Abstract:
A low noise amplifier (LNA) for amplifying an input signal communicated over a transmission line having an impedance. The LNA includes a current sensing amplifier having an input to connect to the transmission line. The current sensing amplifier has an input impedance that matches the transmission line impedance. The current sensing amplifier amplifies the input signal to generate a first output signal. A voltage sensing amplifier receives the input signal and generates a second output signal. A combiner combines the first output signal and the second output signal to generate an LNA output signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of the filing date of U.S. provisional application No. 60/413,595 filed Sep. 24, 2002, the content of which is herein incorporated by reference in its entirety and the content of U.S. non-provisional application Ser. No. 10/072,843 filed Feb. 6, 2002, is herein incorporated by reference in its entirety. 
     
    
     
       TECHNICAL FIELD  
         [0002]    An aspect of this invention relates to low noise amplifiers (LNAs).  
         BACKGROUND  
         [0003]    Today, storage devices, such as hard disk drives, are operating at close to Gigabit/s data rates, and soon will reach 2 Gigabit/s data rates. At these data rates, signal bandwidth may approach Gigahertz levels; similar to wireless applications. As such, it is now important to utilize low noise amplifier circuitry having matched input impedance to achieve a flat frequency response when interfacing the amplifier circuitry to a storage device read head through a transmission line. This is similar to wireless applications, where a matched impedance low noise amplifier is generally needed to interface to the antenna and the corresponding transmission line attached to it.  
           [0004]    However, unlike wireless applications, storage devices are more broadband in nature. In fact, most storage devices such as a hard disk drives produce signals from about DC to the Nyquist frequency. In contrast, wireless devices typically operate over a narrow frequency band (at most up to a few tens of Megahertz) with the signals centered around the carrier frequency. For the purpose of matching the input impedance of a conventional RF wireless amplifier, the impedance only needs to be matched at the carrier frequency. Input impedance matching of conventional wireless amplifiers can simply be achieved using resonance tuning with inductive and capacitive components as is well known to those skilled in the art.  
           [0005]    Tuning schemes do not work on ultra broadband signals such as that found in a disk drive device and the emerging ultra wideband wireless devices that are being debated in the industry. For these ultra broadband and ultra wideband devices to work better, a very low noise amplifier is needed to process the weak signals encountered by the read head or the antenna. Since a transmission line is normally used to couple the read head or the antenna to the low noise amplifier, input impedance matching is needed.  
           [0006]    Conventional very low noise amplifiers and other wideband amplifiers generally include a passive resistor to impedance match to the transmission line. However, using a passive resistor may increase power loss and the input noise figure (sometimes referred to as input referred noise voltage) due to the fundamental noise generated by the resistor. Circuit textbooks universally teach that any resistive components, real resistors or synthesized resistors, generate wideband thermal noise. The value of the fundamental noise is widely known to be {square root}{square root over (4kTR)} for ideal resistors and somewhat higher for synthesized resistors.  
         SUMMARY  
         [0007]    A low noise amplifier (LNA) for amplifying an input signal communicated over a transmission line having an impedance. The LNA includes a current sensing amplifier having an input to connect to the transmission line. The current sensing amplifier has an input impedance that matches the transmission line impedance. The current sensing amplifier amplifies the input signal to generate a first output signal. A voltage sensing amplifier receives the input signal and generates a second output signal. A combiner combines the first output signal and the second output signal to generate an LNA output signal.  
           [0008]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0009]    [0009]FIG. 1 is a block diagram of an aspect of an LNA.  
         [0010]    [0010]FIG. 2 is a detailed diagram of an aspect of an LNA.  
         [0011]    [0011]FIG. 3 is a detailed diagram of an aspect of an LNA including models of noise source.  
         [0012]    [0012]FIG. 4 is a detailed diagram of an aspect of an LNA.  
         [0013]    [0013]FIGS. 5A and 5B are detailed diagrams of aspects of amplifier loads for an LNA. 
     
