltage drift. 
         [0021]      FIG. 4  illustrates a comparison between the ideal clamp signal from the transfer function of  FIG. 2  and a spice simulation of the clamped signal from the prior art video clamp circuit of  FIG. 1  during the active video.  FIG. 4  shows active line periods  410 .  FIG. 4  illustrates that ideal clamp signal  411  remains at a nearly constant voltage during active video interval  410 . Signal  412  illustrates a simulation of the response to the prior art circuit illustrated in  FIG. 1 .  FIG. 4  illustrates that the simulated clamp signal  412  drifts downward in voltage during the active video interval  410 . The leakage current of transistor  103  changes the voltage across AC coupling capacitor  101  causing this voltage drift. 
         [0022]    The time domain video signal artifacts that are caused by leakage in transistor  103  are shown in  FIGS. 3 and 4 . Increasing I L  will cause signal dependent line-droop which is also undesirable. This will not help during blanking period. 
         [0023]      FIG. 5  illustrates the circuit of this invention. The circuit of  FIG. 5  is an improvement over the circuit of  FIG. 1 . The input video signal is coupled to the clamp circuit via AC coupling capacitor  501 . Amplifier  502  compares the video signal following the AC coupling capacitor V CL , with a clamp reference V ref  and generates an output V O  current proportional to the error voltage. This output V O  is coupled to the gate electrode of charge transistor  503 . Charge transistor  503  supplies a corresponding source-drain current I charge  that is fed back to charge the ac coupling capacitor as I clamp . Charge sink  504  carries a small intentional leakage current I L . 
         [0024]      FIG. 5  further includes transistor  505  between clamp transistor  503  and the feedback terminal. Transistor  505  receives a gate bias voltage V bias  causing it to act as a resistor and carry a predetermined current. Transistor  505  may be replaced by a diode, resistor or switch serving the same function.  FIG. 5  also includes current sink  506  having a current of I charge . Current sink  506  introducing an opposite leakage current to I charge  at a cascade node that is isolated from the input V CL  through transistor  505 . The current I charge  of current sink  506  is selected to saturate the undesirable cut-off process leakage of transistor  503 . Transistor  505  isolates the current drawn by current sink  506  from the input V CL . This invention also shifts V ref  slightly introducing an offset to the linear portion of the trans-conductance amplifier while obtaining a very sharp transition point. 
         [0025]    The current I charge  of current sink  506  is selected corresponding to the leakage current of clamp transistor  503  when cut-off. Thus when clamp transistor  503  is cut-off no current will flow through clamp transistor  503  into AC coupling capacitor  501 . The voltage V bias  is selected to permit conducting the current when clamp transistor  503  is operating in region  201  of  FIG. 2 . If transistor  505  is replaced with a resistor it value is selected to permit carrying this current. If transistor  505  is replaced with a diode, this diode should conduct current from clamp transistor  503  to the node. The forward bias voltage of this diode prevents the leakage current of clamp transistor  503  from entering AC coupling capacitor  501 . If transistor  505  is replaced by a switch, the switch should be closed to couple clamp transistor  503  to AC coupling capacitor  501  when operating in region  201  and open to isolate clamp transistor  503  from AC coupling capacitor  501  when operating in region  202 . 
         [0026]      FIGS. 6 and 7  illustrate the improvement of this invention.  FIG. 6  shows the improvement of the invention over the prior art of  FIG. 3 .  FIG. 6  illustrates a comparison between the output signal using an ideal clamping circuit and a simulation of the inventive circuit of  FIG. 5 . During vertical blanking interval  610 , ideal clamped signal  611  and signal  612  clamped by the circuit in  FIG. 5  are virtually identical.  FIG. 7  shows the improvement of the invention over the prior art of  FIG. 4 .  FIG. 7  shows active line periods  710 .  FIG. 7  illustrates that ideal clamp signal  711  remains at a nearly constant voltage during active video interval  710 . Signal  712  illustrates a simulation of the response to the invention illustrated in  FIG. 5 . During active video period non-ideal overshoot is nearly eliminated.  FIG. 7  illustrates that the simulated clamp signal  712  drifts downward in voltage during the active video interval  710  much less than shown in  FIG. 4 . This downward drift comes from the slight leakage current I L  and is intentional and programmable. 
         [0027]      FIG. 8  compares the spice simulations of the transfer function of the trans-conductance amplifier of the prior art illustrated in  FIG. 1  with that of the invention illustrated in  FIG. 5 . These transfer functions should be compared with the ideal transfer function shown in  FIG. 2 . Curve  801  is the transfer function of the prior art. Curve  802  is the transfer function of the invention. Curve  802  shows a sharper corner more nearly the ideal transfer function illustrated in  FIG. 2  than the prior art curve  801 .  FIG. 8  illustrates a small offset shift between curves  802  and  801  due to the intentional leakage introduced to saturate undesired process leakage. This offset shift in curve  802  can be tolerated since it will be in order of few millivolts. Alternatively this offset shift may be compensated with an offset correction circuit since it will be static.