Abstract:
Audio amplifiers, particularly those employed with headphones, use snubbers to suppress or snub signals within a particular frequency range. Conventional resistive and resistor-capacitor (RC) type snubbers have a number of drawbacks (i.e., require external components and high power consumption). Here, an active snubber is provided that allows for suppression in a desired frequency range without the need for external components and with relatively small footprint and a relatively small power increase.

Description:
TECHNICAL FIELD 
     The invention relates generally to headphone amplifiers and, more particularly, to an active snubber for headphone amplifiers. 
     BACKGROUND 
     Turning to  FIGS. 1 and 2  of the drawings, conventional headphone systems  100 - 1  and  100 - 2  can be seen. Headphones  102  can be generally modeled as an LRC circuit having resistor R 1 , inductor L, and capacitor C 1 . The headphones  102  are coupled to a headphone or output terminal HPOUT and a ground terminal GND, where an amplifier (not shown) would apply a signal to the headphones through terminals HPOUT and GND. For system  100 - 1 , a resistive snubber  104 - 1  is employed (which is a resistor R 2  coupled between terminals HPOUT and GND). For system  100 - 2 , an RC snubber  104 - 2  (which is a resistor R 2  and capacitor C 2  coupled in series between terminals HPOUT and GND). Snubber  104 - 1  significantly and aversely affects efficiency, making it poor design choice. Snubber  104 - 2 , on the other hand, can be build to have high impedance in the audible range (20 Hz to 20 kHz) and low impedance for frequencies above 1 MHz (where the amplifier is generally not stable), but this usually requires a capacitor on the order of 50 nF (which generally cannot be put “on-chip”). Therefore, there is a need for an “on-chip” snubber with high efficiency. 
     SUMMARY 
     A preferred embodiment of the present invention, accordingly, provides an apparatus is provided. The apparatus comprises a headphone terminal; a first amplifier that is coupled to the headphone terminal; and an active snubber having: a first transistor coupled between a supply rail and the headphone terminals, wherein the first transistor includes a control electrode; a current source that is coupled to the supply rail; a second transistor that is coupled between the current source and the headphone terminal, wherein the second transistor includes a control electrode; and a second amplifier that is coupled between the control electrodes of the first and second transistor, wherein the second amplifier operates as a follower for a first frequency range of a signal applied to the headphone terminal by the first amplifier, and wherein the second amplifier decreases the of impedance the first transistor for a second frequency range of the signal applied to the headphone terminal by the first amplifier. 
     In accordance with a preferred embodiment of the present invention, the second transistor is diode-connected. 
     In accordance with a preferred embodiment of the present invention, the active snubber further comprises a plurality of impedance networks, wherein each control electrode from the first and second transistors is coupled to at least one of the impedance networks. 
     In accordance with a preferred embodiment of the present invention, the active snubber further comprises: a third transistor that is coupled to the supply rail and the second amplifier; and a current mirror that is coupled to third transistor and the first transistor. 
     In accordance with a preferred embodiment of the present invention, the first and second transistors are NMOS transistors. 
     In accordance with a preferred embodiment of the present invention, the ratio of the sizes of the first transistor to the second transistors is N:1, wherein N is a positive integer. 
     In accordance with a preferred embodiment of the present invention, the active snubber further comprises a resistor that is coupled between the first transistor and the headphone terminal. 
     In accordance with a preferred embodiment of the present invention, an apparatus is provided. The apparatus comprises a headphone terminal; a ground terminal; a first amplifier that is coupled to the headphone terminal; and an active snubber having: a first transistor coupled between a supply rail and the headphone terminals, wherein the first transistor includes a control electrode; a current source that is coupled to the supply rail; a second transistor that is coupled between the current source and the headphone terminal, wherein the second transistor includes a control electrode; and a first impedance network that is coupled between the control electrode of the first transistor and the headphone terminal; a second amplifier having a first input terminal, a second input terminal, and an output terminal, wherein the output terminal of the second amplifier is coupled to the control electrode of the first transistor, and wherein the first input terminal of the second amplifier is coupled to the first impedance network; and a second impedance network that is coupled to the control electrode of the second transistor, the second input terminal of the second amplifier, and the ground terminal. 
     In accordance with a preferred embodiment of the present invention, the first impedance network further comprises: a resistor that is coupled between the control electrode of the first transistor and the first input terminal of the second amplifier; and a capacitor that is coupled between the first input terminal of the second amplifier and the headphone terminal. 
     In accordance with a preferred embodiment of the present invention, the second impedance network further comprises: a resistor that is coupled between the control electrode of the second transistor and the second input terminal of the second amplifier; and a capacitor that is coupled between the second input terminal of the second amplifier and the ground terminal. 
     In accordance with a preferred embodiment of the present invention, the active snubber further comprises: a third transistor that is coupled to the supply rail and the second amplifier; and a current mirror that is coupled to third transistor and the first transistor. 
     In accordance with a preferred embodiment of the present invention, the first and second transistors are NMOS transistors. 
     In accordance with a preferred embodiment of the present invention, the ratio of the sizes of the first transistor to the second transistors is N:1, wherein N is a positive integer. 
     In accordance with a preferred embodiment of the present invention, the active snubber further comprises a resistor that is coupled between the first transistor and the headphone terminal. 
