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The E-pHEMT amplifier represents something of a new breed of amplifier in that it exhibits an extremely low noise figure, yet also delivers high output levels. In general, the choice from an amplifier is one or the other, low noise or high output power—and adding too much gain can lead to conditions of instability (or oscillation) that cannot be overcome even through careful impedance matching. The combination of low noise and high output power would indicate that the model PMA2-33LN+ LNA is well suited for processing digitally modulated or even pulsed signals without degradation of signal linearity. Coupled with its noise figure, it also offers relatively high output power at 1-dB compression (Fig. 4), of typically +17.3 dBm at 400 MHz, +17.8 dBm at 900 MHz, +17.7 dBm at 1.5 GHz, +18.1 dBm at 2.0 GHz, and +17.5 dBm at 3.0 GHz. Similarly, the output IP3 is typically +30.0 dBm at 400 MHz, +34.2 dBm at 900 MHz, +36.1 dBm at 1.5 GHz, +37.4 dBm at 2.0 GHz, and +40.0 dBm at 3.0 GHz.

4. Model PMA2-33LN+ LNA delivers respectable output performance, making it suitable for complex digital modulation formats.

The MMIC LNA has measured input return loss of typically 10.0 dB at 400 MHz, 12.4 dB at 900 MHz, 12.5 dB at 1.5 GHz, 13.3 dB at 2.0 GHz, and 20.0 dB at 3.0 GHz. The measured output return loss is typically 18.0 dB at 400 MHz, 25.6 dB at 900 MHz, 22.7 dB at 1.5 GHz, 22.4 dB at 2.0 GHz, and 20.0 dB at 3.0 GHz.

The new LNA is well suited for receiver applications in base-station infrastructure, portable wireless equipment, Long Term Evolution (LTE), Global Positioning Systems (GPSs), GSM telephones, and airborne radar systems. It achieves considerably lower noise figure than other amplifiers in its frequency range, notably those in a low-cost surface-mount housing. Still, there is no discounting the importance of achieving a good input and output impedance match between a system’s other components, such as the antenna and frequency mixer, and the LNA. Proper impedance matches can ensure reaching the noise figures at each frequency cited by the product data sheet.

Where does all the noise come from? As found in active devices such as transistors and amplifiers, noise stems from a number of different sources. One of these noise components, thermal noise, occurs because of vibrations between conduction electrons and holes at a finite temperature. Some of these vibrations have spectral content and may contribute noise over a specific band of interest, although the noise spectrum produced by thermal noise is nearly uniform across RF and microwave frequencies. The power from a thermal source delivering into an impedance matched load is the well-known relationship kTB (in W), where k = Boltzmann’s constant (1.38 x 10-23 J/K); T = the temperature (in K); and B is = system’s noise bandwidth. The power is independent of the source impedance and is directly proportional to the bandwidth, so that a situation with twice the bandwidth would allow twice the noise power to be delivered to the load.

Another noise component is shot noise, which arises from the quantized nature of current flow. Various other noise components follow this quantized behavior.  The noise floor for an amplifier is also related to the bandwidth, and can be found from the relationship:

Noise floor = -174 dBm + noise figure +10log(bandwidth)

where the noise figure and bandwidth refer to the LNA under investigation.

In any communications system, the signal-to-noise ratio (SNR) is an important parameter in establishing coherent communications, and it is generally at the antenna and the receiver LNA where the lower end of the SNR is established. In an analog system, deterioration of the receiver front-end noise figure will result in a loss of signal sensitivity. In a digital system, the result will be a degradation in system bit-error-rate (BER) performance. In any case, the front-end LNA is a key component in many receivers and the availability of an amplifier with a lower noise figure helps to improve sensitivity and the overall SNR for systems in which the frequency bandwidth is a fit.

In fact, a digital system’s BER performance can be much more reliant on receiver SNR performance than a comparable analog radio. When systems operate with digital modulation, the situation is similar, and unwanted noise can be highly disruptive to system performance. The model PMA2-33LN+ LNA brings a new dimension to some of these systems because it combines such a low noise figure with moderately good output characteristics, such as output power at 1-dB compression and third-order-intercept (IP3) point. This combination makes it a nice fit for modern digital communications systems where  complex digitally modulated signals must be received and preserved.

Mini-Circuits, P.O. Box 350166, Brooklyn, NY 11235-0003; (718) 934-4500, FAX: (718) 332-4661, www.minicircuits.com.

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