Abstract:
A device for generating a regulated DC voltage from a DC power supply voltage source includes a loop formed by a reference voltage generator powering an operational amplifier. The output of the operational amplifier powers the input to the reference voltage generator.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of power supply regulation, and more particularly, to regulated power supplies for low noise microphone preamplifiers. 
     BACKGROUND OF THE INVENTION 
     Devices capable of outputting a regulated DC voltage from an unregulated voltage are known for use in an integrated circuit. One example of a known circuit of this type is the “band gap core” circuit, or the “polarization circuit with a reference voltage output by a difference differential” as described in the “Analysis and Design of Analog Integrated Circuits” manual published by Paul T. GRAY and Robert G. MEYER in 1984, second edition under number ISBN 0471-81454 7, pages 4-181 and subsequent pages. 
     This type of circuit is capable of achieving a noise rejection ratio for noise from an unregulated power supply equal to about 40 to 70 decibels. This ratio is insufficient, particularly in the case of a power supply for a preamplifier for a signal output by the microphone in a camera with a universal standard bus port. In this type of camera, the main power supply has a large amount of noise due to the many sudden changes in power consumption. Typically, the regulated voltage is filtered, for example, using additional R.C. filters, but these filters are not very efficient at the low frequencies present in the audio range. Furthermore, these filters are not necessarily possible on integrated circuit chips. 
     Another disadvantage of self-polarized circuits like those described in the manual mentioned above, is that the output regulated voltage is necessarily less than the unregulated power supply voltage. Consequently, a sufficiently high unregulated voltage is necessary so that the regulated voltage after division is sufficient to be useable by standard circuits on the load side. This is not always the case, particularly on portable equipment such as cameras. 
     Accordingly, there is a need for a regulated power supply with a better noise rejection ratio for noise in the unregulated power supply voltage. This ratio is preferably between 100 dB or more. The better noise rejection ratio should also be at low audio frequencies, and the level of this power supply should be sufficiently high for use by these circuits on the load side. 
     SUMMARY OF THE INVENTION 
     According to the invention, a conventional reference voltage generating circuit will be used and the voltage obtained at the output will be coupled to the input of an operational amplifier. The output of the operational amplifier will form the unregulated voltage input to the reference voltage generator. 
     The operational amplifier performs two functions. Firstly, it amplifies the reference voltage at the output from the reference voltage generator. Secondly, it reduces the noise at the input by about 50 decibels. Considering noise specific to components and possibly to an adapter stage between the reference voltage generator and the operational amplifier, it is impossible to expect that the total rejection will be the sum of the rejections, namely about 120 dB. However, it is possible to approach or exceed 100 decibels. Thus, a voltage at a suitable level is available with a noise rejection ratio of noise from the unregulated power supply of about a 100 dB. 
     The system that has just been described necessitates a startup phase during which the reference voltage generator is powered by the unregulated power supply voltage. This startup phase terminates as soon as the output voltage from the operational amplifier becomes sufficiently large to power the generator in turn. At the end of this startup phase, the power supply for the reference voltage generator is switched from the unregulated power supply voltage to the voltage at the output from the operational amplifier. 
     In sum, the invention relates to a device for generation of a regulated DC voltage starting from a DC power supply voltage source. The device includes a reference voltage circuit with the DC power supply voltage being applied to one input. The reference voltage circuit supplies an output voltage pre-regulated to a level that is a fraction of the DC power supply voltage equal to or less than 1. The device includes the output from the reference voltage generator circuit carrying the pre-regulated voltage being coupled to a first input to an operational amplifier with a second input and one output. The output carries the regulated DC voltage, and a feedback voltage taken from the output from the operational amplifier is applied to the second input. This output is coupled to the input of the reference voltage generator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An example embodiment of this invention will now be described with reference to the FIGURE which shows a block diagram of an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The sole FIGURE shows an example embodiment of a power supply according to the invention. The circuit  1  shown in the FIGURE includes a reference voltage generator  3  with the output  5  coupled to a first input  6  of an operational amplifier  7 . The output  9  of the operational amplifier  7  is coupled in feedback to the input  4  of the reference voltage generator  3 . The reference voltage generator  3 , may include, for example, a first balanced current mirror and a second unbalanced current mirror, with a balancing resistor between the two unbalanced arms of the second mirror, the voltage at the terminals of this second resistor then forming the reference voltage, as would be appreciated by the skilled artisan. This reference voltage is equal to a fraction (less than 1) of the power supply voltage present on an input  4  of the reference voltage generator  3 . 
     The operational amplifier  7 , is polarized by the unregulated power supply voltage source  2  and a fraction of the voltage present on output  9  is applied to its second input  8 . This fraction is determined as a function of the amplification ratio to be obtained between input  2  and output  9 . Operation in steady state mode is as follows. The output  9  from operational amplifier  7  powers the input  4  to reference voltage generator  3 , for example, through a diode and a resistor as shown. Output  5  from this generator powers the first input  6  to the operational amplifier  7 . Consequently, the circuit operates in a closed loop and the power supply voltage received on the first input  6  to the operational amplifier  7  has already been pre-regulated by the reference voltage generator  3 . This pre-regulated voltage is once again regulated by the operational amplifier  7 , such that attenuations of noise in the power supply voltage to source  2  are accumulated. 
     For the startup phase, to obtain a voltage at the output  5  from the reference voltage generator  3 , the source  2  has to be connected to the input  4  of this reference voltage generator  3 . 
     To obtain steady state operation of the loop described above, the output  9  from the operational amplifier is coupled to a circuit  10 ,  14  for disconnecting the source  2  from the input  4  to the reference voltage generator  3 . In the example shown, this circuit includes a comparator  10  with two inputs  11 ,  12  and an output  13 . The input  11  receives a threshold voltage, the input  12  receives the voltage present at the output  9  from operational amplifier  7  and output  13  controls a switch  14  to disconnect the source  2  from the input  4  whenever the value of the voltage present on output  9  from the operational amplifier reaches the value of the threshold voltage present on the first input  11  of the comparator  10 . 
     Advantageously, to increase the value of the rejection ratio of the power supply voltage present on the power supply source  2  of the operational amplifier  7 , the output  5  from the voltage generator  3  is connected to a feedback stage  15  of the output voltage on the input  4  to the reference voltage generator  3 . In the case shown in the FIGURE, a PNP transistor is used in which the base forms the input  16  to the feedback stage and in which the emitter  17  forms the output from this feedback stage  15 . Consequently, the circuit comprises a double feedback loop, one loop being composed of the reference voltage generator  3  and the feedback stage  15 , and a second loop being composed of the voltage generator  3  and the operational amplifier  7 .