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RF Components: Power Amplifiers

100 Watt Performance on a 12 Watt Budget

Please note, the technology described in the following article applies to our complete line of Stealth Microwave linearized amplifiers.

Due to advances in digital compression technology in cable TV systems, renewed interest in the USA and abroad has increased in the frequency range of 2.50 to 2.686 GHz. Multichannel Multipoint Distribution Systems (MMDS) which operate in this band are upgrading from analog to digital TV service. These new formats allow several television channels to be transmitted versus a single analog channel. These multichannel configurations require ultra linear RF subsystems so as not to generate unwanted intermodulation products (IMD). The RF/Microwave high power amplifier (HPA) is typically the leading cause of this type of distortion and therefore dictates the linearity of the entire transmitter.

A common method used to minimize IMDs is to "back-off" the output power of the HPA until the desired carrier to noise ratio (C/I) is achieved. The relationship between IMDs, intercept point and carrier levels is expressed as follows:

Single Carrier Level + (1/2 x C/I3) = Output Intercept Point (OIP3)

With a single carrier level of 32.5 dBm and a desired IMD level of 55 dBc, an amplifier with an intercept point of 60 dBm would be needed. Since the P1dB of an HPA is typically 10 dB below the OIP3, an HPA with a P1dB of 50 dBm would be required. This is approximately a 100 Watt amplifier. If transistors that operate in the 2.6 GHz range are used, such an amplifier would consume approximately 30 amps, be quite large in size, and have a high price tag.

Stealth Microwave has developed the SM2527-41L which has the same performance as a backed off 100W unit , but consumes only 5.5 amps @ 12 Volts in a housing that measures 7.5 L x 4.0 W x 2.0 H inches.

Although the P1dB of this amplifier is 41 dBm (12 Watts), the intercept point measures greater than 60 dBm, resulting in the similar IMD performance as the 100 Watt amplifier. See Figure 1.

Figure 1


The SM2527-41L uses the latest GaAs FET technology along with a built in predistortion linearizer which produces the resulting IMD performance. The predistorter uses phase and amplitude correction to reduce the IMDs created in the HPA. This in turn pushes the theoretical intercept point higher. Additional circuitry allows the linear gain of the unit to only change ± 0.15 dB over a temperature range of 0o to 55o C.

The unit is specifically designed for the MMDS market. Many MMDS transmitters use feedforward techniques which are suitable for base station applications, however, for remote locations, size and power consumption are critical, making the SM2527-41L a better solution.

The amplifier has several built in features, including a single DC supply of 12 Volts, over voltage protection, reverse voltage protection, and thermal protection which will turn the unit off if the case temperature exceeds 60o C, and will automatically turn on once the temperature has fallen back to normal levels. Logic On/Off control is also standard, allowing the end user to turn the unit on or off with a TTL signal. Two optional features are available: Forward/Reverse Power Detection which allows the end user to monitor output power and the VSWR of the transmit antenna and an RF sample port for analog monitoring of the output spectrum.

The price of the amplifier with all of the built in features including the linearizer, is approximately 1/3 the price of a standard 100 Watt amplifier. Delivery is in stock to 10 weeks ARO.