Abstract:
An amplifier is provided that includes an output portion that sources and sinks current associated with an output load and an amplification portion that is biased by a relatively small bias current with respect to an output current of the amplifier. The amplification portion provides an amplified output signal to the output portion. The amplifier further comprises at least one impedance component coupled between the output portion and the amplification portion to alter at least one pole associated with the amplifier to mitigate instability of the amplifier related to the relatively small bias current.

Description:
RELATED APPLICATIONS 
     The present invention claims priority from U.S. Provisional Patent Application No. 61/057,844, filed May 31, 2008. 
    
    
     TECHNICAL FIELD 
     This invention relates to circuits, and more specifically to a low bias current amplifier. 
     BACKGROUND 
     A power efficiency improvement of a Class A type amplifier is amplifier circuit design with two transistors in its output stage in a source follower type amplifier configuration. Source follower operation typically uses two “complementary” transistors, one an NPN-type and the other a PNP-type with both power transistors receiving the same input signal together that is equal in magnitude and phase to each other. In this configuration, the output current handling capability is significantly increased. The input signal is reproduced at the output, except at the midpoint when both transistors are OFF and at either rail. These amplifiers are more commonly known as Class B type amplifiers due to the “dead zone” in the midpoint. At very low biasing currents, Class B type amplifiers can oscillate and become unstable under high load conditions. 
     SUMMARY 
     In accordance with an aspect of the invention, an amplifier is provided that comprises an output portion that sources and sinks current associated with an output load and an amplification portion that is biased by a relatively small bias current with respect to an output current of the amplifier. The amplification portion provides an amplified output signal to the output portion. The amplifier further comprises at least one impedance component coupled between the output portion and the amplification portion to alter at least one pole associated with the amplifier to mitigate instability of the amplifier related to the relatively small bias current. 
     In accordance with another aspect of the present invention, a Class B amplifier is provided that comprises an output portion that includes an N-type metal oxide field effect transistor (MOSFET) (NMOS) that sources current and P-type MOSFET (PMOS) that sinks current associated with an output load and an amplification portion that is biased by a relatively small bias current with respect to an output current of the amplifier, such that the ratio of the bias current of the amplification portion to an output current of the amplifier is about 1:20 to about 1:10,000. The amplification portion provides an amplified output signal to the gates of the NMOS and the PMOS. The Class B amplifier further comprises at least one impedance component coupled between the gates of the NMOS and the PMOS and the amplification portion to alter at least one pole associated with the amplifier to mitigate instability of the amplifier related to the relatively small bias current. 
     In accordance with yet another aspect of the invention, a Class B amplifier is provided that comprises means for sourcing and sinking current associated with an output load and means for providing an amplified output signal to the means for sourcing and sinking current. The Class B amplifier further comprises means for biasing the means for providing an amplified output signal with a bias current that is a relatively small with respect to an output current of the amplifier and means for altering at least one pole associated with the amplifier to mitigate instability of the amplifier related to the relatively small bias current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a block diagram of a low bias current Class B amplifier in accordance with an aspect of the present invention. 
         FIG. 2  illustrates a resistor model that can be employed to determine an increase in impedance of an output pole of the amplifier in accordance with an aspect of the present invention. 
         FIG. 3  illustrates an equival small-signal model that can be employed to determine an increase in impedance of the output pole of the amplifier in accordance with an aspect of the present invention. 
         FIG. 4  illustrates a bode plot that illustrates gain responses and phase responses of the amplifier of  FIG. 1  with and without the one or more impedance components in accordance with an aspect of the present invention. 
