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The RF variable-gain amplifier (VGA) is a major building block in modern high-frequency communications devices. It is used in the transmitter (Tx) and the receiver (Rx) stages for a number of functions.

Different applications put different requirements on the VGA, and an integrated VGA is often either under-designed or over-designed for a variety of applications. It is difficult to find one VGA that will fit all the applications. This is why modular design is popular. The modular VGA design approach has a trade-off, as shown in the example below.

In the modular VGA, the amplifier block and gain-control (GC) block are clearly defined. The amplifier block is used only for power amplification, and the GC block is used only for adjusting the power level.

When each block is designed and tested successfully, the building block can be repeated several times to fit various requirements. For example, say each amplifier stage provides 10 dB gain, while each GC stage provides 20 dB control range. If the application requires a 20 dB gain VGA with a 40 dB control range, it is easy to use two amplifier blocks and two GC blocks to meet the requirement.

The main trade-off is the permutation of the amplifier and gain control stages. In the Tx, a VGA is typically used before the PA. The Tx VGA requires a high-output 1 dB compression point and the amplifier stage should be behind the GC stage. Because the GC stage attenuates the input RF signals, it essentially increases the input 1 dB compression point of the amplifier by the attenuation level. On the other hand, the VGA is also used as a low-noise amplifier (LNA) in Rx. The noise factor (NF) of the overall RF system strongly depends on the NF of the LNA. Therefore, the gain stage should be in front of the attenuator stage.

A 20 dB gain VGA with 40 dB control range illustrates the design trade-off. In Figure 1, the two amplifier blocks come after the two GC blocks. In Figure 2, the order is reversed.

The amplifier block and the GC block are the same. The only difference is the arrangement of the blocks. The RF performance, however, is different. The comparison is in Table 1. It should be observed that the configuration in Figure 1 is better suited for the Tx, while the arrangement in Figure 2 is well-suited for the Rx.

Variable-gain amplifier differences on NF and IIP₃ are already obvious. When the gain control kicks in, the differences of NF and IIP₃ between the two configurations are even greater.

One modular VGA design example at 2.45 GHz is presented. The Tx requires a PA driver with maximum gain greater than 15 dB and a dynamic range of 40 dB. The amplifier block and GC block are designed separately before they are cascaded together.

The GC block or the voltage-controlled amplifier (VCA)^{1, 2} can be designed in a number of ways. The three main configurations for a VCA are the constant-impedance approach6, the resistive-line approach⁷ and the π-configured attenuator approach^4,5.

The variable resistance approach can be done with either a metal semiconductor field-effect transistor (MESFET) or a positive intrinsic negative (PIN) diode. Each design approach focuses on different design parameters. In this particular design, the resistive line approach using a PIN diode is presented.

The building blocks of the resistive line approach are a quarter-wave transmission line (TL) and a shunt resistor (a PIN diode in this design).

The quarter-wave transformer is a popular way to transform impedance. The design equation is simple:

The impedance of one end (Z_in) of the transformer is inversely proportional to the impedance at the other end (Z_out). For example, if Z_in = 1Ω and the TL has a characteristic impedance (Z_O) of 80Ω, Z_out will be 6400Ω, which is a high impedance that will reflect most of the incoming RF signal. The typical values of Z_O of the TL range from 50Ω to 90Ω.

To obtain variable attenuation using this approach, the fixed value Z_in is replaced with a variable-resistance PIN diode. A typical shunt-configured PIN diode can archive maximum attenuation about 20 to 30 dB at 2.45 GHz. In this particular design, a dynamic range of 50 dB is desired. Any number of variable resistors can be used, depending on the desired range of attenuation. Four shunt diodes are used in this design, as shown in Figure 3. The middle two shunt diodes provide the bulk of the attenuation by reflecting incoming RF signals.

However, a reflective VCA is not desirable in most applications, especially in high-power Tx applications. The reflected RF energy needs to be absorbed inside the VCA to provide low return loss. This will require its internal resistance to be different from its external resistance.

