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Although often associated with power amplifiers, E-pHEMT devices are also quite capable of supporting the design of efficient low-noise amplifiers.
Low-noise amplifiers (LNAS) for high-frequency applications have been based on GaAs metal-epitaxial-semiconductor field-effect-transistor (MESFET) and depletion-mode pseudomorphic-high-electron-mobility-transistor (pHEMT) technologies for some time. Semiconductor technologies such as GaAs heterojunction-bipolar-transistor (HBT) and the newer enhancement-mode pHEMT (E-pHEMT) technologies have been used primarily for power-amplifier (PA) applications. Still, the many outstanding characteristics of E-pHEMT devices also make them suitable for use in high-frequency LNAs capable of wide frequency coverage, including a 100-to-500-MHz LNA which will be revealed here.
For PAs, the performance of E-pHEMT technology offers many well-suited characteristics, including:
1. Saturated drain-source current(I^sub dss^) of less than 10 µA at room temperature.
2. Drain current (I^sub d^) of approximately 0 at a gate-source voltage (V^sub gs^) of 0.
3. Quiescent drain current (I^sub dq^) of less than 30 mA in code-division-multiple-access (CDMA) communications applications.
4. Superior output power (P^sub out^) and high efficiency with bias voltages of less than +3 VDC.
5. No thermal runaway effects (common to bipolar transistors).
6. No secondary breakdown mechanism.
7. The ability to survive under high mismatch conditions.
However, E-pHEMT technology can also provide a combination of high gain, low noise, and wide dynamic range in high-linearity LNA applications, such as intermediate-frequency (IF) amplifiers for commercial communication systems and preamplifiers for magnetic-resonance-imaging (MRI) systems. These types of applications have been made practical with the availability of low-cost plastic-packaged surface-mount E-pHEMT devices specifically designed for LNA applications. This article demonstrates why E-pHEMT technology can economically provide superior electrical performance in the VHF and UHF wireless communications bands commonly associated with other technologies, such as GaAs MESFETs and depletion-mode pHEMTs.
The goal of the design project is to produce a 100-to-500-MHz LNA with an output third-order intercept point (QIP^sub 3^) of +36 dBm, a noise figure below 2.0 dB, and gain of 20 dB with flat gain response. Resistive-capacitive (RC) feedback was used to provide good input and output impedance matching to the active device and to ensure unconditional stability. The matching was also required to reduce the overall stage gain to the specified 20 dB level and maintain flat gain across the 400-MHz operating bandwidth. The amplifier design specification includes...