Amplifier With Improved Input Resistance and Controlled Common Mode

Abstract

An amplifier includes a first pair of transistors (the first pair) that defines a first output, each transistor of the first pair having a gate coupled to a first input terminal; a second pair of transistors (the second pair) that defines a second output, each transistor of the second pair having a gate coupled to a second input terminal; a first capacitor coupled to the second output terminal and to the gate of a first transistor of the first pair; a second capacitor coupled to the second output terminal and to the gate of a second transistor of the first pair; a third capacitor coupled to the first output terminal and to the gate of a third transistor of the second pair; and a fourth capacitor coupled to the first output terminal and to the gate of a fourth transistor of the second pair.

1 . An amplifier comprising:
 a first pair of transistors that defines a first output, each transistor of the first pair of transistors having a gate coupled to a first input terminal;   a second pair of transistors that defines a second output, each transistor of the second pair of transistors having a gate coupled to a second input terminal;   a first pair of capacitors that minimizes degradation of input resistance at the first input terminal, a first capacitor of the first pair of capacitors coupled to the second output terminal and to the gate of a first transistor of the first pair of transistors, and a second capacitor of the first pair of capacitors coupled to the second output terminal and to the gate of a second transistor of the first pair of transistors; and   a second pair of capacitors that minimizes degradation of the input resistance at the second input terminal, a third capacitor of the second pair of capacitors coupled to the first output terminal and to the gate of a third transistor of the second pair of transistors, and a fourth capacitor of the second pair of capacitors coupled to the first output terminal and to the gate of a fourth transistor of the second pair of transistors.   
     
     
         2 . The amplifier as claimed in  claim 1 , wherein
 the first pair of capacitors that defines paths for correction currents, the correction currents being equal in magnitude and opposite in polarity to currents flowing due to corresponding parasitic capacitances of the first pair of transistors; and   the second pair of capacitors that defines paths for correction currents, the correction currents being equal in magnitude and opposite in polarity to currents flowing due to corresponding parasitic capacitances of the second pair of transistors.   
     
     
         3 . The amplifier as claimed in  claim 1  and further comprising:
 a biasing circuit, coupled to the first pair of transistors and the second pair of transistors, that biases the first pair of transistors and the second pair of transistors. 
 
     
     
         4 . The amplifier as claimed in  claim 3 , wherein the biasing circuit comprises:
 a pair of diodes; and   a pair of current sources coupled to the pair of diodes.   
     
     
         5 . The amplifier as claimed in  claim 4 , wherein the pair of current sources comprises:
 at least one digital to analog converter that controls a common mode voltage associated with the first output and the second output.   
     
     
         6 . The amplifier as claimed in  claim 5  and further comprising:
 a comparator, coupled to the first output terminal and the second output terminal, that compares the common mode voltage with a reference voltage. 
 
     
     
         7 . The amplifier as claimed in  claim 1 , wherein
 the first transistor and the third transistor comprise positive metal oxide semiconductor (PMOS) type transistors, and   the second transistor and the fourth transistor comprise negative metal oxide semiconductor (NMOS) type transistors.   
     
     
         8 . The amplifier as claimed in  claim 1 , wherein the amplifier comprises an output stage amplifier of a multistage amplifier circuit. 
     
     
         9 . The amplifier as claimed in  claim 1 , wherein the amplifier is coupled to an input circuit, the amplifier functioning as a load to the input circuit and reducing loading of the input circuit by generating correction currents using the first pair of capacitors and the second pair of capacitors. 
     
     
         10 . A control circuit for controlling a common mode voltage associated with an output of a circuit, the control circuit comprising:
 a comparator coupled to the circuit to compare the common mode voltage with a reference voltage;   a digital to analog converter (DAC) coupled to the comparator and responsive to a transition at a comparator output to configure the DAC to a setting corresponding to the transition; and   a biasing element, coupled to the circuit and to the DAC, that enables biasing of the circuit based on the setting, thereby controlling the common mode voltage.   
     
     
         11 . The control circuit as claimed in  claim 10 , wherein the biasing element is a diode. 
     
     
         12 . The control circuit as claimed in  claim 11  and further comprising:
 a clock coupled to the comparator and is responsive to the comparator output and a clock input to generate a clock signal; and 
 a counter, coupled to the DAC and the clock, that is responsive to the clock signal to configure the DAC. 
 
     
     
         13 . A method for controlling a common mode voltage of a circuit, the method comprising:
 comparing the common mode voltage with a reference voltage;   configuring a digital to analog converter (DAC) to a setting based on the comparing; and   biasing the circuit based on the setting.   
     
     
         14 . The method as claimed in  claim 13 , wherein configuring the DAC comprises:
 configuring the DAC by a digital block, the digital block comprising a clock and a counter.   
     
     
         15 . The method as claimed in  claim 13 , wherein configuring the DAC comprises:
 configuring the DAC to the setting corresponding to crossing of the reference voltage by the common mode voltage.