A Simple VVA RFIC Design with a Focus on Repeatability and Stability

Technical Feature

A Simple VVA RFIC Design with a Focus on Repeatability and Stability

A novel voltage variable attenuator (VVA) integrated circuit reduces the sensitivity of the transfer curves to temperature and process variations. The new solution is achieved without compromising the simplicity of a single positive voltage control design and a large attenuation range. This article describes FET-based VVA characteristics, design trade-offs and common topologies. Following a sensitivity analysis discussion, measured results are presented to show the lower sensitivity of the suggested circuit over a traditional design. Measurements of a large quantity of VVAs, based on the new topology, confirm the robustness of the solution.

Yair Shemesh
Alpha Industries
Woburn, MA

Voltage variable attenuators are important control elements, widely used for automatic gain control both in receive and transmit chains.^1,2 When used in handsets, their overall cost and size are of paramount importance. A single positive voltage control scheme and a simple circuit design help in achieving this goal. When operated as part of an open loop gain control, a linear transfer curve simplifies the design of the chain's gain. The unit-to-unit transfer curve repeatability and its stability over temperature are key factors in the ability to design and predict the overall system gain. In receive chains, the VVA is used to control the signal that enters the mixer. It must have a high enough third-order intercept point (IP3) by itself in order not to degrade the overall power linearity. FET VVA topologies such as TEE, PI and bridged TEE are often used,^2,3 each with its own merits and problems.

This article reviews the trade-offs among VVA features, and focuses on the issue of stability and repeatability. The reasons for high parameter sensitivities are identified, and based on these identifications, a new VVA topology is suggested. Compared to traditional topologies, the new configuration significantly reduces the sensitivity to process and temperature variations, without sacrificing simplicity, single positive voltage control approach, attenuation range and transfer curve linearity. Several VVAs have been designed and measured based on the new topology. The results clearly show the predicted stability and repeatability improvements.

VVA Design Parameters

The VVA performance is a trade-off among contradicting requirements, as shown in Table 1 . Several FET-based VVA topologies are commonly used. An emphasis on a specific requirement affects the topology selection. For example, the bridged-TEE has superior return losses and a very linear transfer curve, at the cost of complex circuitry and sensitivity to process variations and temperature changes.

The large volume handset production in recent years has emphasized the need for repeatability when selecting a VVA topology. In FET-based VVAs, the device is used as a variable resistor at zero drain source voltage (V_ds ). The device parameters vary over temperature, across the wafer and from wafer-to-wafer. Since channel resistance changes sharply for gate-source voltage (V_gs ) close to the pinch-off voltage (V_p ), the stability and repeatability of the transfer curve is governed mostly by V_p . Other FET parameters, such as the ON resistance (R_on ), have a second-order influence on the VVA transfer curve. These parameters are usually correlated to V_p changes in a predicted manner.

In the commonly used topology of a single positive voltage-controlled TEE VVA, shown in Figure 1 , the control voltage V_c is translated into two gate-source voltages.

For the series devices it is given by

V_gsser = Div · V_c - V_center (1)

and for the shunt device, by

V_gssh = -Div · V_c (2)

where

Div = resistor divider ratio R1/(R1+R2)
V_center = fixed DC voltage applied to the series path at the TEE junction

Proper selection of Div and V_center for a given V_p allows a roughly linear attenuation dependence on V_c .

At V_c = V_{top knee} , the shunt device start conducting (V_gssh = V_p ), while at Vc = V_{bottom knee} , the series devices start conducting (V_gsser = V_p ). A V_p variation will translate, according to Equations 1 and 2, into a shift of the top and bottom knees. A V_p change by an amount of DV_p , shifts the nominal point N by a voltage DV_c , to the point R, where

(3)

The result, as seen in the ideal curve shown in Figure 2 , is that at V_c = V_top knee, the attenuation will drop by an amount D[dB] where

D = S · DV_c (4)

S is the transfer curve slope given by DA/DV_eff . Therefore, minimizing the attenuation variation D is possible by designing a VVA with small attenuation range DA and a wide control voltage range DV_eff . A 12 dB range VVA for a 5 V control voltage was demonstrated by Boglione and Pavio.⁴

Unfortunately, an attenuation range of more than 30 dB is a more common requirement. Cascading several low attenuation range VVAs is possible for an overall wide attenuation range; however, the overall transfer curve sensitivity will increase as well.

Applying two separate positive controls for the series and shunt devices, and setting V_center with a pinch-off follower, can reduce the sensitivity to V_p variations. This can be done at the cost of additional circuitry, based on an operational amplifier, which generates the proper two control voltages.⁴ In many applications, the need for a simple, low cost, low size VVA solution rules out this option.

