Abstract:
Linear-in-dB current-steering VGAs with an adaptive bias current operable so that as the gain of the amplifier decreases, the DC current consumption also decreases. The modified VGA circuits result in power consumption savings, which are of particular value in wireless (battery powered) applications.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Patent Application No. 60/636,551 filed Dec. 17, 2004. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to variable gain amplifiers, and more specifically to high frequency, variable gain amplifiers with a linear-in-dB gain control. 
   2. Prior Art 
   The use of variable gain amplifiers (VGAs) is prevalent, for example, in connection with communications, networking, and other electronic devices. Commonly VGAs may be found in various radio frequency. (RF) applications, including the handling of intermediate frequency (IF) and baseband circuits. Many VGAs are designed to provide a constant output signal amplitude even if the input signal amplitude changes. Such changes include the change in the operating parameters of the circuit. 
   A form of a current-steering VGA  100  is shown with respect to  FIG. 1 . It comprises an amplifier core of three active devices, transistor  110 , transistor  120 , and transistor  130 . Transistor  110  and transistor  120  form a current mirror stage. The area of transistor  120  is ‘m’ times the area of transistor  110 . Device transistor  110  is biased at a DC current I 1  produced by an exponential current source  140 , having a control input V ctrl . Exponential current sources are well-known in the art and therefore the discussion herein addresses only the usage of such current sources. The collector of transistor  120  is tied to the supply voltage through the load impedance Z load    150 . The collector of transistor  130  is tied to the supply voltage while its base is biased at a constant voltage V BIAS . This sets the DC voltage of the common emitter connection. In one embodiment, the input signal is applied to the common emitter of the three devices; in another embodiment the input signal is supplied through transconductance amplifier  160 . The current I 2  is the tail DC bias current. The transconductance amplifier Gm adds the amplified input signal Gm*Vin to the tail DC bias current I 2 . The AC output signal is taken across impedance  150  at the collector of transistor  120 . The input AC signal is divided among devices transistor  110 , transistor  120 , and transistor  130 , according to their emitter admittances Y i , which depend on the bias current of each device. 
   The transistors draw the following currents: transistor  110  draws I 1 , transistor  120  draws m·I 1 , and transistor  130  draws I 2 −(m+1)·I 1 . Since Y i =I i /V T  and the thermal voltage V T =KT/q, the gain of VGA  100  can be reached by using the following equations:
 
 V   out   /V   in (dB)=20·log 10   [Y   120 /( Y   110   +Y   120   +Y   130 )· G   m   ·Z   load ]=20·log 10   {m·I   1   /[I   1   +m·I   1 +( I   2 −( m+ 1)])· G   m   ·Z   load }=20·log 10   [m·G   m   ·Z   load ·( I   1   /I   2 )]
 
   While I 2  is a fixed current value, i.e., independent of V ctrl , I 1  has an exponential function dependency of V ctrl  such that I 1 =I 10 ·10 (α/20·Vctrl) . Substituting this into the gain equation above results in the VGA gain being:
 
 V   out   /V   in (dB)=20·log 10 ( m·G   m   ·Z   load   ·I   10   /I   2 )+α· V   ctrl  
 
