Abstract:
A transconductance amplifier and method for improving the phase response and linearity. A differential amplifier circuit receives differential signals for amplification on respective bases of input bipolar transistors. The transistors amplify a small signal received on the based connections to produce an amplified output current. The differential amplifier circuit is connected to load impedances which form a cascode transconductance amplifier output stage. Feedback transistors provide a feedback voltage from the emitters of each of the different bipolar transistors to the base, improving the linearity of the differential amplifier. Phase compensation is provided by cross coupling through first and second capacitors a portion of each individual differential signal component to the base connections of the differential amplifier input transistor.

Description:
The present invention relates to transconductance amplifiers which are used in high frequency signal processing systems specifically, a transconductance amplifier having a linear amplified response and wide operational bandwidth is disclosed. 
     Solid state transconductance amplifiers are used as a basic circuit element for many types of signal processing devices. For instance, in the implementation of high order active filters, transconductance amplifiers may be used as an integrator. An ideal integrator provides a transfer function which is proportional to the gain of the transconductance amplifier and an integration capacitance terminating the output of the amplifier. 
     In order to implement signal processing devices such as high order active filters, the transconductance amplifier must provide a high degree of amplitude linearity over the frequency range of interest. Certain applications for processing signals in the 10 MHZ to 1GHZ frequency spectrum require a linearity of up to −80 dBc for third order intermodulation products (IM3) Additionally, to operate as an ideal integrator, the phase response for the transconductance amplifier terminated with an integration capacitor must be maintained at 90° over the bandwidth of interest. The requirement for a high linearity, and a flat phase response of substantially 90° over the bandwidth of interest, are particularly important when high order active filters requiring high linearity are implemented using such transconductance amplifiers. 
     BRIEF SUMMARY OF THE INVENTION 
     A highly linear and phase compensated transconductance amplifier implemented from a differential amplifier circuit is provided for by the invention. The differential amplifier circuit has first and second bipolar transistors which receive a differential signal for amplification. In order to improve the linearity of the gain response of the differential amplifier circuits, first and second input transistors comprise an input stage to each of the differential amplifier input transistors. The first and second input transistors supply one-half of the differential signal to a respective base of the differential amplifier circuit transistors, and provide feedback from the emitter of the respective differential circuit transistors to their respective bases. The effect is to improve the overall amplitude response linearity for the transconductance amplifier. 
     The phase of the resulting transconductance amplifier is compensated so that a phase response of 90° is maintained over the bandwidth of interest. First and second capacitors cross couple a portion of each half of the differential signal to the bipolar transistor which is receiving the other half of the differential signal. The first and second capacitors provide a pair of dominant poles in the frequency response of the transconductance amplifier, which dominates the pole produced from parasitic capacitances of the input transistors, and produce a pair of zeroes in the complex frequency plane which essentially cancels the resulting dominant poles. The compensated amplifier has a linear amplitude output and a relatively constant phase over the frequency range of interest. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 represents a transconductor amplifier of a preferred embodiment used to implement an integrator in a high order active filter. 
     FIG. 2 illustrates the amplitude response of the uncompensated transconductance amplifier. 
     FIG. 3 represents the uncompensated phase response of a transconductance amplifier. 
     FIG. 4 represents a preferred embodiment of the invention having a linearized amplitude function and a compensated phase response. 
     FIG. 5 is the small signal equivalent circuit for the linearizing circuit  5 . 
     FIG. 6 is the amplitude response of the transconductance amplifier of FIG. 4 which has been phase compensated. 
     FIG. 7 is the phase response of the transconductance amplifier of FIG. 4 which has been phase compensated. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, a functional block diagram is disclosed showing an integrator circuit which is used as an element for a high order active filter. The integrator comprises a transconductance amplifier  2  connected to an integrating capacitor  4 . The signal to be integrated is applied to input terminal  1 , which is connected to the input of the transconductance amplifier  2 . The gain of the transconductance amplifier is gm, and the response of the integrator circuit may be represented as:            V   out       V   in       =         g   m     /   c     S                            
     where c is the value of the integrating capacitor and S is the S plane operator. 
     The idealized integrator of FIG. 1 should have a frequency response W o /S, or in the frequency plane W o /jω, and the phase response for the idealized integrator should be a continuous −90°, where W o  is the unity gain frequency of the integrator. 
     The transconductance amplifier  2  is selected as the gain element because, over the high frequency region of interest, ie. 10 MHZ-1GHZ, a wide operational bandwidth may be achieved. Improvements of linearity for small signal operation may be implemented by applying feedback to the input signals from each half of the differential amplifier  2  by boost circuits  5  and  6 . While the resulting response provides a linear magnitude response as shown in FIG. 2, a phase response as set forth in FIG. 3 is obtained which is inadequate over the bandwidth of interest. The phase response is preferably a constant 90°, and any deviation from this idealized figure represented by the doted line in FIG. 3, reduces the total operational bandwidth for the integrator. 
     A more detailed illustration of the transconductance amplifier in accordance with the preferred embodiment is shown in FIG.  4 . Referring now to FIG. 4, a basic differential amplifier is shown having bipolar input transistors  22  and  23 , connected to a bipolar current source transistor  29  through emitter resistors  26  and  27 . The gain of the amplifier is set by the values of resistors  26 - 27 . The output stage of the differential amplifier is shown as  11 . The output stage  11  provides a cascode output impedance, where PMOS transistors  12 ,  13 ,  14  and  15 , operate in a saturation mode because of the bias voltage Vpc, and Vpb, applied to respective gates of these devices, which conduct a current from voltage supply Vdd. Bipolar transistors  18  and  19  similarly operate in the active mode under control of a bias potential Vabc to provide cascode output impedance for the differential amplifier. 
     The circuits  5  and  6  include input PMOS transistors  31  and  34  driven from a current source  36 , and supply a differential signal component to the base of differential amplifier transistors  23  and  22 . In order to provide the signal boost for linearizing the output of the differential amplifier, NMOS transistors  32  and  35  supply a feedback current from the emitters of bipolar transistors  22  and  23  to the base connections of bipolar transistors  22  and  23 . The feedback forces the emitter voltage to linearly track input differential voltage, thus tending to further linearize the output currents produced from each of bipolar transistors  22  and  23  over an increased frequency bandwidth. Integrating capacitor  4  is connected between the collectors of transistors  22  and  23  which constitute the output terminals of the amplifier. 
     NMOS transistors  32  and  35  and PMOS transistor  31  and  34  of the linearizing boost circuits  5  and  6  introduce objectionable parasitic capacitance C p  at their drain nodes between the base of transistors  23 ,  22  and ground. This results in a pole created in the complex frequency domains for the amplifier circuit, reducing the net effective operational bandwidth for the device. 
     Compensation of the parasitic capacitances produced by NMOS transistors  32  and  35  are provided by capacitors  40  and  41 . Each of these capacitors cross couple a portion of the differential signal components applied to the transconductance amplifier, VIP and VIN, to the base of the respective bipolar transistors  23  and  22  which receive VIP and VIN. By cross coupling a portion of each differential signal component to the bipolar transistor  22 ,  23  amplifying the other differential component, a dominant pole is produced which dominates the pole produced by the parasitic capacitance C p . 
     The foregoing effects are illustrated more particularly with respect to the small signal equivalent circuit of the linearizing circuit  5  shown in FIG.  5 . The small signal equivalent circuit demonstrates an output current i out  from the linearizing circuit which is produced from one-half of the differential amplifier current through bipolar transistor  23 . The base resistance of transistor  23  is represented by r π , and the transconduction of transistor  23  is g mQ . The current through NMOS transistor  32 , and PMOS transistor  31 , are respectively: 
     
