Patent Publication Number: US-5530399-A

Title: Transconductance scaling circuit and method responsive to a received digital code word for use with an operational transconductance circuit

Description:
TECHNICAL FIELD 
     This invention relates in general to operational transconductance amplifiers and more specifically to the tuning of metal-oxide-semiconductor (MOS) operational transconductance amplifiers. 
     BACKGROUND 
     Integrated operational transconductance amplifier (OTA) circuits are used in a wide array of applications such as filtering or signal level regulation (i.e., gain or attenuation blocks). A commonly used topology for an OTA is given in FIG. 1 of the accompanying drawings. The OTA 100 includes two functional elements: an input voltage-to-current converter 102 characterized by transconductance gm 0  and a programmable linear current scaling circuit 104 with an input to output current gain ratio A I . The current gain A I  is a function of bias currents I 1  and I 2  as given in the following equation: 
     
         A.sub.I =k(I.sub.2 /I.sub.1) 
    
     where k is a constant of proportionality. The resulting transconductance for the OTA 100 is given by the following equations: ##EQU1## 
     In the application of the OTA 100 in an integrated transconductance-capacitor (Gm-C) filter, Gm is tuned and/or programmed to achieve some desired bandwidth. The tuning circuit is often a phase lock loop which tunes Gm so that the ratio of Gm/C is some desired value where C is the filter capacitance. For the OTA 100, the bias current I 1  is typically set by the tuning circuit, and I 2  is typically a programmable value that enables linear scaling of the bandwidth with respect to a reference current set by I 1 . A common implementation of the OTA 100 uses a current steering digital-to-analog converter (D/A) to set the value for I 2 , thus enabling digital programming of the filter bandwidth. 
     In bipolar or Bipolar-CMOS technology, the current scaling element is typically a bipolar &#34;translinear amplifier&#34; such as the one depicted in FIG. 2 of the accompanying drawings. In bipolar transistor technology, the output current, I out , of the translinear amplifier 200 is proportional to the exponential of the input voltage, I out  ∝exp(V be  /V T ), where V T  is the thermal voltage. As a result, the current gain of the bipolar translinear amplifier 200 is exactly proportional to the ratio of I 2  /I 1  as in the first equation. Thus, the desired linear scaling of Gm can be performed by adjusting the I 2  /I 1  ratio. 
     In MOS technology, however, the output current of the transistor is proportional to the quadratic of the input voltage, I out  ∝(V gs  -V T ) 2  where V T  is the threshold voltage. As a result, the current gain of a MOS translinear amplifier shown in FIG. 3 of the accompanying drawings is not exactly proportional to I 2  /I 1 , but is a non-linear function of this ratio. Furthermore, the current gain is also dependent on the nominal value of the input current I in  as well as the carrier mobility, μ, which is highly process and temperature dependent. 
     FIG. 4 of the accompanying drawings shows an example of a typical voltage tunable complementary MOS OTA 400 implementing a translinear amplifier current scaling circuit 402, similar to the one shown in FIG. 3. Here the nominal Gm is set by resistor Rgm, and the Gm &#34;tuning&#34; is performed by adjusting the tuning bias voltage, Vtune, to the N-channel MOS differential pairs, MN1, MN2 and MN3, MN4. Bias currents Iss represent the DC biasing for the MOS OTA 400. As a result of the non-linear transistor gain, wide dynamic range current scaling (and consequently Gm scaling) is more problematic for MOS technology than bipolar technology. 
     Hence, there is a need for a circuit in MOS technology that emulates the linear behavior of the bipolar &#34;translinear amplifier&#34; in order to obtain deterministic scaling of the OTA transconductance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a prior art operational transconductance amplifier (OTA). 
     FIG. 2 is a circuit diagram of a prior art bipolar translinear amplifier. 
     FIG. 3 is a circuit diagram of a prior art MOS translinear amplifier. 
     FIG. 4 is a circuit diagram of a prior art voltage tunable CMOS operational transconductance amplifier. 
     FIG. 5 is a MOS transconductance scaling circuit in accordance with the present invention. 
     FIG. 6 is an OTA filter circuit in accordance with the present invention. 
     FIG. 7 is an OTA attenuator circuit in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An operational transconductance amplifier (OTA) is a device that outputs a current which is proportional to a differential voltage input. Transconductance, Gm, is defined as the differential of the output current divided by the differential of the input voltage. 
     Referring now to FIG. 5, there is shown a MOS OTA scaling circuit 500 in accordance with the present invention. The OTA scaling circuit 500 includes first and second OTAs 502 and 504 the OTA scaling circuit further preferably includes a reference voltage generator 503 and a turning voltage generator 505. Each OTA 502, 504 is characterized by its respective transconductance, Gm 1  and Gm 2 , which is controlled by a tuning voltage, V tune1  for Gm1 and V tune2  for Gm2. The pair of OTAs 502, 504 are driven from a DC voltage reference generator 503 which generates the reference voltage Vref. As the result, OTA 502 sources an amount of current given by the equation: 
     
         Isource=Gm.sub.1 ×Vref. 
    
