Abstract:
The invention concerns a power amplifier consisting of three stages: a first feedback-free stage amplifying the input signal in current; a second stage amplifying in voltage the signal output from the first stage and comprising two direct current feedback half-circuits symmetrically implanted and operating over the whole frequency spectrum; a third feedback-free stage amplifying the current derived from the second stage and powering a loudspeaker.

Description:
FIELD OF THE INVENTION 
   The object of this invention is a hi-fi bipolar transistor power amplifier for audio frequencies. 
   SUMMARY OF THE INVENTION 
   This amplifier, which is disclosed in the application for this patent, can be used for a number of applications, particularly in information technology and the electronics industry. As such, it can be described as universal. 
   Its main characteristics are as follows:
         Very broad bandwidth combined with negligible phase shift.   Reproduction of dynamic range exempt from transient distortion.   Linear group propagation delay over the whole frequency range.   High level of immunity vis-à-vis electrostatic phenomena.   Excellent reliability due to the small number of electronic components.       

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The sole FIGURE in the appended drawing is the wiring diagram of a non-limitative embodiment of a power amplifier according to the invention. The amplifier represented is made up of three stages. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The first stage comprises two symmetrical bipolar transistors biased to class A Q 1  and Q 2  and two resistors R 1  and R 2  used to set the input impedance at source. Its bandwidth is very broad. It operates in an open loop with low phase shift at a single pole. Its very low input capacitance allows for linear frequency response even with a high-impedance source. 
   The second stage comprises six bipolar transistors Q 3 , Q 4 , Q 5 , Q 6 , Q 7 , Q 8  and resistors R 3 -R 18 . 
   Its voltage gain is determined by the two symmetrical direct-current negative feedback potential dividers formed by the resistors R 9 /R 11  and R 10 /R 12 . The transistors Q 3  and Q 4  provide initial voltage amplification and are then buffered by the transistors Q 5  and Q 6 . The signal power is then further amplified by the transistors Q 7  and Q 8  connected in common-collector mode. 
   In this second stage, the signal is therefore amplified successively as follows: one voltage amplification, one current amplification and one power amplification. This arrangement allows for the signal to be increased gradually to the level required by the following stage. 
   This second stage presents a linear open loop gain, a low output impedance and a high propagation velocity guaranteeing linear transient behaviour exempt from dynamic distortion. 
   The third stage comprises four transistors biased to class AB Q 9 , Q 10 , Q 11 , and Q 12 , two resistors R 19  and R 20  and two diodes D 1  and D 2 . 
   The transistors Q 9  and Q 10 , which are powered by a current source, cause a drop in voltage V BE  at the terminals of the transistors Q 11  and Q 12  that is directly proportionate to the drop in voltage V BE  at the terminals of the transistors Q 9  and Q 10 . 
   The quiescent current passing through the transistors Q 11  and Q 12 , generated by their voltage VBE, is therefore directly dependent on the voltage VBE of the transistors Q 9  and Q 10 . 
   This circuit constitutes a simplification of the conventional bias circuit, thus improving the static and dynamic characteristics of this stage compared with the known analogous stages. 
   The resistors R 19  and R 20  are used to act directly on the working range (classes A, AB or B) depending on the type of application required. 
   The diodes D 1  and D 2  provide a very fast pulse response through their p-n junction capacitance and protect the resistors R 19  and R 20  against heating. 
   This third stage, which is a novel design in itself, is electrically and thermally stable and its very low output impedance allows for open loop operation. 
   The combination of these three stages allows for the production of power amplifiers with the following specific features and advantages:
         assembly capacity   adaptability to a variety of applications such as microphone amplifiers and low-noise phono amplifiers   distortion-free operation over very wide voltage ranges   repeatability of characteristics in mass production   minimal test procedures   availability of the electronic components   ease of maintenance   unrivalled attack reproduction   dynamic range reproduced without compression   high fidelity sound image.       

   The invention is obviously not limited to the diagram shown and the application described. 
   In particular, other applications may be envisaged, particularly due to the high speed of the circuit. 
   For example, in information technology, it can be used for the fast and reliable positioning of hard disk write heads. The very short settling time is obtained in this case without any parasitic oscillation, and the desired position is reached without error. 
   Miniaturisation of the circuit would make it even faster. 
   Also in information technology, its low output impedance means that it can be used to transmit data at high speeds over long distances. The standard distance (100 m) for hardwired links could thus be increased considerably. 
   In information technology and the electronics industry, it can be used, due to its high switching rate, in very high frequency switched mode power supplies. 
   Its high input impedance combined with a very low capacitance makes it very easy to control. 
   These applications are not exhaustive and any system requiring accuracy, high speed and/or immunity to external agents would benefit from this “universal” circuit.