       
         i NMOS =g mn ·V e   
       
     
     
       
         i PMOS =g mp ·V in   
       
     
     where g mn  is the transconductors of NMOS transistors  32 , g mp  is the transconductor of PMOS transistor  31 , V e  is the emitter voltage of transistor  23 , V in  is the differential signal component applied to the gate of PMOS transistor  31 . 
     The small signal equivalent circuit demonstrates that there are two reactances, C c  of capacitor  40 , and C p  of NMOS device  32 . The small signal output current i out  flowing through transistor  23  and the emitter degeneration resistor  26  can be written as follows:          i   out     =           g   mp     +     SC   c           g   mn     +     S        (       C   c     +     C   p       )           ·       V   in       R   e                                
     where g mp  is the transconductance of PMOS transistor  31 ; g mn  is the transconductance of NMOS transistor  32 ; R e  is the emitter resistance  26  of bipolar transistor  23 . 
     The transfer function of the small signal equivalent circuit includes a pole, located at g mn /(C c +C p ). The effect of the compensation capacitor  40  is to produce a pole in the complex frequencies domain which dominates the pole produced by the parasitic capacitance C p  of NMOS device  32 . The dominant pole thus created is canceled by a substantially equivalent zero represented by.            g   mp       C   c       .                          
     Accordingly, the effects of the parasitic capacitance C p  on the overall amplifier response are effectively neutralized. 
     The amplitude response for the circuit which has been phase compensated and the compensated phase response are shown in FIG.  6  and FIG.  7 . The linearity of the amplitude response has been maintained in FIG. 6, and the phase response has a constant phase of 90° over an increased bandwidth compared to the uncompensated phase response of FIG.  3 . 
     A similar response is provided by the boost and linearizing circuit  6 . Because of the presence of capacitor  41 , a portion of the differential component signal VIN is cross-coupled to the base of the transistor  22  of the input differential amplifier which receives differential component VIP. A dominant pole is produced which is effectively canceled by a zero which was created at the same time. 
     Thus there has been described with respect to one embodiment of the invention, a transconductance amplifier having an improved linearity, and phase response. 
     The foregoing description of the invention illustrates and describes the present invention. Additionally, the disclosure shows and describes only the preferred embodiments of the invention but, as mentioned above, it is to be understood that the invention is capable of use in various other combinations, modifications, and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein, commensurate with the above teachings and/or the skill or knowledge of the relevant art. The embodiments described hereinabove are further intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with the various modifications required by the particular applications or uses of the invention. Accordingly, the description is not intended to limit the invention to the form disclosed herein. Also, it is intended that the appended claims be construed to include alternative embodiments.