         FIG. 5  illustrates a detailed schematic diagram of a low bias current Class B amplifier in accordance with an aspect of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a block diagram of a Class B amplifier  10  in accordance with an aspect of the present invention. The amplifier  10  includes an amplification portion  12  and an output portion  14 . The output portion  14  includes an N-type metal oxide semiconductor field effect transistor (MOSFET) MN 1  (hereinafter referred to as an NMOS) coupled to a power supply voltage (VPWR) and an output node  22 . The output portion  14  includes a P-type MOSFET MP 1  (hereinafter referred to as a PMOS) coupled to a power supply ground (VGND) and the output node  22 . The output portion  14  is configured to source current through the NMOS MN 1  during a push phase and sink current through the PMOS MP 1  during a sink phase. The output node  22  is coupled to an output capacitor CLOAD and an output load RLOAD. The amplification portion  12  is powered by a supply voltage (VDD) and a supply ground (VGND). The amplification portion  12  receives a reference signal VREF and an output signal VOUT from the output node  22  and provides an amplified output signal (AMPOUT) to a gate of the NMOS MN 1  and to a gate of the PMOS MP 1 . 
     In accordance with an aspect of the present invention, the amplification portion  12  is biased at a relatively low bias current with respect to the amount of output current IOUT provided at the output of the amplifier  10 . For example, an output current can be provided that is about 20 times to about 10,000 times the bias current of the amplification portion  12 . However, at heavy load conditions an input pole  18  formed from an output resistance (ROUT 1 ) of the amplification portion  12  and an input capacitance (CIN 1  and CIN 2 ) of the NMOS MN 1  and PMOS MP 1  moves toward an output pole  20  formed from an output resistance (ROUT 2 ) of the NMOS MN 1  and the output capacitor CLOAD during the source phase. It is to be appreciated that the input pole  18  also moves toward an output pole formed from an output resistance (ROUT 3 ) of the PMOS and the output capacitor CLOAD during the sink phase. 
     In either situation, if the output impedance of the amplification portion  12  is not small enough, the amplifier  10  becomes unstable. In order to push the input pole to a higher frequency, the amplification portion  12  could consume more current by increasing the bias current of the amplification portion  12  to lower the output impedance and consequently the gain. However, in accordance with an aspect of the present invention, one or more impedance components  16  are coupled between the output of the amplification portion  12  plus the gates of the NMOS MN 1  and PMOS MP 1  of the input portion  14  to push the input pole to a higher frequency and the output pole to a lower frequency without consuming more bias current at the amplification portion  12 . The one or more impedance components  12  can be a diode, a transistor, an inductor, a resistor or combination thereof. The addition of one or more impedance components  16  effectively splits and moves the input pole  18  away from the output pole  20 . With the one or more impedance components  16 , the output of the amplification portion  12  sees another current path to the source of MN 1  or MP 1  so that the impedance at its output is reduced. Therefore, the input pole is moved to a higher frequency without increasing the bias current in the amplification portion  12 . While the input pole  18  is moved to higher frequency, the output pole  20  is pushed to a lower frequency because of the increased impedance looking into the source of MN 1  or MP 1  and hence splitting the poles. 
     The increase in impedance, which causes a decrease in frequency, for the output pole  20  can be derived by applying a resistor model  30  of  FIG. 2  and a small-signal model  40  of  FIG. 3  looking into the source of the NMOS MN 1  and utilizing the equations below. A similar analysis can be employed to determine the decrease in impedance of the input pole  18 . 
                   Vg   =           -   Vx     ⁢     R       ROUT   ⁢           ⁢   1     +   R         ⁢     
     -   Ix   -   gmVg   +     Vx   ⁢               ⁢   1       R   +     ROUT   ⁢           ⁢   1             =   0             Equation   ⁢           ⁢   1               Ix   =       gmVx   ⁢     R     R   +     ROUT   ⁢           ⁢   1           +     Vx   ⁢     1     R   +     ROUT   ⁢           ⁢   1                     Equation   ⁢           ⁢   2                 Vx   Ix     =       1       gm   ⁢     R     R   +     ROUT   1           +     1     R   +     ROUT   1             =       R   +     ROUT   ⁢           ⁢   1         1   +   gmR                 Equation   ⁢           ⁢   3               Rx   =         1   gm     +       ROUT   ⁢           ⁢   1     gmR       =       1   gm     +       ROUT   ⁢           ⁢   1     β                 Equation   ⁢           ⁢   4               
where R L  is the output load, R S  is output impedance of the amplification portion  12 , R is the resistance of the one or more impedance components  16  and R X  is the output impedance associated with the output pole  20 . An analogy to bipolar transistor characteristics can be employed such that:
 