     In accordance with a preferred embodiment of the present invention, an apparatus is provided. The apparatus comprises an audio source that generates an audio signal; an integrated circuit (IC) having an input terminal, an output terminal, and a ground terminal, wherein the audio source is coupled to the input terminal of the IC, and wherein the IC includes: a supply rail; a first amplifier that is coupled to the input terminal and the output terminal of the IC; a resistor that is coupled to the output terminal; a first NMOS transistor that is coupled to the resistor at its source and the supply rail at its drain; a current source that is coupled to the supply rail; a second NMOS transistor that is coupled to the resistor at its source and the current source at its drain, wherein the second NMOS transistor is diode-connected; a first impedance network that is coupled between the gate of the first NMOS transistor and the output terminal; a second impedance network that is coupled between the gate of the second NMOS transistor and the ground terminal; and a second amplifier having a first input terminal, a second input terminal, and an output terminal, wherein the first input terminal of the second amplifier is coupled to the first impedance network, and wherein the second input terminal of the second amplifier is coupled to the second impedance network, and wherein the output terminal of the second amplifier is coupled to the gate of the first NMOS transistor; and headphones that are coupled to the output terminal and the ground terminal of the IC. 
     In accordance with a preferred embodiment of the present invention, the resistor further comprises a first resistor, and wherein the first impedance network further comprises: a second resistor that is coupled between the gate of the first NMOS transistor and the first input terminal of the second amplifier; and a first capacitor that is coupled between the first input terminal of the second amplifier and the first resistor. 
     In accordance with a preferred embodiment of the present invention, the second impedance network further comprises: a third resistor that is coupled between the gate of the second NMOS transistor and the second input terminal of the second amplifier; and a second capacitor that is coupled between the second input terminal of the second amplifier and the ground terminal. 
     In accordance with a preferred embodiment of the present invention, the current source is a first current source, and wherein the supply rail is a first supply rail, and wherein the active snubber further comprises: a second supply rail; a third NMOS transistor that is coupled to the supply rail at its drain and the output terminal of the second amplifier at its gate; a first PMOS transistor that is coupled to the source of the third NMOS transistor at its source, wherein the first PMOS transistor is diode-connected; a second current source that is coupled between the drain of the first PMOS transistor and the second supply rail; and a second PMOS transistor that is coupled to the source of the first NMOS transistor at its source, the gate of the first PMOS transistor at its gate, and the second supply rail at its drain. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a diagram of an example of a conventional system using a resistor snubber; 
         FIG. 2  is a diagram of an example of a conventional system using an RC snubber; 
         FIG. 3  is a diagram of a system using an active snubber in accordance with a preferred embodiment of the present invention; 
         FIG. 4  is a bode plot depicting gain and phase for the system of  FIG. 3 ; and 
         FIG. 5  is a diagram depicting the output impedance and phase for the system of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     Refer now to the drawings wherein depicted elements are, for the sake of clarity, not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
     Referring to  FIG. 3  of the drawings, the reference numeral  300  generally designates a system in accordance with a preferred embodiment of the present invention. The system generally comprises an audio source  310 , an integrated circuit (IC)  312 , and headphones  102 . In operation, the audio source  310  generates an audio signal which is provided to the input terminal IN of IC  312 . IC  312  amplifies (and filters) the audio signal and provides it to the headphones  102  through headphone terminal or output terminal HPOUT and ground terminal GND. 
     Of interest, however, is the IC  312 . IC  312  generally comprises an amplifier  308  and an active snubber  302 . Additionally, snubber  302  generally comprises resistor R 4 , impedance networks (resistor/capacitor R 5 /C 3  and resistor/capacitor R 6 /C 4 ), current source  306 , amplifier  304 , and NMOS transistors Q 1  and Q 2 . 
     In operation, the snubber  302  allows signals output from amplifier  308  within the audible frequency range (about 20 Hz to about 20 kHz) to pass to the headphones  102 . Preferably, current source  306  (which is coupled to supply rail VDD) generates a bias current I BIAS , which is provided to diode-connected NMOS transistor Q 2 , so to generate a small quiescent current through resistor R 4  (which is coupled to output terminal HPOUT). When a signal within an audible range is provided by amplifier  308 , capacitors C 3  and C 4  have high impedance, causing amplifier  304  to have unity gain (operating as a follower). Essentially, for this low frequency range, the gate voltage (V G ) for transistor Q 1  follows the voltage output through terminal HPOUT (plus a DC bias which is generally equal to a gate-source voltage drop across transistor Q 2 ). Because the gates-source voltage of transistor Q 1  is generally constant, the effective impedance of transistor Q 1  looking into the source terminal is high, and in order to function in this manner, transistors Q 1  and Q 2  are operating in a saturated region. 
     The gate of transistor Q 2  is also biased at the same voltage as the gate of transistor Q 1 , and because transistor Q 1  is N times larger than transistor Q 2 , transconductance (g m1 ) is higher than transconductance (g m2 ) of transistor Q 2  for the same bias voltage. Additionally, as the frequency rises (generally above a few hundred kilohertz), snubber  302  can suppress or snub the signal from amplifier  308 . With this increase in frequency, the impedance of the capacitor C 3  decreases so that the node N 1  no longer follows the voltage (signal) at terminal HPOUT. Consequently, resistor R 5  and capacitor C 3  in combination with amplifier  304  generate an increased, inverted gain (G) to cause the gate voltage (V G ) on transistor Q 1  to increase while being out of phase with the voltage (signal) at terminal HPOUT. Ideally, the phase shift is 180° to obtain an impedance (Z OUT ) of 
                     Z   OUT     =         1     g     m   ⁢           ⁢   1         ⁢     (     1   +            V   G     HPOUT            )       =         (     1   +   G     )       g     m   ⁢           ⁢   1         .               (   1   )               
As an example a bode plot of the gain (dB) and phase (degrees) can be seen in  FIG. 4 , and as shown, the phase is near 180° at 1 MHz (which is also where the gain begins to plateau). Also, the output impedance Z OUT  (Ω) and phase (degrees) is shown in  FIG. 5 , where it can be seen that the impedance greater than 7 kΩ in the audible range (between about 20 Hz and about 20 kHz) and about 150Ω near 1 MHz (where the amplifier  308  tends becomes unstable if the load impedance is larger than a few hundred ohms).
 