The designer can vary the values of R₁, Z_O1, Z_O2, θ₁ and θ₂ to trade off size, dynamic range and input/output return loss. In this design, the following values are used: R₁ = 50Ω, Z_O1 = 70Ω Z_O2 = 95Ω, θ1 = θ2 = 90°. Note that the use of a fixed resistor in series with a PIN diode at the input and output of the network results in lowered distortion at maximum attenuation because the incoming RF signal dissipates in a passive device, rather than a diode.

A low-cost, plastic-package PIN diode is used. Parasitic diode elements (package inductance and capacitance, junction capacitance) are significant at 2.45 GHz. The reduction of the effects of package parasitic inductance can turn an ordinary design into a high-performance circuit. The parasitic inductance cancellation scheme does not have to be complicated. The main contributors are package leads, bond wires and via-hole inductance. The inductance can be canceled by simply using a shortened radial microstrip stub (capacitive impedance), in place of the via holes, to resonate out the parasitic inductance. The dimension of the radial stub is determined during simulation. The schematic, layout and test result is shown in Figures 4, 5 and 6, respectively.

As shown in Figure 6, the attenuation range of the VCA is from -2 dB to -50 dB with good port matching for all the attenuation range.

The gain block is based on Agilent's pseudomorphic high electron mobility transistor (PHEMT). PHEMTs were chosen because they are known for their high OIP₃ and ultra-low NF. The high OIP₃ is desirable in this design.

The transistor is biased with V_ds=4 V DC and I_ds=60 mA for high OIP₃. Active bias circuitry is used to stabilize the bias point. The actual design is simple with feedback technique⁸. Feedback is added at the source to improve the stability and to reduce the matching complexity. The feedback is performed with two pieces of TLs at the source. Then simple LC matching is provided at the input and output to further improve matching performance. The design is shown in Figure 7. The test results are shown in Table 2.

The PA driver requires maximum gain greater than 15 dB and dynamic range of 40 dB. After comparing the requirement with the test results of each building block, it should be easy to conclude that one GC block and two amplifier blocks are needed for the complete VGA.

The modular VGA approach offers maximum flexibility and optimum performance for different applications.

Figure 1 and 2 component. parameters:

CG block:

Insertion loss = 2 dB

GC = 20 dB

OIP₃ = 25 dBm

Each gain block:

OIP₃ = 30 dBm

Gain = 10 dB

NF = 1.5 dB

1. “Application of PIN diodes,” Agilent Technologies Application Note AN922.

2. “Design with PIN diodes,” Alpha Industries Application Note APN1002.

3. Jack Lepoff and Raymond Waugh, “The PIN Diode - A Tutorial,” Proceedings of the RF EXPO WEST, Santa Clara, California, February 1991.

4. Raymond W. Waugh, “A Low Cost Surface Mount PIN Diode Pi Attenuator,” Microwave Journal, Vol. 35, No. 5, May 1992.

5. Louis Fan Fei, “A Low-Cost, Compact, Pi-Configured PIN Diode VCA,” Applied Microwave & Wireless, November 2000

6. Louis Fan Fei, Raymond W. Waugh, “A VCA Using the Constant Impedance Approach,” Applied Microwave & Wireless, January 2001.

7. Louis Fan Fei, Raymond W. Waugh, “A VCA Using Resistive Line Approach with Parasitic Inductance Cancellation Circuit,” Applied Microwave & Wireless, January 2001

8. Guillermo Gonzalez, “Microwave Transistor Amplifiers,” John Wiley & Sons, Inc., 1998.

Louis Fan Fei is a RF design engineer at Agere Systems (formerly Lucent Technologies Microelectronics group) in Atlanta. He has been with Agere since July 1998. He received his B.E.E and M.S.E.E degrees from Georgia Tech in 1996 and 1998. He also worked as an intern for Hewlett Packard's Colorado Springs division in the summer of 1997. He can be reached at ffei@agere.com.

Ming-Ju Ho received his M.S. and Ph.D. degrees in electrical engineering from the Georgia Institute of Technology in 1991 and 1996. Currently, he is a distinguished member of technical staff at Agere Systems' wireless communications and networking division, Atlanta He can be reached at 404-253-1844, or hymjho@agere.com.

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