A TEE VVA Sensitivity Analysis

In an ideal TEE attenuator, as shown in Figure 3 , the resistors R_sh and R_ser should satisfy the matching and attenuation conditions given in Equations 5 and 6, respectively.

where

A = attenuation in dB

Since for V_gs < V_p , the FET resistance is much higher than 50 W, the series devices should be shunted by a resistor (r_ser ) that limits the overall series arm resistance to 50 W. For a maximum attenuation of 30 to 40 dB, r_ser is of the order of 60 to 110 W, depending on the device's periphery and characteristics.

Figure 4 shows the required R_ser and R_sh for an attenuation range of 0 to 40 dB, together with the series FET resistance, r_net , for the case of r_ser = 110 W in shunt with the device.

Figure 5 shows the deviation from the nominal attenuation of the transfer curve, caused by a change of 10 percent in the shunt device (curve a) and 10 percent in the series device, with the resistor in shunt r_ser = 100 W (curve b) and r_ser = 60 W (curve c).

The transfer curve shape is more sensitive to the shunt device resistance variation than to the series device variation, as the shunting resistor r_ser decreases.

Though the deviation in dB from the nominal transfer curve is smaller for higher V_c (lower attenuation levels), the relative deviation dA/A is larger at low attenuation levels, with a maximum at A = 0.

The New Circuit

One possible approach⁴ for minimizing the variations is to apply a V_p follower voltage (-V_p ) to V_center .

In this case, the R_ser deviation from the nominal R_ser due to the V_p changes is ideally eliminated since

R_ser = f(V_gsser - V_p ) = f(Div · V_c ) (7)

Hence, stabilization is achieved for the bottom knee position. The disadvantage of this approach is that the top knee, affected mostly by R_sh , is very sensitive to V_p variations, especially for wide attenuation range, steep slope VVAs. In the newly proposed circuit, shown in Figure 6 , a V_p follower applies a -V_p voltage to the drain and source of the shunt device. The sensitivity of the shunt arm is eliminated since

R_sh = f(V_gssh - V_p ) = f{[Div · V_c - (-V_p )] - V_p } = f(Div · V_c ) (8)

Hence, stabilization is achieved for the top knee position. As previously explained, the series device resistivity mainly affects the bottom knee, which is less critical to the transfer curve shape. Selection of shunting r_ser resistors low and close to 50 W reduces attenuator sensitivity at the high attenuation levels. Such selection requires high OFF impedance, that is, small gate periphery. On the other hand, a small gate periphery for the series devices increases the loss at the minimum attenuation state. This shunting r_ser resistor is finally set as a compromise between these two requirements.

The conflict of these requirements is easy to resolve for low attenuation range VVAs and for devices with low normalized drain-to-source capacitance (C_ds ).

Results

Several VVAs have been designed, fabricated and measured based on both old and new topologies. Measured transfer curves of the two types of 900 MHz VVAs built to similar specifications are shown in Figure 7 .

While the old topology (design A) has a maximum attenuation at low control voltage, the new one (design B) has maximum attenuation at high control voltage (inverted slope). Other measured parameters are similar for both designs.

V_p variations are caused by process variations and temperature changes.V_p is also the major cause for transfer curve changes. Therefore, transfer curve stability over temperature is an indicator for process variations.

Figure 8 shows the measured change in the transfer curve when the temperature is raised from +20 to +85°C for the two designs.

It is clearly seen that the new design B deviation is smaller by a factor of 4, compared to design A. Furthermore, since design A maximum deviation is at the low attenuation point of Vc = 3.5 V, the relative deviation dA/A is even worse compared to the dA/A of design B.

VVAs based on the new topology with 3 V control and steeper slope have also been designed and built in large quantities. Measurements confirm the robustness of the solution.

Conclusion

A new VVA topology has been suggested, offering an improvement in stability and repeatability over temperature and process variations.

Acknowledgment

The author wishes to acknowledge the support of D. Johnson and the lab assistance of J. Bedrosian.

References
1. R. Goyal, Monolithic Microwave Integrated Circuits Technology and Design , Artech House Inc., 1989.
2. I. Bahl and P. Bhartia, Microwave Solid State Circuits Design , John Wiley & Sons, New York, 1988.
3. B. Maoz, "A Novel, Linear Voltage Variable MMIC Attenuator," IEEE Transactions on Microwave Theory and Techniques , Vol. 38, No. 11, November 1990, pp. 1675-1683.
4. L. Boglione and R. Pavio, "Temperature and Process Insensitive Circuit Design of Voltage Variable Attenuator IC for Cellular Band Applications," IEEE Microwave and Guided Wave Letters , Vol. 10, No. 7, July 2000, pp. 279-281.

Yair Shemesh received his BScEE and MScEE degrees from the Technion, Haifa, Israel, and has worked for Rafael in Israel and Alpha Industries in the US. His specialties are MW devices, modeling, PAs, RF to MMW MMICs, modules and RF front-ends. He is currently a senior principal engineer at Alpha and can be reached via e-mail at yshemesh@alphaind.com.