     The transconductance Gm.sub. 1 is set by the tuning voltage V tune1 , (Gm 1  =f(Vtune1). The first OTA 502 behaves essentially as a reference OTA which sets a stable transconductance and source current with respect to temperature and process. The tuning voltage generator 505 which generates the tuning voltage V tune1  can be some type of reference transconductance setting circuit such as a bandgap voltage reference or a transconductance tuning phase locked loop. The source current, Isource, is used as the input current for a current mode digital to analog converter (D/A) 506. The D/A circuit 506 converts the source current, Isource, using an arbitrary function, into an output sinking current, Isink, that is characterized by the equation: 
     
         Isink=Isource×f(W.sub.m), 
    
     where Wm is an m-bit digital programming word, such as from (W m  : 0, 2, . . . , 2 m  -1). The relationship f(W m ) can be any desired function such as a linear function or some arbitrary non-linear function. The Isink current is then provided to the output of OTA 504 while OTA 504, which is being driven by the same Vref input as OTA 502, produces an output current, Iout. The OTA 504 output current, Iout, is a function of the fixed input voltage, Vref, multiplied by the transconductance Gm 2 . The tuning voltage, V tune2 , tunes the transconductance, Gm 2 , therefore, the output current, Iout, can also be varied by adjusting the tuning voltage V tune2 . 
     An integrator consisting of an operational amplifier 508 and capacitor 510 forces the OTA 504 output current Iout to equal the D/A output current Isink by regulating the transconductance tuning voltage V tune2 . The integrator acts as a negative feedback loop that adjusts V tune2  in order to keep the current entering into the operational amplifier 508, Idiff, at zero, thus forcing Iout to equal Isink. As a result, the transconductance Gm 2  is given by: ##EQU2## This equation indicates that Gm 2  can be programmed relative to Gm 1  through the digital input to the D/A circuit 506. So, based on the digital code word, the output tuning voltage V tune2  indirectly represents the scaled transconductance of OTA 504. The tuning voltage V tune2  can then be used as a scaling output to drive other OTAs. 
     The OTA scaling circuit 500 of the present invention allows the tuning voltage V tune2  to compensate for variations in the source current while still allowing the scaling to be controlled by the digital code word. The scaling function can therefore be characterized by the following equation: 
     
         Isink/Isource=f(W.sub.m), 
    
     which overcomes the problems associated with the variations of Isink over process and temperature normally associated with integrated MOS OTA circuits. 
     By feeding the source current into the D/A 506 and changing the current within the D/A as a function of the digital word, W m , the OTA scaling circuit 500 can scale other OTA circuits either linearly or non linearly. The scaling circuit 500 provides a means of taking any voltage tunable OTA and digitally controlling its transconductance. 
     As an example, the arbitrary D/A function, f(W m ), can be linear as given by the following equation: 
     
         Gm.sub.2 =k(Wm+1)Gm.sub.1, 
    
     where k is a scaling constant and again Wm is the m-bit digital programming word. This type of linear Gm scaling can be used to program the -3 dB bandwidth of an OTA-capacitance (OTA-C) filter. 
     Referring now to FIG. 6, there is shown an MOS OTA-C filter 602 employing the G m  scaling circuit 500 in accordance with the present invention. The scaling circuit 500 is also referred to as the master portion of the circuit while the OTA filter 602 is referred to as the slave portion of the circuit. Here, the V tune2  tuning voltage sets the transconductance, Gm 2 , for all three OTAs 604 in this third order active filter. By using a linear D/A, such as described in the previous equation, Gm 2  can be scaled from (k)Gm 1  up to (2 m  k)Gm 1 . Since the OTA-C bandwidth is proportional to Gm/C, this produces a scaling in the -3 dB bandwidth from (kGm 1 )/(2πC) to (2 m  kGm 1 )/(2πC). Again, V tune1  sets the stable reference transconductance, Gm 1 , which is scaled by the arbitrary D/A function, f(Wm). 
     In prior art OTA-C filters if only a phase locked loop (PLL) were used for bandwidth programming, then the reference frequency would have to be continuously adjusted. However, this would be impractical in a real system. By using the scaling circuit described by the invention, the source current is adjusted by the PLL such that Gm 1  /C is a fixed known quantity which is stable over temperature and process. By feeding the current into the D/A and converting the current within the D/A as a function of a digital word, the filter can be scaled linearly. 
     As another example, refer to FIG. 7, where there is shown an attenuator circuit 702 being scaled by the Gm tuning circuit 500 in accordance with the present invention. Here the OTA attenuator stage 702 can be operated with a digitally programmable voltage attenuation by tuning the transconductance Gm 2  of the input OTA 704 relative to the fixed transconductance, Gm 1 , of the voltage follower OTA 706. 
     Here Gm 2  can be an exponential transfer characteristic with respect to Gm 1  as given by the following equation: 
     
         Gm.sub.2 =k1 exp(-k2W.sub.m)Gm.sub.1, and 
    
     the voltage attenuation is given by: ##EQU3## 
     where k1 and k2 are constants and Wm is the m-bit programming word. Thus, a &#34;linear-to-dB&#34; digital programming of the voltage attenuation is implemented. Furthermore, the bandwidth of the attenuator circuit 702 remains essentially impervious to the attenuation setting. The transconductance tuning circuit 500 as described in combination with the attenuator circuit 702 eliminates the need for resistor-divider networks in attenuator circuits and thus offers a significant savings in silicon die area. 
     By taking a digital word and providing a tuning voltage that indirectly represents transconductance, in the manner described by the invention, other MOS OTA circuits can be driven with high precision and little variation over process and temperature changes. The transconductance of other OTAs slaved off of this scaling circuit are thus forced to a precision transconductance. 
     In today&#39;s integrated circuits (IC) it is not uncommon to have multiple OTAs performing various filtering functions and attenuation functions within a single IC. The scaling circuit as described by the invention provides a way for controlling each one of these OTA functions using a digital word to independently program each OTA circuit. Each OTA circuit used in an integrated circuit can be slaved off of a single master OTA using Gm 1 , regardless of the function of the slaved circuit. Thus, a &#34;local&#34; regulation circuit is provided that can program, for example, the attenuation or bandwidth of multiple OTA circuits. 
     Hence, a MOS integrated circuit has been provided that uses feedback to implement a digitally programmable current scaling function.