Abstract:
An amplifier with increased bandwidth by current injection and a method thereof. The amplifier includes an input stage for receiving a first input signal, a load stage coupled to the input stage for generating a first output signal, a first current source coupled to the input stage for allowing a predetermined current to flow, and a second current source, coupled to the input stage, for injecting a first current into the input stage for outputting the first output signal.

Description:
BACKGROUND OF INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an amplifier, and more particularly, to an amplifier with increased bandwidth by current injection.  
         [0003]     2. Description of the Prior Art  
         [0004]     Operational amplifiers are one of the most widely used structures in modern day engineering. They can have many applications ranging from buffers and filters to analog-to-digital converters. Where high performance is required, such as a high speed, high resolution analog-to-digital converter, an operational amplifier having both high gain and high bandwidth is necessary.  
         [0005]     Please refer to  FIG. 1 .  FIG. 1  shows a conventional telescopic operational amplifier  10 . It comprises a plurality of transistors M 1 -M 9 . Matched transistors M 1  and M 2  serve as an input stage  12  for respectively receiving a non-inverting input V ip  and an inverting input V in , while transistors M 3 -M 8  biased by voltages V bn , V bp1 , V bp2  act as a load stage  14  (i.e., an active load). The remaining transistor M 9  is biased by voltage V cmfb , and is the tail transistor acting as a current source for sinking a constant reference current I Q , thereby steering the currents I 1  and I 2 . Since transistors M 5  and M 6  are a matched pair and transistors M 7  and M 8  are a matched pair, currents I 1  and I 2  therefore have the same current value. If both transistors M 1  and M 2  are turned on, I 1  is equal to half I Q , and I 1  is equal to I 2 . As shown in  FIG. 1 , both the non-inverting output V on  and the inverting output V op  are coupled to an output capacitor C 0 . A telescopic configuration is often a preferred configuration because telescopic operational amplifiers consume less power than other topologies. They also have the added advantage of high speed. The telescopic configuration is well known to those skilled in the art, and therefore is not described in detail here.  
         [0006]     The frequency response of an operational amplifier is the response of the circuit in the frequency domain. In other words, if the circuit is given a sinusoidal input the response should be a sinusoidal output of the same frequency, but amplified by the open loop gain, A 0 . This frequency response can be modeled as a low pass function.  
                 H   ⁢           ⁢     (   s   )       =       A   0       1   +     s     ω   p             ⁢     
     ⁢       where   ⁢           ⁢     ω   p     ⁢           ⁢   is   ⁢           ⁢   the   ⁢           ⁢   dominate   ⁢           ⁢   pole   ⁢           ⁢   and   ⁢           ⁢   s     =     j             ⁢   ω                 Equation   ⁢           ⁢     (   1   )               
 
         [0007]     For frequencies much greater than ω p , i.e. ω&gt;&gt;ω p , we can approximate the gain, A, to be:  
               A   ⁡     (   jω   )       =         A   0     ⁢     ω   p       jω             Equation   ⁢           ⁢     (   2   )                        A   ⁡     (   jω   )            =         A   0     ⁢     ω   p       ω             Equation   ⁢           ⁢     (   3   )               
 
 Therefore, putting this result back into equation (1), we can obtain the unity-gain bandwidth ω u : 
 
ω u =A 0 ω p   Equation (4) 
 
         [0008]     The unity gain bandwidth can also be written in terms of device transconductance, gm, and output resistance, r 0 . Equation (4) then becomes:  
               ω   u     =         gm   ×     r   0           r   0     ⁢     C   0         =     gm       C   op     +     C   load                   Equation   ⁢           ⁢     (   5   )               
 
