Patent Publication Number: US-5892399-A

Title: Current boost circuit for reducing crossover distortion in an operational amplifier

Description:
This application claims priority under 35 U.S.C. § 119 (e) (1) of provisional application number 60/022,082, filed Jul. 29, 1996. 
     FIELD OF THE INVENTION 
     This invention generally relates to electronic circuits and in particular it relates to current boost circuits for operational amplifiers. 
     BACKGROUND OF THE INVENTION 
     FIG. 1 shows a prior art operational amplifier output stage. The prior art circuit of FIG. 1 includes transistors M 10  -M 17 , resistors R C1  and R C2 , capacitors C C1  and C C2 , negative rail V SS , positive rail V DD , input voltage V IN , output voltage V OUT , and bias voltages V B1  and V B2 . When the input voltage V IN  is close to the negative rail V SS , transistor M 16  has the potential to source much more current than M 17  sinks so the output voltage V OUT  is close to the positive rail V DD . The gate of transistor M 17  is then at a lower voltage than the drain. If there is a large positive voltage step at the input V IN  of this stage, transistor M 16  turns off quickly. However, to drive the output voltage close to the negative rail V SS , M 17  must provide current to the load. Thus the voltage from gate to drain of M 17  must change from a large negative value to a positive value. This change requires a large current to pass through C C1 . The only source for this current is transistor M 11 . Due to a constant voltage at its gate, transistor M 11  provides a constant bias current for M 13  and M 15 , but this current is not sufficient to charge C C1  quickly enough for large signal transitions. Therefore this output stage exhibits crossover distortion under heavy loading conditions. 
     SUMMARY OF THE INVENTION 
     Generally, and in one form of the invention, the current boost circuit includes: a first transistor having a gate coupled to an input node; a second transistor; a first capacitance coupled between a gate of the second transistor and a drain of the first transistor; a third transistor having a gate coupled to a drain of the second transistor; and a second capacitance coupled between the drain of the second transistor and a drain of the third transistor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a schematic diagram of a prior art operational amplifier output stage; 
     FIG. 2 is a schematic diagram of a preferred embodiment operational amplifier output stage. 
    
    
     Corresponding numerals and symbols in the different figures refer to corresponding parts unless otherwise indicated. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 2, a circuit diagram of a preferred embodiment operational amplifier is shown. The circuit of FIG. 2 includes transistors M 10  -M 17 , resistors R C1 , R C2 , and R C3 , capacitors C C1 , C C2 , and C C3 , negative rail Vss, positive rail V DD , input voltage V IN , output voltage V OUT , and bias voltages V B1  and V B2 . The circuit of FIG. 2 includes the circuit of FIG. 1 with the addition of resistance R C3  and capacitance C C3 . On a positive transition at the input V IN , the drain voltage of transistor M 10  decreases, causing capacitance C C3  to pull the gate of transistor M 11  down. As the voltage from the source to the gate of transistor M 11  increases, transistor M 11  provides more current, and capacitance C C1  charges faster. After a certain time constant determined by the product of resistance R C3  and capacitance C C3 , the current flowing through transistor M 11  returns to its quiescent value. 
     Alternative versions of the preferred embodiment include many variations in resistance R C3  such as leaving it out of the circuit completely or forming an equivalent resistance with other components (for example, a MOSFET biased in the linear region). There are also ways to use parasitic circuit components to form a capacitance that replaces capacitance C C3 . These alternative embodiments provide the same improvement in performance described above. The same principle applies to other amplifier stages as well. The deficiency in the prior art circuit shown in FIG. 1 is the lack of current available for charging capacitance C C1  during a large output voltage swing. This lack of current is a problem in other amplifiers as well, and this preferred embodiment applies to amplifiers that charge a capacitor from a fixed current source. 
     The preferred embodiment produces no increase in the speed (bandwidth) of the circuit, since the product of the current available from a previous stage and the value of capacitor C C2  determines the bandwidth. Neither of these determining quantities changes with the addition of the preferred embodiment. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.