     Additionally, to further reduce the impedance of the snubber  302 , additional circuitry is provided. In particular, NMOS transistor Q 3  (which is about the same size as transistor Q 2 ) is coupled at its gate to the amplifier  304 , so the gate voltage of transistor Q 3  is generally the same as the gate voltage of transistor Q 1 . The source of transistor Q 3  is coupled to the source of diode-connected PMOS transistor Q 4 , and the drain of transistor Q 4  is coupled to a second current source  314  (which is coupled to supply rail VSS). The ratio of currents in first current source and second current source is 1:1. Additionally, the gate of transistor Q 4  is coupled to the gate of PMOS transistor Q 5  to form a current mirror (with transistor Q 5  being N times larger than transistor Q 4 ), while the source of transistor Q 5  is coupled to the source of transistor Q 1 . This arrangement allows the transconductance (g m5 ) of transistor Q 5  to add in parallel with transconductance (g m1 ) of transistor Q 1  to reduce the impedance of the snubber  302  to 
                     Z   OUT     =       (     1   +   G     )       (       g     m   ⁢           ⁢   1       +     g     m   ⁢           ⁢   5         )               (   2   )               
The output impedance Z OUT  (Ω) and phase (degrees) for active snubber  302  is shown in  FIG. 5 , where it can be seen that the impedance greater than 7 kΩ in the audible range (between about 20 Hz and about 20 kHz) and about 150Ω near 1 MHz (where the amplifier  308  tends becomes unstable if the load impedance is larger than a few hundred ohms)
 
     To examine the effectiveness of snubber  302 , a comparison between snubber  302  and other conventional designs (i.e., snubbers  104 - 1  and  104 - 2 ) can be seen in Table 1 below. In particular, Table 1 shows simulations results for each of snubbers  104 - 1 ,  104 - 2 , and  302  with a 10 mW audio amplifier at 1 kHz into 16Ω headphones, and clearly, base on these results, snubber  302  provides significantly better performance with reduced area. It should also be noted that the area calculator for capacitor C 2  used for snubber  104 - 2  assumes the largest density capacitor available “on-chip” was used. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 No 
                 Snubber 
                 Snubber 
                 Snubber 
               
               
                 Parameter 
                 Snubber 
                 104-1 
                 104-2 
                 302 
               
               
                   
               
             
             
               
                 Worst Case 
                 33° 
                 66.7° 
                 68.8° 
                 64.4° 
               
               
                 Phase 
               
               
                 Margin 
               
             
          
           
               
                 Worst Case 
                 8.9 dB 
                 17.5 
                 dB 
                 17.1 
                 dB 
                 15.4 
                 dB 
               
               
                 Gain Margin 
               
               
                 Additional 
                 0 
                 2.67 
                 mA 
                 0.05 
                 mA 
                 0.081 
                 mA 
               
               
                 Current 
               
               
                 (Dynamic 
               
               
                 and 
               
               
                 Quiescent) 
               
               
                 Effective 
                 0 
                 3,500 
                 μm 2   
                 3,000,000 
                 μm 2   
                 30,000 
                 μm 2   
               
               
                 area on chip 
               
               
                   
               
             
          
         
       
     
     Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.