         [0009]     From equation (5) it can be seen that the unity gain bandwidth ω u  is a function of device transconductance gm, and total output capacitance C 0 . The total output capacitance includes parasitic junction capacitance C op , and capacitive load C load  driven by the operational amplifier  10 . Therefore, the way to increase bandwidth is by increasing gm or by decreasing the output capacitance C 0 . C load  cannot be decreased because it is determined by circuit specification and hence cannot be decreased for a specific application. Furthermore, the parasitic capacitance dominates the capacitive load, so decreasing C load  would only have a minimal effect. To increase gm would require a larger device, which then leads to more parasitic capacitance, as the device size cannot be increased without limits. This gain-bandwidth relation therefore creates an upper level bandwidth limit.  
       SUMMARY OF INVENTION  
       [0010]     It is therefore one of the objectives of the claimed invention to provide an amplifier with increased bandwidth by current injection and a related method thereof.  
         [0011]     It is therefore one of the objectives of the claimed invention to provide an amplifier. The amplifier with current injection has an increased bandwidth while maintaining a high gain.  
         [0012]     It is therefore one of the objectives of the claimed invention to provide an amplifier. The amplifier does not require increased power consumption or circuit area.  
         [0013]     Briefly described, the present invention discloses an amplifier. The amplifier comprises an input stage for receiving a first input signal; a load stage coupled to the input stage for generating a first output signal; a first current source coupled to the input stage for allowing a predetermined current to flow; and a second current source, coupled to the input stage, for injecting a first current into the input stage for the first output signal.  
         [0014]     In addition, a method for increasing the bandwidth of an amplifier is disclosed. The method comprises providing the amplifier with an input stage for receiving a first input signal, a load stage coupled to the input stage for outputting a first output signal, and a predetermined current source coupled to the input stage for allowing a fixed current to flow; and providing a first current and injecting the first current into the input stage for outputting the first output signal.  
         [0015]     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0016]      FIG. 1  is a diagram illustrating a conventional telescopic operational amplifier.  
         [0017]      FIG. 2  is a diagram of a telescopic operational amplifier according to a first embodiment of the present invention.  
         [0018]      FIG. 3  is a diagram of a telescopic operational amplifier according to a second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0019]     Please refer to  FIG. 2 .  FIG. 2  is a diagram of a telescopic operational amplifier  20  with current injection according to a first embodiment of the present invention, for increasing the bandwidth while maintaining a high gain. The telescopic operational amplifier  20  includes an input stage  22 , which has two transistors M 1  and M 2 , and a load stage  24 , which has a plurality of transistors M 31 , M 41 , M 51 , M 61 , M 71 , M 81 . The transistor M 9  acts as a current source for sinking a constant reference current I Q , thereby steering the currents I a -I d . If both transistors M 1  and M 2  are turned on, the collective currents I a  and I c  are equal to the current I 1  shown in  FIG. 1 , and the collective currents I b  and I d  are equal to the current I 2  shown in  FIG. 1 . Please note that the circuit architecture of the input stage  22  and load stage  24  is substantially the same as that of the input stage  12  and load stage  14  shown in  FIG. 1 . The amplifier  20  of the present invention further comprises extra current sources  26  and  28 .  
         [0020]     As shown in  FIG. 2 , a series of electrically connected transistors M 72 , M 52  and M 32  are coupled to the drain of the transistor M 1 , and a series of electrically connected transistors M 82 , M 62  and M 42  are coupled to the drain of the transistor M 2 , for respectively injecting a current I c  and a current I d  into the input stage  22 . The transistors M 72  and M 82  are biased by a voltage V bp1 , M 52  and M 62  are biased by a voltage V bp2 , and M 32  and M 42  are biased by a voltage V bp3 . Additionally, an inverting output V on  coupled to an output capacitor C 0  is coupled to the drain of the transistor M 31  and an non-inverting output V op  coupled to an output capacitor C 0  is coupled to the drain of the transistor M 41 . In an embodiment, transistors M 32 , M 42 , M 51 , M 52 , M 61 , M 62 , M 71 , M 72 , M 81 , M 82  are PMOS transistors, and transistors M 1 , M 2 , M 9 , M 31 , M 41  are NMOS transistors. Please note this amplifier configuration is merely an embodiment of the present invention and is not meant to be a limitation.  
         [0021]     In an embodiment, both of the current sources  26 , and  28  are p-type current sources. Since the transistor M 9  still sinks the reference current I Q , the original currents I 1 , I 2  shown in  FIG. 1  are changed due to the injected currents I C , I d . In order to maintain the same bias condition, the size of the devices is scaled, according to the change of current of each device. Assume that I a  is set to k*I 1 . Therefore, the channel aspect ratio (W/L) is adjusted accordingly. In other words, for transistor M 71  as compared to transistor M 7  in  FIG. 1 , the channel aspect ratio is:  
                 (     W   L     )       M   ⁢           ⁢   71       =       k   *       (     W   L     )       M   ⁢           ⁢   7       ⁢   where   ⁢           ⁢   0     ⁢           &lt;   k   &lt;   1             Equation   ⁢           ⁢     (   6   )               
 
