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_dss) of less than 10 µA at room temperature.
2. Drain current (I_d) of approximately 0 at a gate-source voltage (V_gs) of 0.
3. Quiescent drain current (I_dq) of less than 30 mA in code-division-multiple-access (CDMA) communications applications.
4. Superior output power (P_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 (OIP₃) 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 operation from a +5-VDC supply with current consumption of less than 65 mA.

The ATF-54143 from Agilent Technologies (San Jose, CA) was selected as the active device for the 100-to-500-MHz LNA. The ATF-54143 is one of a family of high-dynamic-range, low-noise enhancement-mode PHEMT discrete transistors designed for use in low-cost commercial applications in the VHF through 6 GHz frequency range. It is housed in a four-lead SC-70 (SOT-343) surface-mount plastic package and operates from a single regulated supply. If an active bias is desirable for repeatability of the bias setting—particularly desirable in high-volume production—the ATF-54143 requires only the addition of a single PNP bipolar junction transistor. Compared to amplifiers using depletion mode devices, the E-pHEMT design has a lower part count and a more compact layout. Besides having a very low typical noise figure (0.5 dB) at 2 GHz, the ATF-54143 is specified at 2 GHz and +3-VDC bias to provides a +36 dBm output third-order intercept point at 60 mA drain current. A data sheet for this device may be downloaded from: http://literature.agilent.com/litweb/pdf/5989-0034EN.pdf

Using the Advanced Design System (ADS) suite of computer-aided-engineering (CAE) software simulation and analysis tools from Agilent/EEsof (Santa Rosa, CA), the amplifier circuit can be simulated in both linear and nonlinear modes of operation. For the linear analysis, transistors can be modeled with a two-port S-parameter file using the Touchstone format. More information about Agilent electronic-design-automation (EDA) software may be found at: http://www.agilent.com/eesof-eda .The appropriate ATF54143.s2p file can be downloaded from the Agilent Wireless Design Center website:

http://www.semiconductor.agilent.com (type ATF-54143 in the Quick Search at the top of the page. Under Search Results click on the underlined ATF-54143. Scroll down to the S-parameters listing for 60 mA).

For the nonlinear analysis, a harmonic−balance (HB) simulation was used. The HB simulation was preferred over other nonlinear methods because it is computationally fast, handles both distributed and lumped-element circuitry, and can easily include higher-order harmonics and intermodulation products. The HB approach was used for the simulation of the 1-dB compression point (P_-1dB) and OIP₃.

Although this nonlinear transistor model closely predicts the DC and small-signal behavior (including noise), it does not correctly predict the intercept point. To properly model the exceptionally high linearity of the E-pHEMT transistor, a better model was required.

Besides providing information regarding gain, P_-1dB, noise figure, and input and output return loss, the simulation provides very important information regarding circuit stability. Unless a circuit is actually oscillating on the bench, it may be difficult to predict instabilities without actually presenting various VSWR loads at various phase angles to the amplifier. Calculating the Rollett stability factor (K) and generating stability circles are two methods made considerably easier with computer simulations. Simulated and measured results show the stability factor, K > 1 (Fig. 1), at the cost of reduced third-order intercept point and output power, through the use of a series resistor on the output.

To meet the goals for noise figure, intercept point and gain, the drain source current (I_ds) was chosen to be 60 mA. The characterization data in the device data sheet shows that 60 mA gives the best IP₃ combined with a very low minimum noise figure (F_min). Also, as shown in the data sheet, a 3-V drain-to-source voltage (V_ds) gives a slightly higher gain and easily allows the use of a regulated +5-VDC supply.

The use of a controlled amount of source inductance—usually only a few tenths of a nanohenry—can often be used to enhance LNA performance. This is effectively equivalent to increasing the source leads by approximately 0.025 inch or so. The effect can be easily modeled using an RF simulation tool such as ADS. The usual side effect of excessive source inductance is gain peaking at a high frequency and resultant oscillations.