                   Ib   =     Vg   R             Equation   ⁢           ⁢   5               Ic   =   gmVg           Equation   ⁢           ⁢   6                 Ic   Ib     =     gmR   =   β             Equation   ⁢           ⁢   7               
This results in
 
     
       
         
           
             
               
                 
                   Rx 
                   = 
                   
                     
                       
                         1 
                         gm 
                       
                       + 
                       
                         
                           Rout 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                         gmR 
                       
                     
                     = 
                     
                       
                         1 
                         gm 
                       
                       + 
                       
                         
                           ROUT 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                         β 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   8 
                 
               
             
           
         
       
     
       FIG. 4  illustrates a bode plot  50  that illustrates gain responses and phase responses of the amplifier  10  of  FIG. 1  with and without the one or more impedance components  16  in accordance with an aspect of the present invention. A gain versus frequency graph  52  is disposed above a phase versus frequency graph  54  such that the frequencies are aligned between the graphs. The dashed lines indicate the responses associated with the amplifier  10  without one or more impedance components coupled between the amplification portion  12  and the output portion  14  of the amplifier  10 . The solid lines indicate the responses associated with the amplifier  10  with one or more impedance components  16  coupled between the amplification portion  12  and the output portion  14  of the amplifier  10 . The general rule for stability is that the phase shift of the output should not reach 180° phase rolloff before the gain of the amplifier has reached zero gain due to gain rolloff caused by the poles of the amplifier. 
     As illustrated by the dashed line in the gain graph, the input pole and the output poles overlap such that a double overlapping pole  54  is a result. The doubling of the poles causes an increase in the slope of the phase rolloff since each pole causes a  902  shift in the phase of the output signal. For example, a gain rolloff of −20 db/decade causes a phase rolloff of 45°/decade that starts at a decade in frequency prior to a pole and ends a decade in frequency after the pole. Therefore, the phase will rolloff −40 db/decase for the double overlapping pole  54 . As a result of the double overlapping pole  54 , the phase has rolled off to −180° before the gain of the amplifier has reached zero. This is unacceptable since it will cause the amplifier to oscillate and become unstable. As illustrated by the solid line in the gain graph, the input pole  58  and the output pole  56  have been pushed apart from one another. This causes two spaced apart sloping portions that have less steep slope (i.e., −20 db/decade) during the first rolloff. As a result, the phase has rolled off to −155° when the gain of the amplifier has reached zero. This provides a phase margin of 35° that assures stability of the amplifier during normal operation. 
       FIG. 5  illustrates a detailed schematic diagram of a Class B amplifier  70  in accordance with an aspect of the present invention. The amplifier  70  includes an amplification portion  72  and an output portion  74 . The output portion  74  includes an NMOS MN 1  coupled to a power supply voltage (VPWR) and an output node  76 . The output portion  74  includes a PMOS MP 1  coupled to a power supply ground (PGND) and the output node  76 . The output portion  74  is configured to source current through the NMOS MN 1  during a source phase and sink current through the PMOS MP 1  during a sink phase. The amplification portion  72  is powered by a supply voltage (VDD) and a supply ground (VGND). The amplification portion  72  receives a first input signal INP to a gate of a first input NMOS MN 3  and a second input signal INM to a gate of a second input NMOS MN 2 . A current source sinks a current 2*IB 1 . When the inputs (INP and INM) are equal, then half of the current flows through MN 2  and half flows through MN 3 . Each of these currents is equal to IB 1 . This current is then mirrored through two sets of current mirrors: MP 3  with MP 2  and MP 4  with MP 5 . The bias current IB 1  is mirrored through a current mirror formed of NMOS MN 4  and NMOS MN 5 . An amplified output signal AMPOUT is provided to a gate of the NMOS MN 1  and the gate of the PMOS MP 1 . This amplifier provides a push-pull output and is also known as a transconductance amplifier. 
     The bias current IB 1  is selected to be relatively low in respect to the amount of output current IOUT provided at the output of the amplifier  70 . For example, a bias current of 3 microamps can provide an output current of 3 ma. In one aspect of the invention, an output current IOUT can be provided that is about 20 times to about 10,000 times the bias current IB 1  of the amplification portion  72 . A resistor R is coupled between the output of the amplification portion  72  and the gates of the NMOS MN 1  and PMOS MP 1  of the output portion  74  to push the input pole to a higher frequency and the output pole to a lower frequency as previously discussed without consuming more bias current at the amplification portion  72 . The size of the resistor R can be selected based on a given bias current to output current ratio. For example, a 1 MOhm resistor can be used for a 1:1000 bias current to output current ratio, while a 100 MOhm resistor can be used for a 1:10,000 bias current to output current ratio. 
     It is to be appreciated that although the above examples of the present invention are illustrated with amplifiers, the present invention can be employed with other circuits that have a substantially identical poles in which one is associated with a low bias current and the other is associated with a second bias current linked together through a source follower configuration. 
     What have been described above are examples of the invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the invention are possible. Accordingly, the invention is intended to embrace all such alterations, modifications, and variations that fall within the scope of this application, including the appended claims.