Click here for the enhanced PDF version of this article

The high linearity variable gain amplifier (VGA) VG025 is designed for use in transmitter and receiver automatic gain-control (AGC) circuits, and as variable gain blocks following low-noise amplifier stages in high dynamic range receiver front-ends. The amplifier integrates a high-performance amplifier and an analog attenuator within a single surface-mount package and is optimized for various current and next-generation wireless applications that demand high dynamic range and small size.

This high dynamic range variable-gain amplifier is capable of achieving an analog attenuation range of up to 21 dB. Alternative solutions such as digital attenuators require much more complex driving circuitry and do not offer the precision that an analog attenuator offers. Discrete configurations can also be undesirable because of the large number of components required and the large amount of space needed on a PC board.

The amplifier offers 1 dB compression point (P1dB) of +21 dBm and an output third-order intercept point (OIP3) of +40 dBm. A key feature for the device is in its ability to maintain constant OIP3 and P1dB performance over the entire gain control range (Figure 1). This allows the VGA to be used in an RF chain without worrying about system dynamic range degradation with varying attenuation levels. FET-based attenuation circuitry will rapidly degrade system linearity with increasing attenuation levels.

This amplifier can be adjusted to operate anywhere in the 50 MHz to 2200 MHz frequency range using several off-chip components. Consequently, it can be used in both the RF and IF sections of a radio or transceiver. For frequencies of less than 1 GHz, the gain is approximately 15 dB with the gain control range being about 20 dB.

Since VG025 is a multichip module using a GaAs MESFET process for the amplifier and PIN diode for the attenuator, the variation in gain over temperature is minimal. The gain variation range is controlled by a single 0 V to +4.5 V analog input requiring up to 28 mA for the full attenuation range. It operates from a +5 V supply voltage with a typical bias current of 150 mA. Capable of operating over the -40 °C to +85 °C temperature range, the VGA achieves an MTTF greater than 1000 years at a mounting temperature of +85 °C.

Because the VGA's attenuator can be driven in a variety of ways for different applications, this article discusses a few applications.

An inexpensive dual-operational amplifier (op-amp) circuit allows the VG025 amplifier to operate as an RF chain temperature-compensation stage that features independent and adjustable offset and slope control, while providing lower losses and larger slopes than can be achieved with thermistor networks or thermal compensation pads. It can be set to account for some amplifier gain variation and the gain change with temperature for amplifier chains of up to eight amplifiers. Each amplifier in a chain will vary over the commercial temperature range (-40 °C to 85 °C) by about ±0.5 dB. The VG025 can automatically compensate the gain fluctuations over temperature with a properly configured driver circuit shown in Figure 2. Only two resistors need to be changed for different chain compensation. Electrically erasable potentiometers (EEPOTs) can be used to tune and align different IF and RF units for the variation of other components in the RF chain. To maintain a constant gain over temperature for the system, calibration steps only need to be performed at two temperature levels, typically at room and cold temperature.

The constant current of (+5 Vcc - V_ref)/R_offset is set at room temperature. The reference voltage is a simple voltage divider set to +5 Vcc/2. When the drive loop is locked, the offset current flows through the attenuator and into the op-amp U2a output. The maximum attenuator drive current is set by the maximum current sinking of the op-amp U2a and can be limited by increasing R6.

Temperature compensation uses the negative temperature coefficient change in a biased P-N junction voltage. The anode voltage is multiplied by the op-amp gain to equal the reference voltage at room temperature. It can be adjusted by changing the diode bias with R1 or the op-amp U2b gain with R2. For lower temperatures, the voltage change (ΔV/Rslope) gives additional attenuator current and decreases the gain. For higher temperatures, part of the offset current flows through Rslope and is sinked by op-amp U2b, reducing the current through the attenuator and increasing the gain.

For the temperature-compensation circuit, a diode junction voltage, which varies with temperature, varies the amount of current flowing in the control pin. The change in voltage gives a linear attenuation change with temperature with the amount depending on the value of the temperature slope resistor R_slope. A fixed offset attenuation (and current) can be sourced using the power rail voltage, +5 V and the voltage drop to a fixed-voltage reference, made by R4 and R5, through the offset resistor, R_offset. The offset current at room temperature is needed to decrease the attenuation at higher temperatures. The offset current also inserts some loss, which gives better return losses at room temperature. In a voltage-variable gain application, as a control voltage is applied to the base of the external NPN current control transistor, the transistor turns on and allows current flow. As current is applied, the resistance of the attenuator decreases and shunts RF energy to ground.

Figure 3 shows the VG025 expected gain variation over temperature at 240 MHz for different settings of R_slope and R_offset. The positive gain slope of the VG025 with increasing temperature offsets the expected decrease in system gain due to the other components used in the chain lineup. The R_offset value should be adjusted at room temperature to attain the targeted system nominal gain. When the system is set at a different temperature level (hot or cold), the R_slope value can be adjusted so that the same gain level is achieved. With the assumption that amplifiers and mixers will vary in gain at a constant gain/deg slope, this driver circuitry along with the VG025 should offer a constant system gain value over the operating temperature range.

A single op-amp circuit can be used as a voltage variable driver, providing stable gain operation over temperature with the use of the VG025. Figure 4 shows the circuitry required to give a temperature-compensated and semi-linearized attenuation curve. The attenuation range varies with a control voltage from 0 to +4.5 V controlling the attenuator current through the collector current of Q1. Pin 1 is biased from the same +5 Vcc, which biases the amplifier. It is sufficiently decoupled by L1 and C3 for the attenuator and L2 and C4 for the amplifier. Temperature compensation is achieved by the variation of the current limiting resistor network of R1, R3, R4 and thermistor R2.

With a simple NPN driver, the VG025 amplifier is suitable for automatic gain control (AGC) circuits in transmitters and receivers. This is a simple circuit that uses a minimal number of external components for biasing the amplifier and driving the attenuator. The gain variation starts occurring when the transistor Q1 turns on. As V_agc is increased, it creates a larger emitter voltage in the NPN transistor, which in turn, sources more current into the VG025 attenuation circuitry. The collector current of the transistor directly biases the attenuator while the 120 ΩR1 limits the current and sets the V_agc range from 0.7 V to +5 V for full attenuation (Figure 5).

Various ways of implementing the analog attenuator in the VG025 amplifier have been presented. Lower-cost alternatives involving variable attenuators have been described. The combination of attenuation circuitry along with a high linearity amplifier in a small package gives designers a desirable component for use in highly integrated system designs.

The authors would like to thank Rob VonDeisenroth and John Bellantoni for providing the measured performance data.

Tuan Nguyen is a director of marketing for WJ Communications Inc. in San Jose, Calif. He has been with WJ since 1998, and has worked with MESFET, HFET and HBT semiconductor components. He received his BSEE and MSEE degrees from the University of Illinois at Urbana-Champaign.