   Hence, there is shown a linear-in-dB VGA having a rate of α dB/V. However, even though prior art shows the linear-in-dB capability, the prior art suffers from at least the fact that the current I 2  remains constant and hence, regardless of the decrease in gain of the linear-in-dB VGA  100 , the I 2  current consumption remains the same. It would therefore be advantageous to provide a linear-in-dB VGA that is capable of adjusting I 2  current consumption in accordance with the linear-in-dB VGA gain. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a prior art linear-in-dB VGA. 
       FIG. 2  is a schematic diagram of a linear-in-dB VGA in accordance with the disclosed invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The deficiencies of the prior art require a technique that allows the DC bias current of a linear-in-db VGA to decrease as the gain decreases and vice versa. In accordance with the disclosed invention, a technique is shown for the control of the fixed current portion (DC tail current) of a linear-in-dB VGA by providing a second exponential current source (ECS) such that the fixed current is provided by that second ECS. The result of the implementation of the circuit of a VGA in accordance with the disclosed invention is the decrease of current consumption of the circuit with the decrease in gain, which is of particular importance to systems that derive their power from an independent power source, such as a battery. 
   Reference is now made to  FIG. 2  where an exemplary and non-limiting schematic diagram of a linear-in-dB VGA  200 , in accordance with the disclosed invention, is shown. In addition to the components described in detail in connection with  FIG. 1 , and which form a linear-in-dB VGA  100  amplifier core, there is added an ECS  270  as described in further detail below. In the prior art solutions the current consumption of the fixed-bias VGA, for example VGA  100 , is equal to I 2  and does not change as the gain of the VGA, for example VGA  100 , changes. At low gain settings most of the current I 2  flows through transistor  130  and is therefore wasted to the supply. In accordance with the disclosed invention, it is possible to modify the current I 2  so that it tracks the current I 1  while the gain control characteristics are maintained. This is achieved by providing a variable current I 3  from ECS  270 . ECS  270  may be of the same design as ECS  140  or not, as desired. In that regard, ECSs of various designs are well known in the prior art, having been used in prior art VGAs for which the present invention is a substantial improvement, as well as elsewhere. In accordance with the modified VGA  200 , the input signal is applied to a transconductance stage  160  with a gain G m  that converts the input signal to a current Gm*Vin that is added onto the output I 3  of the ECS  270  and then the sum applied to the common emitter of the transistors  110 ,  120 , and  130 . 
   With I 3  being an exponential function of V ctrl , its function is:
 
 I   3   =I   30 ·10 (β/20·Vctrl)  
 
   The gain of modified VGA  200  may then be derived to be:
 
 V   out   /V   in (dB)=20·log10( m·G   m   ·Z   load   ·I   10   /I   30 )+(α−β)·V ctrl  
 
   It is now easily noticed that the gain of VGA  200  is linear-in-dB with a rate of (α−β) dB/V. In this equation, α and β must have the same sign. Either α&gt;β&gt;0 so that α−β&gt;0 (positive gain), or α&lt;β&lt;0 so that α−&lt;0 (attenuation). 
   The advantage of the modified VGA  200  would now be apparent to those skilled-in-the-art. In particular, at the maximum gain, I 3  may equal I 2  of the prior art of  FIG. 1 , but the total bias current consumption of the amplifier is exponentially decreasing at a rate of β dB/V as the gain decreases. As a result, less current is steered to the supply through transistor  130  as compared with the fixed-bias VGA, for example VGA  100 . Furthermore, because the rate by which the gain decreases is a function of the difference (α−β), the rate the current I 3  decreases can be set independently, allowing for independent optimization of other performance parameters of the circuit. The current savings are extremely important in certain applications, and particularly, for example, in an RF transmitter that, when implementing the VGA circuit in accordance with the disclosed invention, can provide a range of gain control with minimum impact on the overall current consumption. In one non-limiting embodiment of the disclosed invention the value of α is significantly larger than the value of β, for example, β has a value of 10% of that used for α. As a result the linear-in-dB behavior of the exemplary embodiment is predominately the result of the value of α. However, there is still a benefit in the reduction of the current at a rate of β. Specific values of α and β are selected so as to reach the goals of the linear-in-dB behavior of the modified VGA  200  as well as the reduction of the tail DC current required. 
   The description provided hereinabove is of a single-ended VGA. However other usages of the techniques disclosed by this invention are possible, including but not limited to, differential structures, without departing from the spirit of the disclosed invention. A person skilled-in-the-art would further note that while bipolar NPN transistors are shown herein, this invention is not limited to such an implementation, and would further include, but not limited to, bipolar pnp transistors, metal-oxide semiconductor (MOS) transistors, including but not limited to MOSFETs, and heterojunction bipolar transistors (HBTs), as well as any other transistor devices, without departing from the spirit of the disclosed invention. Thus the transistor symbol shown in the Figures is to be understood to be used in the general sense and not as being limited as to the transistor type usable in the present invention.