         [0022]     Similarly, for transistor M 81  as compared to transistor M 8  in  FIG. 1 :  
                 (     W   L     )       M   ⁢           ⁢   81       =         (     1   -   k     )     *       (     W   L     )       M   ⁢           ⁢   8       ⁢   where   ⁢           ⁢   0     ⁢           &lt;   k   &lt;   1             Equation   ⁢           ⁢     (   7   )               
 
 The frequency response for the operational amplifier  20  can still be modeled as a low-pass function as shown by equation (1). The dominate pole ω p , however, is now:  
               ω   p     =       1       r   0     ⁢     C   0         =     1       r   0     ⁡     (       kC   op     +     C   load       )                   Equation   ⁢           ⁢     (   8   )               
 
         [0023]     This is because the device size has decreased by a factor of k, where  0 &lt;k&lt;1, and therefore the parasitic capacitance C op  has also decreased by a factor of k. 
 
 Putting this result in the equation for unity-gain bandwidth ω u  it can be shown that:  
               ω   u     =         gm   ×     r   0           r   0     ⁢     C   0         =     gm       r   0     ⁡     (       kC   op     +     C   load       )                   Equation   ⁢           ⁢     (   9   )               
 
 Due to the smaller device parasitic capacitance C OP , the unity-gain bandwidth ω u , compared with the prior art unity-gain bandwidth, has been increased by a factor: 
 
         [0024]     (C op +C load )/(kC op +C load ). Please note that the circuit size of the operational amplifier  20  is substantially the same as that of the operational amplifier  10  shown in  FIG. 1 .  
         [0025]     Please refer to  FIG. 3 .  FIG. 3  is a diagram of a telescopic operational amplifier  30 , with gain boosting and current injection, according to a second embodiment of the present invention, for increasing the bandwidth while also increasing the gain. The telescopic operational amplifier  30  is similar to the above telescopic operational amplifier  20 . The lengthy description of the same components, i.e., the transistor M 9 , the input stage  22  and the current sources  26 ,  28 , is not repeated. As shown in  FIG. 3 , the key difference is that the telescopic operational amplifier  30  has a load stage  32  including a plurality of amplifiers  33 ,  34 ,  35 ,  36  used to enable the gain boosting. Since the gain boosting is well known to those skilled in this art, further description is omitted for brevity. Compared with the telescopic operational amplifier  20 , the telescopic operational amplifier  30  has an increased DC gain A 0  due to the gain boosting. In addition, with the currents injected into the input stage  22  by the current sources  26  and  28 , the bandwidth of the operational amplifier  30  is thereby increased.  
         [0026]     Please note that, in the above-mentioned embodiments, the current injection is applied to a differential pair. However, both of the current injection configurations shown in  FIG. 2  and  FIG. 3  are just exemplary embodiments, and are not meant to be limitations.  
         [0027]     The embodiments of the present invention have an increased bandwidth without sacrificing gain. The circuit area and power consumed are still approximately the same but the parasitic capacitance has been decreased. The current injection technique can also be used in a gain boosted operational amplifier, therefore allowing both the bandwidth and the gain to be increased at the same time.  
         [0028]     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.