MMIC amplifiers that are efficient and deliver high power output over a very broad frequency range are extremely desirable in applications ranging from microwave instruments to electronic warfare systems. In addition to reducing the number of devices required to produce a desired RF power level, they consume less current and reduce heat sinking requirements. However, achieving these goals presents significant design challenges, which is why there are currently few such devices available. Two devices, one in die form (the iT2007) and the other packaged (the iT2001P), achieve this performance over bandwidths as broad as 10 MHz to 22 GHz, with power output as high as 1 W and gain as high as 13 dB.
The iT2007 GaAs MMIC traveling-wave amplifier chip has the broadest bandwidth of the two, and can achieve a power-added efficiency (PAE) greater than 12 percent up to 12 GHz and greater than 10 percent up to 18 GHz. The iT2007 is typically biased at 40 percent IDSS for best combination of power and linearity. Power-added efficiency versus frequency at 1 dB gain compression is shown in Figure 1.
The standard iT2007 device covers 1 to 22 GHz. However, the input and output ports of the iT2007 are DC coupled (no internal DC blocking capacitor), which gives the designer the choice of extending the low end of the frequency coverage to 10 MHz with the use of external components. This could not have been achieved if the device was AC coupled and incorporated an internal blocking capacitor. It is currently the only available MMIC amplifier that can deliver power output of 29 dBm at such low frequencies while also providing such broadband frequency coverage.
To achieve the frequency extension, the designer employs external coupling capacitors at the RF input and output ports, and auxiliary gate pads are provided for the capacitors to be connected to ground in order to maintain low VSWR. Positive drain voltage (Vdd) can be provided by using an on-chip pad with a choke to ensure good matching over the bandwidth. The designer can optionally connect an external bias tee directly from the RF output pad, along with a large value of inductance in parallel with resistors to optimize power, gain and return loss.
The iT2007 provides saturated output power of 1 W up to 6 GHz and greater than 28 dBm up to 18 GHz. Gain is typically 7 dB ±1 dB up to 22 GHz. In a biased condition, DC power consumption can be as low as 2.7 W; its nominal DC bias condition is 9 VDC at 300 mA. In maximum output power condition, power dissipation increases to about 3.8 W. A plot of power output at 1 dB compression and saturated power output is shown in Figure 2.
The high efficiency of the iT2007 makes it appealing for use in the design of amplifiers with higher power outputs. Its low current consumption allows more devices to be combined in parallel before the limits of a typical heat sink are reached. For example, when operated in class AB, dissipation is very low in the standby condition, which increases reliability and operating life because of lower channel temperatures. As a result, it is possible to build a power amplifier of modest dimensions (thanks to a relatively small heat sink) that delivers nearly 4 W CW from 2 to 18 GHz using only four devices. The bandwidth limitation is mainly due to the broadband combining capability of the user.
The iT2007 also includes dynamic gain control over a range of 25 dB, which is useful in many applications, such as when the amplifier is used as a predriver, and the ability to vary gain makes it easier to accommodate the needs of succeeding amplification stages.
The device employs Ti-Pt-Au gate metallization, silicon nitride passivation and polyamide for scratch protection. The entire active area of the device is passivated as well as above the air bridges. This process makes the device very robust because the area is coated with an oxide layer that protects against contamination, increases electrical stability and makes it highly resistant to humidity.
The iT2001P is iTerra’s packaged amplifier, which covers 1 to 20 GHz and differs in several ways from the iT2007. It is not DC coupled, so its low frequency limit cannot be extended. It also does not contain variable gain control. However, the device still provides saturated output power of 1 W up to 6 GHz and greater than 27 dBm up to 18 GHz. Gain is 13 dB, nearly double that of the iT2007. Its optimum DC power consumption of 5.4 W is obtained in a biased condition, which delivers an optimum combination of output power and linearity. Table 1 lists the performance specifications for both the iT2007P and the iT2007 devices.
The device employs two cascaded devices and is fabricated using pHEMT technology, along with the same gate metallization, passivation and polyamide as the iT2007. The base of the package is composed of copper to minimize thermal resistance while also ensuring compatibility between materials. The feedthroughs are realized on a ceramic frame, which maintains the device’s broadband performance. The iT2001P is an excellent general-purpose amplifier, and is well suited to any application in which high power output over a very broad bandwidth is desired.
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