    
       [0014]    Like reference symbols in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0015]    [0015]FIG. 1 shows a low noise amplifier (LNA)  10  for amplifying an ultra broadband input signal  12  communicated to the LNA  10  over a transmission line (not shown). The LNA  10  may be fabricated using any suitable process including Metal Oxide Semiconductor (MOS) and Galium Arsenide (GaAs) or any other compound semiconductor devices. The LNA  10  includes a voltage sensing amplifier  14  and a current sensing amplifier  16  to amplify the input signal  12 . The current sensing amplifier  16  may synthesize the input matching impedance for matching the impedance of the transmission line. The outputs of the voltage and current sensing amplifiers  14  and  16  are combined to generate the output signal, Vout, 18. The voltage and current sensing amplifiers  14  and  16  advantageously generate opposite and equal noise levels so that the noise generated by the input matching circuit substantially cancels out.  
         [0016]    [0016]FIG. 2 shows an aspect of an LNA  20  for amplifying an ultra broadband input signal  22 . The input signal  22  may be generated by an input source  23 . The LNA  20  includes a current sensing amplifier  24  for synthesizing the input matching impedance. In one aspect, the current sensing amplifier  24  may be configured as a common-base amplifier with current sink Ib 1 . Here, the transconductance, Gm, of the current sensing amplifier  24  equals VT/Ib 1 , where VT stands for the volt equivalent of temperature (kT/q where k is the Boltzman constant, T is absolute temperature, and q is coulomb charge). In a typical application, an impedance value of 50 Ohms may be used. The 50 Ohms input impedance may be obtained by biasing the current sensing amplifier such that the transconductance, Gm, is equal to the inverse of 50 Ohms (20 milliMhos).  
         [0017]    Although the current sensing amplifier  24  provides an input matching impedance to match the transmission line, half of the input signal may be lost. This is evident by looking at the voltage present on the input of the current sensing amplifier  24 . In this case, the voltage is exactly half of the source value.  
         [0018]    A voltage sensing amplifier  26  may recover the other half of the input signal. An input of the voltage sensing amplifier  26  may be connected to the input of the current sensing amplifier  24 . The outputs of the current and voltage sensing amplifiers  24  and  26  may be combined in a summer  28 . By combining the outputs of the current and voltage sensing amplifiers  24  and  26 , full signal amplification may be achieved. In addition, the noise generated by the current sensing amplifier may be exactly cancelled out by summing the voltage sensing amplifier  26 . In one aspect, the voltage sensing amplifier  26  may be configured as an emitter-follower amplifier. The current and voltage sensing amplifiers  24  and  26  may each include resistive loads  25  and  27 . The ratio of the load resistors  25  and  27  may determine the values of k1 and k2 associated with the current and voltage sensing amplifiers  24  and  26 .  
         [0019]    AC coupling of the input signal may also be used to bias the operating point of the current sensing and voltage sensing amplifiers  24  and  26 .  
         [0020]    [0020]FIG. 3 shows how an LNA  30  may cancel out the noise. Here, transistor equivalent noise sources have been added to the aspect of the LNA shown in FIG. 2. For simplicity, two noise sources  32  and  34  may be added, one noise source, v1,  32  at the input of the current sensing amplifier  36  and the other noise source, v2, at the input of the voltage sensing amplifier  38 . Noise generated by other circuitry (input referred noise) is lumped into the noise sources  32  and  34  for each of the amplifiers  36  and  38 .  
         [0021]    The polarity of the amplified noise at the output of the two amplifiers may be traced as follows. In the current sensing amplifier  36 , the output noise is equal to −k1*v1 where k1 is the gain of the current sensing amplifier  36 . This number is a negative number since signal inversion occurs from the base of the amplifier  36  to the collector of the amplifier  36 .  
         [0022]    In the voltage sensing amplifier  38 , the output noise contributed by the noise generated by the current sensing amplifier  36  is equal to −½ of v1*k2, where k2 is the gain of the voltage sensing amplifier  38 . The factor of ½ is introduced because by the time the noise from the current sensing amplifier  36  reaches the emitter of the current sensing amplifier  36 , half of the noise is lost due to the input impedance seen at the source of the signal which is substantially equal to the transconductance of the current sense amplifier  36 . In other words, when the load seen by an emitter follower is equal to the Gm of an emitter follower, the gain of the emitter follower is ½.  
         [0023]    The other contribution of the noise of the voltage sensing amplifier  38  is −k2*v2. After subtracting the output voltage of the amplifiers  36  and  38 , and scaling k1 and k2 appropriately, the noise generated by v2 is left.  
         [0024]    Meanwhile, the input signal is amplified by the amplifiers  36  and  38 . The output voltage of the current sensing amplifier  36  due to the input signal is ½ k1*vin. There is no inversion because there is no inversion of signal communicated from the emitter to the collector of the current sensing amplifier  36 . On the other hand, the output voltage of the voltage sensing amplifier  38  due to the input signal is −½*k2*vin. Note that there is an inversion of the input signal in the case of the voltage sensing amplifier  38 .  
         [0025]    By subtracting the output signals from the current and voltage sensing amplifiers  36  and  38 , the components of the output signals associated with the input signal add together, while the components of the output signals related to noise source v1  32  substantially cancel out.  
         [0026]    Noise source v2 remains as substantially the only source of noise components present in the output signal. Since noise source v2 represents noise generated by a voltage sensing amplifier, the noise can be made arbitrarily small. This may be done by making the Gm of the voltage sensing amplifier  38  very large. Note that this is not possible with a current sensing amplifier, since the input matched impedance dictates the Gm of the current sensing amplifier  36 .  
         [0027]    [0027]FIG. 4 shows another aspect of an LNA  40  for amplifying an ultra broadband input signal  22 . The LNA  40  is similar in function to the LNA  20  with corresponding elements numbered from  22 - 28 , except that in LNA  40  the resistive loads of the current sensing amplifier  44  and voltage sensing amplifier  46  may be replaced with transimpedance amplifiers  50  and  52 . This may result in a significant increase in the achievable bandwidth. Alternatively, referring to FIG. 5A, a nested transimpedance amplifier structure  60  may be used as disclosed in pending U.S. non-provisional patent Ser. No. 10/072,843, filed Feb. 6, 2001, which is hereby incorporated by reference in its entirety. Also, referring to FIG. 5B, a low gain amplifier  72  may be inserted in the transimpedance load  70  to increase the Gm of a transimpedance device  74  while minimizing the input capacitance and resistance of the transimpedance load. A factor of two to four of improvement can easily be achieved with a corresponding increase in bandwidth. Other types of amplifier loads may also be employed including amplifier loads that are not broadband or semi-broadband in nature.  
         [0028]    Input capacitance cancellation may also be included to improve input matching over a wider frequency band.  
         [0029]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.