Abstract:
The present invention discloses a gain amplifier with DC offset cancellation circuit. The multiple feedback factors are employed in the DC offset cancellation circuit for canceling DC offset by increasing convergence time.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to a gain amplifier with DC offset cancellation circuit, and, more particularly, the present invention relates to a gain amplifier with DC offset cancellation circuit applied in digital communications systems.  
         BACKGROUND OF THE INVENTION  
         [0002]    Programmable gain amplifier is an indispensable component in digital communications systems. A programmable gain amplifier is provided for boosting gain of signals such that received input signals from radio frequency front-end circuits can be adjusted to suitable level. Accordingly, by doing so, demand for dynamic range of a base band analog-digital converter (ADC) is reduced. Moreover, if input signal have DC offset, a gain amplifier can be utilized for boosting DC offset of the input current. After processed by a gain amplifier, resulting signals will saturate the following baseband ADC stage. Due to coupling of RF input to local oscillators input of mixers in the applied architectures of low intermediate frequency (Low IF) and Zero intermediate frequency (Zero IF), it is highly likely that such coupling may introduce interference where signals are mixed and result in unavoidable DC offset. Therefore, it is desired to incorporate a DC offset cancellation circuit in the design for resolving the aforementioned problem. Furthermore, due to mismatch of components or manufacturing process flaw, DC offset can also be observed in integrated circuits used in gain amplifier circuits. The architecture proposed in the present invention offers a solution for resolving such DC offset problem and a method for allowing the system to be rapidly converged to stable status.  
         SUMMARY OF THE INVENTION  
         [0003]    The main objective of the present invention is to provide a gain amplifier with DC offset cancellation circuit. By applying multiple sets of feedback factors for reducing convergence time, the DC Offset of the signals can be cancelled.  
           [0004]    The other objective of the present invention is to provide a gain amplifier with fast DC offset cancellation circuit, wherein the gain amplifier with DC offset cancellation circuit comprises a gain amplifier having an input end coupled to an input signal source, a buffer having an input end coupled to the output end of the gain amplifier; and an active low-pass analogue filter coupled to the output end of the buffer, for filtering input analogue signals at high frequencies and feeding the filtered output signals to the input of gain amplifier to be deducted from the input signals, so as to cancel DC bias.  
           [0005]    With these disadvantages of the various display technologies in mind, a better type of display would be desirable which requires less voltage, is more efficient and is generally more advantageous for all types of display applications. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a schematic circuit diagram of a gain amplifier with DC offset cancellation circuit according to a preferred embodiment of the present invention;  
         [0007]    [0007]FIG. 2 is a circuit diagram of a variable resistor of the gain amplifier shown in the FIG. 1;  
         [0008]    [0008]FIG. 3 is a schematic frequency response diagram according to the present invention;  
         [0009]    [0009]FIG. 4 is a schematic diagram illustrating simulation result under different temperatures when 48 dB gain is provided;  
         [0010]    [0010]FIG. 5 is a schematic diagram illustrating simulation result under different temperatures when 48 dB gain is provided and a lower capacitance is feedback to a low-pass filter; and  
         [0011]    [0011]FIG. 6 is a schematic diagram illustrating test simulation result when a DC offset input is provided. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    Refer to FIG. 1, a schematic circuit diagram of a gain amplifier with DC offset cancellation circuit is illustrated according to a preferred embodiment of the present invention. The gain amplifier comprises a gain amplifier, a buffer  2  and an active low-pass analogue filter  3 . The gain amplifier  1  has an input end coupled to an input signal source  10 . The buffer  2  has an input end coupled to the output end of the gain amplifier  1 . The active low-pass analogue filter  3  is coupled to the output end of the buffer  2  for filtering input analogue signals at high frequencies and providing a negative feedback by outputting filtered input signals to the input end of input signal source  10  to be deducted from the input signals, so as to cancel DC bias.  
         [0013]    The active low-pass analogue filter  3  implemented in the preferred embodiment according to the present invention comprises: a variable resistor  30 , an amplifier  31 , a capacitor pair  32  and a comparator 33 . The amplifier  31  is coupled to the output end of the variable resistor  30 . A negative feedback is provided from the output end of the variable resistor  30  to the input signal source  10 . The capacitor pair is coupled to the amplifier  31 . One input end of the comparator  33  is coupled to the output end of the amplifier  31 . The other input end of the comparator  33  is coupled to a reference voltage source Vref. Signals are output to the variable resistor  30  after comparing with the reference voltage Vref in the comparator  33 .  
         [0014]    The preesnt invention operates with low capacitance value initially. Such configuration converges system loops to a stable status. Then operation is applied with higher capacitance value, whereas DC bias drops to a low level. Such a sufficient low DC bias does not interfere bit-error-rate (BER) by over-filtering DC signals.  
         [0015]    [0015]FIG. 2 is a circuit diagram of a variable resistor of the gain amplifier shown in the FIG. 1. The variable resistor  30  applied in the present embodiment comprisesa four metallic oxide semiconductor field effect transistors (MOSFETs)  40 ,  41 ,  42 , and  43 . The gates  400  and  430  of the MOSFETs  40  and  43  are coupled to a output end VC 1  of the comparator  33  (not shown) for receiving a fixed reference voltage Vref. The gates  410  and  420  of the MOSFETs  41  and  42  are coupled to the other output end VC 2  of the comparator  33  (not shown). The sources  401  and  421  of the MOSFETs  40  and  42  are coupled to a positive output end  20  of the buffer  2 . The sources  411  and  431  of the MOSFETs  41  and  43  are coupled to a negative output end  21  of the buffer  2 . The drains  402  and  412  of the MOSFETs  40  and  41  are coupled to a negative output end  310  of the amplifier  31 . The drains  422  and  432  of the MOSFETs  42  and  43  are coupled to a positive output end  311  of the amplifier  31 .  
         [0016]    The simulation result is generated by a programmable gain amplfier of a communications system where low intermediate frequency architecture is adopted. The system is provided with a 48 dB gain by the programmable gain amplfier. FIG. 3 is a schematic frequency response diagram according to the present invention. The gain result shown in FIG. 3 is generated by a programmable gain amplfier where a higher capacitance value feedback is provided to a low pass filter. FIG. 4 is a schematic diagram illustrating simulation result under different temperatures when 48 dB gain is provided. FIG. 5 is a schematic diagram illustrating simulation result under different temperatures when 48 dB gain is provided and a lower capacitance is feedback to a low-pass filte. FIG. 6 is a schematic diagram illustrating test simulation result when a DC offset input is provided. When DC bias of ±400 mV input is provided, system output is rapidly congerged to ±50 mV and the system then switch to adopt a higher reactance value and assure the stability of signals.  
         [0017]    As shown in the preferred embodiment, the circuit volume of integrated circuit of a gain amplifier is reduced by applying the aformentioned architecture according to the present invention. Multi bandwidth offered by multi loops is implemented by applying variable capacitors for controlling carious feedback factors. Such configuration facilitates the convergence of all loops. Moreover, the configuration allows the system loop to be rapidly converged to stable status.  
         [0018]    Although the invention is illustrated and described herein with reference to particular embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention.