Linear amplifiers with high gain serve a multitude of uses in wireless communications and other applications. Gain is generally applied in a system to overcome signal losses from other receiver components, such as power dividers, antenna couplers, and filters. Often, several stages of amplification are needed to compensate for passive-component receiver losses, although the new model MAR-8ASM monolithic amplifier from Mini-Circuits (Brooklyn, NY) provides more than 30-dB small-signal gain from a single surface-mount package, enabling engineers to save a few stages and simplify their designs in the process.
The MAR-8ASM (Fig. 1) is a monolithic microwave integrated circuit (MMIC) that incorporates reliable heterojunction-bipolar-transistor (HBT) devices fabricated with InGaP GaAs technology. InGaP transistors are formed on GaAs substrates with GaAs collectors and bases and InGaP emitters; material processing is performed by means of metal-organic chemical vapor deposition (MOCVD).
Compared to GaAs MESFET devices commonly used in wireless applications, HBTs are easier to design with and use. Because they are vertical devices, HBTs can be made very compact, with extremely high yields from a single wafer. HBTs require only a single positive voltage supply for operation, unlike GaAs MESFET devices which require both positive and negative voltage supplies. HBTs consume no power when they are turned off, eliminating the need for switch components used with other types of transistors.
In recent years, controversy has developed over the use of AlGaAs emitters on GaAs for HBTs, which have been found to be unstable under certain conditions. InGaP materials exhibit lower initial defect densities compared to AlGaAs, resulting in higher yields per wafer and much longer operating lifetimes, on the order to 10 to 20 times longer. InGaP devices feature a much lower conduction-band offset between the InGaP in the emitter and the GaAs in the base, resulting in excellent uniformity of current gain with changes in applied current and operating temperature.
All of these factors account for the high power density, high efficiency, and superior linearity of InGaP HBTs compared to AlGaAs HBTs or even GaAs MESFETs at similar frequencies. By using thick interconnection metals in the InGaP HBT fabrication process, it is possible to produce devices that are small in size but exhibit excellent thermal properties. For example, the MAR-8ASM features thermal resistance of typically 140°C/W. With a maximum rated junction temperature of +150°C, the rugged amplifier can easily withstand input power levels to +13 dBm without damage or degradation of operating lifetime (more than 20 years).
The MAR-8ASM InGaP HBT amplifier is a versatile component, with usable bandwidth of DC to 1000 MHz and better than 20 dB gain across that full range (Fig. 2). It achieves typical gain of 31.5 dB at 100 MHz, dropping to a still-impressive 25 dB typical gain at 1000 MHz. Although designed nominally as a small-signal gain stage, the amplifier exhibits +12.5-dBm typical output power at 1-dB compression (the output level at which the amplifier's "power" gain is 1-dB less than the gain under small-signal conditions) at 1000 MHz. It boasts an impressive dynamic range, using the limits of noise figure at one end and third-order intercept point at the other end. By this definition, the amplifier's typical noise figure of 3.1 dB at 1000 MHz and typical third-order intercept point of +25 dBm at 1000 MHz yield a nearly 22-dB linear dynamic range.
The stable MAR-8ASM amplifier is a "ready-to-use" device, matched at input and output ports for 50-Ω circuits. Two of its four package pins are ground connections; the output connection doubles as the DC bias connection. As expected, the amplifier features low typical input and output VSWRs, of 1.40:1 and 1.80:1, respectively, to minimize mismatches between source and load connections. The MAR-8ASM requires few additional external components: a blocking capacitor on the input pin, a blocking capacitor on the output pin, a bias resistor on the RF output port, a bypass capacitor on the output port, and optional RF choke for the output port.
The amp is rated for operating temperatures from −40 to +85°C. It is designed for bias voltages of +3.2 to +4.2 VDC, and consumes 36 mA. It includes internal protection against power-supply transients, and is a form-fit replacement for the earlier MAR-8SM and MSA-0886 amplifiers. P&A: $1.19 each (30 qty.). Mini-Circuits, P.O. Box 350166, Brooklyn, NY 11235-0003; (718) 934-4500, FAX: (718) 332-4661, Internet: www.minicircuits.com.
