Abstract:
A variable gain analog amplifier is described that uses pulse-density modulation in the form of a sigma-delta modulator (SDM) to produce a gain by modulating the selection of a switch that selects the amount of resistance in a negative feedback loop of the amplifier. The output of the SDM is dithered to increase the gain resolution of the analog amplifier, wherein the increased resolution produces a quiet, inaudible transition between changes in gain setting at an output of the variable gain amplifier and in addition produces a quiet, inaudible mixing and merging of audio signals..

Description:
[0001]    This application is related to U.S. patent application docket number DS10-028, Ser. No. 13/065,301, filed on Mar. 18, 2011, assigned to the same assignee as the present invention, and which is herein incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The present invention is related to audio amplifiers and in particular to increasing gain resolution of output stages of audio amplifiers. 
         [0004]    2. Description of Related Art 
         [0005]    Present day audio systems and integrated audio devices can have a plurality of sources that play into a plurality of different types of outputs, for example, an auxiliary input may be reproduced in an earphone amplifier or a DAC output may be played into an earpiece amplifier. In home high fidelity systems the selection is made by a switching device where the transition between sources is controlled so as not to introduce auditable effects of switching between input and output devices. In integrated audio devices a large number of distinct paths can be configured through summing amplifiers that perform a mixing operation. Unwanted sounds, e.g. pops and clicks, may be audible when the different source signals are at a different level when switched into the summing amplifier. A solution to the unwanted sounds could be to effectively mute the output, perform the required switching and then raise output amplitude back to where it was before muting. 
         [0006]    The gain accuracy of audio amplifiers is usually determined by fixed value resistance in the feedback of the amplifier where the step size is dependent upon the number of elements in the feedback network and the intended range of the amplifier gain. If the gain is changed with a step size that is too large, for instance &gt;0.01 dB for a 60 dB range, a zipping noise will be introduced. The large number of resistors needed to produce such an effect would make the amplifier device expensive and noncompetitive. Unwanted audio effects can often be eliminated by using discrete components on a PCB, but the cost of components can be in the order of magnitude of the integrated audio device itself. 
         [0007]    US 2010/0166084 A1 (Galton et al.) is directed a successive re-quantizer that replaces a delta sigma modulator in a fractional-N PPL or DAC, which avoids spurious tone problems in non-linear analog circuitry. US 2006/0092059 A1 (Guimaraes) is directed to an automatic gain control using a sigma delta ADC. An amplifier is within a sigma delta time continuous loop prior to quantization and an attenuator in the feedback prior to summation with incoming signals. U.S. Pat. No. 7,821,341 B2 (Kim et al.) discloses a gain device with an amplifier that uses the gain device, which uses a linearly variable resistance to control the gain of the amplifier. In U.S. 7,583,213 B2 (Wang et al.) a signal processing system that is directed to changing the level of an input signal to produce an output signal, which includes a shifter, a sigma delta modulator and a level adjuster. U.S. Pat. No. 7,148,829 B2 (Inukai) is directed to a sigma delta modulation circuit having a gain control function, wherein a control unit controls the gain of a variable gain amplifier using a sigma delta modulator and a filter that operates on the output of the sigma delta modulator. In U.S. Pat. No. 7,102,441 B2 (Lee et al.) a variable gain amplifier circuit is directed to a use of a resistor ladder for obtaining a precise gain. 
         [0008]    U.S. Pat. No. 6,404,367 B1 (Van der Zwan et al.) is directed to a sigma delta modulator that is used to interchange (chop) the input network and the feedback network to average out the differences in the two gain stages and produce a more accurate gain. U.S. Pat. No. 6,127,893 (Llewellyn et al.) is directed to a control circuit for controlling a level of an audio signal, wherein the control circuit is based on a R-2R resistive network having a plurality of resistor nodes and a plurality of switches to connect the resistors to a low impedance node. An article by Kevin. J Wang, “Spurious Tone Suppression Techniques Applied to a Wide-Bandwidth 2.4 GHZ Fractional-N PPL”, IEEE Journal of Solid State Circuits, Vol. 43, No. 12, December 2008, is directed to spurious tones in the output of a fractional-N PLL that are reduced by replacing a sigma delta modulator with a digital quantizer with a charge pump offset and a sampled loop filter. An article by James A. Kaehler, “Periodic-Switched Filter Networks—A means of Amplifying and Varying Transfer Functions”, IEEE Journal of Solid State Circuits, vol. sc-4, no. 4, August 1969, is directed to a technique of periodically switching filter networks to allows continuously variable filter parameters. 
         [0009]      FIG. 1  shows a variable gain analog amplifier of prior art. An operational amplifier  10  is connected between an input signal, Vin, and an output signal, Vout, created by the operational amplifier. Switches S 1  and S 2  select the amount of negative feedback resistance connected between the output of the amplifier  10  and the negative input (summing junction) of the operational amplifier  10 . The positive input terminal of the amplifier is connected to Vref, which is often ground potential. Selection of either switch S 1  or S 2  determines the amount of resistance between the input signal Vin and the negative input of the operational amplifier  10 . Assuming that Rin=R 1 =R 2 , then the gain of the amplifier  10  is G1=−(R1+R2)/Rin=−2 when switch S 1  is selected, and when switch S 2  is selected the gain G2=−(R2)/(R1+Rin)=−½. 
