1. Field of the Invention
This invention relates to the design of a high quality hybrid pulse width modulated (PWM) audio amplifier that is constructed in a single miniature package using what the semiconductor industry terms "hybrid" techniques.
2. Prior Art
The design of linear audio power amplifiers has followed the normal technological base in the electronics industry i.e., there were electron tube amplifiers when electron tubes were the only active power devices, there were transistor amplifiers when descrete transistors were the best available active power devices, there were low power one-chip monolithic amplifiers when technology permitted fabricating several transistors and other passive components together on a single monolithic semiconductor chip, then finally with technology advanced to where it is today there are miniature high power hybrid linear amplifiers. The simplicity of using both the monolithic and hybrid modular linear amplifiers is astonishing. This fact coupled with lower costs, higher reliability, and ease of production has spearheaded these devices into virtually every consumer product requiring some type of audio amplifier.
Although these linear modular power amplifiers obviously exhibit many desirable qualities, they all have one major disadvantage--power efficiency. The efficiency of any device is defined as ratio of the useful output power to the total input power. By definition, any practical linear amplifier, no matter how it is designed, when coupled with a sinewave input, can only have an absolute maximum efficiency of about 60% which occurs at the maximum output power point of the amplifier. As total useful power decreases, so does the efficiency. Therefore, with an ideal linear amplifier we can expect the efficiency range of 0% to 60%. Also, in a real environment, a power audio amplifier cannot be operated near its maximum power output because occasional program spikes would drive the amplifier into saturation causing severe distortion. Taking this into consideration, if we operate the amplifier at its half power point to avoid saturation distortion on peaks in the program material, we end up with an efficiency of only about 40%. Low efficiency results in several detrimental effects: 1. Energy consumption is increased. 2. The requirements for the direct current (D.C.) power supply that normally operates a linear amplifier are greatly increased. 3. Large amounts of wasted heat energy must be dissipated in the output stage making some kind of special heat sinking necessary. Although these detrimental effects are of little consequence in low power amplifiers, they contribute to the fact that high power linear amplifiers are very material intensive i.e., they use large amounts of raw materials such as iron, copper, and aluminum which contributes to their large size, weight, and cost.
In an effort to keep the efficiency as high as possible, especially in the mid to high power range, a pulse width modulated (hereafter referred to as PWM) technique is used. Operating efficiencies of 90% to 95% over all output levels are not unusual when using this technique. Although using this technique to build an audio amplifier is not new, the state of the art in electronics components has just advanced to a point where a complete, high power, PWM audio amplifier can be assembled in a single miniature package using what the semiconductor industry terms "hybrid" techniques. This packaging technique of this PWM amplifier constitutes a significant advance to the state of the art for the following reasons: Although PWM audio amplifiers are not new, their circuit design complexities and assembly costs have held them from the consumer market place which is still totally dominated by linear amplifiers. The unique combination of the arrival of the complex PWM control integrated circuits and high speed output switch transistors coupled with the large scale production savings of the hybrid circuit will, for the first time, allow the more efficient PWM audio amplifiers to enter into a competitive arena with the now dominant high power linear amplifier. Once the hybrid PWM amplifiers receive acceptance in the market, the manufacturers will gain by wider profit margins due to simpler assembly and decreased raw material costs and the consumer will reap the attendant savings of lower cost, higher efficiency smaller size, lighter weight, and more reliable operation.
3. Objects
It is the object of this invention to draw upon two developing technologies i.e., the electronic circuit technology of pulse width modulated (PWM) audio amplifiers and the manufacturing technology of "hybrid" circuit construction and define a new commercially saleable product.
It is also an object of the invention to define a product that has improved technical specifications over the present state of the art, primarily in the areas of power efficiency, size, weight, and reliability.
Another object of the invention is to define a product that will be cost effective to an amplifier manufacturer purchasing the product primarily in the areas of assembly and raw materials costs. The assembly gain is from the ease of implementation of the entire basic audio amplifier in modular form with addition of of only a few external components and a scaled-down power supply. The raw materials cost decrease will be attributed to high power efficiency (90% to 95% typical) which virtually eliminates all heat sinks and reduces the maximum power output required from the power supply by about 50%.
It is the object of the invention to state that the quality, shape, size, and power dissipation required by use of the state of art electronic components to accomplish either a single or multi-channel PWM audio amplifier is an ideal match for integration into a single, compact, power hybrid package.