Abstract:
A circuit configuration and a method enables less-expensive bipolar switching devices to be employed in high-accuracy class D amplifiers. A constant-bias current is provided between the switching devices of a class D amplifier, and, through feedback or appropriate alternative implementation, the output levels of the class D amplifier achieve deterministic voltage levels regardless of current.

Description:
REFERENCE TO RELATED APPLICATION 
   This application claims priority to U.S. Provisional Patent Application Ser. No. 60/565,746, filed Apr. 27, 2004, the entire content of which is incorporated herein by reference. 

   FIELD OF THE INVENTION  
   This invention relates generally to electronic amplifiers and, in particular, to a circuit configuration and a method whereby less-expensive bipolar switching devices may be employed in high-accuracy class D amplifiers. 
   BACKGROUND OF THE INVENTION 
   Class A-B amplifiers ensure accurate amplification through the balance of non-saturated sink and source (push-pull) output stage sections which each provide current of a single polarity through both polarities of the output. 
   Class D amplifiers yield higher efficiency than class A-B amplifiers, through use of saturated mutually-exclusive sink and source switching devices. Being reliant on such devices, however, this class of amplifier enjoys no balancing mechanism. In that current in the output stage of a class D amplifier is not always in phase with the voltage, bulk devices which can conduct current in two directions, such as MOSFETs, are thus used to ensure that higher-potential devices may sink current, and that lower-potential devices may source current. MOSFETs, however, are more expensive than their bipolar counterparts, which conduct current only in a single direction. 
   The need exists for a circuit configuration and a method whereby less-expensive bipolar switching devices may be employed in high-accuracy class D amplifiers. 
   SUMMARY OF THE INVENTION 
   The present invention broadly resides a circuit configuration and a method whereby less-expensive bipolar switching devices may be employed in high-accuracy class D amplifiers. 
   In the preferred embodiment, a constant-bias current is provided between the switching devices of a class D amplifier, and, through feedback or appropriate alternative implementation, the output levels of the class D amplifier achieve deterministic voltage levels regardless of current. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  shows a schematic diagram of a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION  
   Referring now to  FIG. 1 , pulsewidth modulator  102  receives incoming data stream  101  as input, and produces digital pulsewidth stream  103  as output. Pulsewidth stream  103  is constrained to two invariant output levels, as is known in the art. 
   Error amplifier  104  receives pulsewidth stream  103  as a non-inverting input, and outputs a voltage to transistor  108  at its base input. The output of error amplifier  104  is similar in nature to pulsewidth stream  103 , alternating between two narrowly-constrained voltage ranges. 
   Constant current source  106  provides a controlled current through transistor  107 , while constant current source  112  provides a controlled current through transistor  108 . The net effect is that a constant current flows through both transistors  107  and  108  at all times, in addition to any load current in either device. 
   Capacitor  105  couples the AC component of error amplifier  104  output to transistor  107  at its base input, while not disturbing the DC operation of constant current source  106 . Transistor  107  receives positive supply voltage V+ at its collector and provides output at its emitter. Transistor  108  receives negative supply voltage V− at its collector and provides output at its emitter. In that the emitters of transistors  107  and  108  are connected, transistor  107  can be seen to be minimally sourcing a small current against transistor  108  at all times, and transistor  108  can be seen to be minimally sinking a small current against transistor  107  at all times. 
   Inductor  109 , in conjunction with capacitor  110 , filters high-frequency switching noise from the output signal of the emitters of transistors  107  and  108 , and directly powers load  111 . Note that the connected emitters directly drive the inverting input of error amplifier  104  as well. This ensures that the outputs of said transistors  107  and  108  will exactly replicate the invariant voltage levels of pulsewidth stream  103  output by pulsewidth modulator  102 . 
   The preceding circuit description contains fundamental elements of both Class AB amplification, in the form of constant output stage bias current, and Class D amplification, in the form of switching operation. That is, if output current sourcing is predominant, a small sink current is always present; and if current sinking is predominant, a small source current is always present. This balance of the two (push-pull) output stage sections ensures that output transitions through zero always occur in a controlled fashion. This balance of two output stage sections yields another less-obvious benefit. Namely, in that current source and sink sections are always balanced against one another, each output stage section need only produce current in a single direction. That is, the current source section is never required to sink current, and the current sink section is never required to source current. This characteristic is extremely important for operation points where output current is not in phase with output voltage. 
   The switching nature of Class D amplifiers is well known in the art, but the output voltage and current in Class D amplifiers are often out of phase due to the common use of output filter inductors. Although early Class D amplifiers suffered high distortion by ignoring this fact, high-quality Class D amplifiers now almost universally employ switching elements with bidirectional current control, such as MOSFETs, to make current direction transparent to the pulsewidth modulation. 
   The circuit description of  FIG. 1  essentially depicts a Class D modulator coupled to a Class AB output stage. The use of pulsewidth modulation in the present invention brings acceptance of digital input data and improved efficiency. The use of constant output bias current allows use of inexpensive transistors with unipolar current control. Note that output voltage and current in a Class D amplifier are out of phase approximately 50 percent of the time. Resultantly, the theoretical efficiency of the present invention is found to be approximately between that of a Class AB amplifier and a Class D amplifier. 
   Although bipolar transistors are shown herein, operation with alternative control elements is anticipated. The present invention finds utility in both bridged and single-ended applications.