         [0010]    If the two switches S 1  and S 2  are toggled (turned on and off) at a relatively rapid rate with a fifty percent duty cycle, an average gain of Gavg=−(2+½)/2=−1.25 as shown in  FIG. 2  where the two switches are opened and closed out of phase with each other. When Vin  20  applied to the amplifier is a ramp over a span of time t, Vout becomes a ramp  21  interrupted by the two different gains resulting from G1=−2 and G2=−½. The effective, or average output voltage  22  is twenty five percent higher than the input voltage Vin. 
         [0011]    A portion of the control signal that drives the switches S 1  and S 2  can appear at the output of the amplifier, but this can be attenuated by the ratio of the switched resistance to the total resistance, or Rmod=Rsw/Rtot where Rmod is the control signal attenuation , Rsw is the switched resistance and Rtot=Rin+R1+R2. Also some of the high frequency switch control signal can be coupled to the output of the amplifier through parasitic capacitance of the circuitry. The higher the modulating frequency compared to the amplifier gain bandwidth (Fmod=fclk/fbw, where Fmod is the modulating frequency, fclk is the clock rate and fbw is the frequency bandwidth of the amplifier) and the lower Rmod, the lower the amplitude of the control signal that will appear at the output of the amplifier Vout. 
       SUMMARY OF THE INVENTION 
       [0012]    It is an objective of the present invention to increase the gain resolution of an output stage of an audio amplifier. 
         [0013]    It is further an objective of the present invention to increase the gain resolution of a programmable gain amplifier beyond that resolution provided by the fixed number of gain elements forming the feedback network of the programmable gain amplifier. 
         [0014]    It is still further an objective of the present invention to allow a pop-and-click free switching or mixing between different audio paths without the need of muting the output of the audio amplifier. 
         [0015]    It is also further an objective of the present invention to attenuate the switch control signal at the output of the of the audio amplifier. 
         [0016]    It is also still further an objective of the present invention that a pulse-density-modulator (PDM) control the gain determining switches of the variable gain audio amplifier. 
         [0017]    In the present invention an audio amplifier is formed from a operational amplifier in which negative feed back resistors are selected/deselected by a pulse-density-modulator (PDM) to form the output gain of the operational amplifier used as an audio amplifier. There are a plurality of resistors which can be connected to the summing junction (negative input) of the operational amplifier. If a resistor is not included in the negative feedback network, that resistor is part of the input resistance between the signal input and the negative input of the operational amplifier. 
         [0018]    The PDM, preferably a sigma delta modulator (SDM), operates at a frequency higher than the audio bandwidth of the amplifier and selects switches (selecting feedback resistance or gain) of at least two of the plurality of switches that form the feedback of the output of the operational amplifier to the negative input and thus the gain of the amplifier circuit. It should be noted that other forms of pulse density modulation such as a successive quantizer circuit can be used to control the selection of the switches that make up the feedback network of the audio amplifier. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    This invention will be described with reference to the accompanying drawings, wherein: 
           [0020]      FIG. 1  is a circuit diagram of prior art of a variable gain operational amplifier; 
           [0021]      FIG. 2  is a signal diagram of the operations of the variable gain operational amplifier of prior art; 
           [0022]      FIG. 3  is a circuit diagram of the audio amplifier of the present invention; 
           [0023]      FIG. 4  is a timing diagram of a pulse stream from a sigma-delta modulator modulating the width of a switch control signal; and 
           [0024]      FIG. 5  is a diagram demonstrating usage of a dithered gain amplifier in an audio mixer. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0025]    In  FIG. 4  is shown a circuit diagram of the variable gain analog amplifier  40  of the present invention. An operational amplifier  41  and a network of resistors Rin and R 1  to Rn establish the gain of the analog amplifier. The operational amplifier  41  has the positive input terminal connected to ground; although a reference voltage could be connected to the positive input terminal. There are a plurality of resistors, R 1  to Rn, which can be connected between the output and the negative input (summing junction) of the amplifier  41  by the selection of one of a plurality of switches, S 1  to Sn. A resistor that is not selected by a switch to be in the negative feedback resistor network of the operational amplifier  41  will form a part of the input resistance with Rin connected between the signal input Vin and the negative terminal of the operational amplifier  41 . 
         [0026]    The selection of the gain of the analog amplifier  40  by the selecting the switches is controlled by a pulse-density modulator (PDM)  42 , which is preferably a sigma-delta modulator (SDM), in response to a gain control signal. The output of the PDM/SDM  42  is dithered  43  to increase the gain resolution of the modulated signal selecting the switches S 1  to Sn. The modulator  42  can have multiple outputs, which can be used to select a plurality of resistors simultaneously in the switch network noted by switches S 1  to Sn connected to the negative input of the operational amplifier  41 , and the modulator  42  uses both oversampling and noise shaping to increase the resolution of the output pulse width without increasing the required timing. 
         [0027]    The clock, CLK, is modulated in width by the PDM/SDM  42  will peak above the spectrum of the quantized noise, but will be limited by the modulated gain of the switched resistors. Further reduction of the clock signal can be attained by a spread spectrum  44  that is applied to the clock to spread the power of the clock below the noise floor. The spread spectrum  44  of the clock can also smear the spectrum of an input signal tone depending on the amount of spread spectrum applied. The spectrum of the output of the amplifier Vout (dithered gain) contains at low frequencies the audio content that has been amplified, higher frequencies that are the result of the increased gain resolution, which is shaped by quantization noise (high pass filter response), and a tone, which is the clock frequency that is used to clock the SDM. 
         [0028]    High frequency signals at the output connected to earphones are not desired since these high frequency signals could combine with FM signals of the headphone wire that could be used as an antenna. Therefore, it is important to keep quantization noise and the frequency spectrum of the clock at an acceptably low amplitude. The amount of quantization noise can be controlled by the SDM and static gain step used. The amplitude of the clock spectrum at the output is not as easily controlled. The use of a spread spectrum  44  on the clock that drives the SDM is an approach that spreads the clock over a wider frequency spectrum to reduce the amplitude of the clock frequency components that are present in the amplifier output, Vout. 
         [0029]    In  FIG. 5  is an example of the timing diagrams for the output pulse stream of a sigma-delta modulator (SDM)  42  that modulates a switch control signal. The use of the SDM to increase the variable gain resolution of the variable gain amplifier  40  eliminates the need for increasing the timing or semiconductor area requirements. In  FIG. 5  there is shown a low duty-ratio pulse of 25% and a high duty-ratio pulse of 75%. The pulse width is toggled between duty-ratios such that the average over a period of time is a desired high-resolution value. In the output stream “1” there are two low duty-ratio pulse and two high-duty ratio pulses yielding 200% over four pulses or an average duty-ratio of 50%. In output stream “2” there are one low duty-ratio and three high duty-ratio pulses that yields an average duty-ratio of 62.5%. Thus a single-bit output can be modulated to a higher effective pulse-width resolution. This effect is limited by the rate of change of the pulse-width relative to the rate of change of the gain, or oversampling ratio, which is the number of pulses that are average over an audio time period. 
         [0030]    Sigma-delta modulation employs both over sampling and noise-shaping to increase the resolution of the output pulse-width ratio without increasing the timing requirement. Quantizing noise may be shaped at the modulator with feedback such that it is lowered in the band of interest and increased out of band. Lowering of the in-band quantizing noise leads to the increase of pulse-width resolution and, in-turn, gain resolution. 
         [0031]    The quantization noise will be present at the output of the amplifier, but it will be limited in amplitude by: the amount/order of the noise shaping used, the amount of amplifier gain modulation by the switched resistor (Rmod), and the ratio of switching frequency with the gain bandwidth of the amplifier. For a low number of static units in the resistor feedback network and a high resolution in gain, this will result is a small amount of quantization noise, i.e. much below what is normally seen from a headphone output when driven from a typical SDM in a high-fidelity DAC. 
         [0032]    In  FIG. 6  is shown a diagram of an application that demonstrates usage of a plurality of dithered gain amplifiers  40  in an audio mixer  60 . From a variety of audio sources  61  a plurality of dithered gain amplifiers  40  having gains Gx 1  to GxN are connected to summing junctions  62  from which the summed audio sources are connected to audio playback devices  63  through a second plurality of dithered gain amplifiers  40  with gains Gy 1  to GyM. The audio playback devices  63  range from headphones and speakers to recording devices. 
         [0033]    As a first example, audio content (e.g. using path gain Gx 1 ) is currently been played into an output (e.g. gain Gy 1 ) and the user requires another content to be added to that output (e.g. from path gain Gx 2 ), without muting the output. This is made possible by performing a dithered increase of the gain of the added path (gain Gx 2 ) without changing any other gains. Without the dithered gain the addition of the second content would have unavoidably produced unintended audible effects. 
         [0034]    As a second example, before any audio sources or paths are enabled into a given output, the gain of the output stage that is intended (e.g. gain Gy 1  in  FIG. 6 ) is dropped in a fashion that is not unpleasant to the user. The dithered gain steps at the output can have a resolution higher than 16 bit precision over the entire gain range, which will make the gain transitions virtually continuous, with no additional audible effects like pop and click. The next step will be to enable all the paths that are intended to be played into that output stage, by increasing their gain (e.g. gain Gx 1 ). This can be done crudely since the intended output is effectively muted. The gain of the intended output gain (gain Gy 1 ) can then be turned on again smoothly with dithering. The small step change in gain at the output will avoid any issues due to circuit offset or signal synchronization, for the mixing or muxing of paths. 
         [0035]    While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.