A Highly Efficient 7 to 14 GHz Monolithic <br>Class-E Power Amplifier

Highly efficient monolithic power amplifiers, such as those operating under switching-mode conditions, that is class-E, -F, are suitable for the realization of large-scale phased-array radar systems.¹ In these systems, the number of radiators can easily reach 100,000 or more. Such large-scale arrays are ideal for applications in space or mobile ground systems. By utilizing highly efficient, single chip, T/R modules that are based on class-E power amplifier technology, significant savings in cost, weight and volume for these aforementioned systems can be attained.

The concept of switching-mode power amplifiers was originally introduced by Sokal² and subsequently by the work of Raab,³ who introduced comprehensive design methodologies for a broad family of RF switching-mode power amplifiers. However, the first monolithic version of a class-E switching-mode amplifier operating at 835 MHz was reported in 1994,⁴ followed by the work of several authors pushing the operating frequency of these circuits to ever-higher frequencies,^5–8 albeit over a rather limited frequency band.

Design Methodology

Class-E Load Network

In previous publications, methodologies for the design of monolithic class-E amplifiers having lumped⁷ or distributed⁸ output loads were described in detail. In these designs, the load topologies were of the inverted “L” type (series L, shunt C). Furthermore, little design efforts were made to optimize these circuits for broadband operation.

Hence, in the previous amplifier designs, the drain bias line was treated independent of the load circuit, merely acting as a choke realized by a quarter-wavelength transmission line, as shown in Figure 1. It is worth mentioning that the function of the load network in a class-E amplifier is to shape the voltage and current waveforms. Therefore, in the design of a broadband class-E amplifier, care should be taken to ensure that class-E waveforms exist over the entire frequency band.

Choices of a suitable device nonlinear modeling tool as well as a design environment tool capable of time domain analysis, robust harmonic balance and envelope simulation, are critical for the successful design of highly nonlinear circuits such as switching-mode amplifiers. All aspects of the nonlinear device modeling and circuit simulations were successfully performed by using Agilent CAD tools, namely ICCAP⁹ and ADS,¹⁰ respectively.

In this article, a new distributed broadband class-E load topology is presented, as shown in Figure 2. The load offers superior performance over an octave bandwidth (7 to 14 GHz) when compared with the classical class-E load network. Using the time domain simulation capability of ADS,¹⁰ the load network was optimized to obtain near ideal class-E current and voltage waveforms over several frequency points, as shown in Figure 3.

The voltage waveform across the switch rises slowly at switch-off and falls to zero at the end of the half-cycle. It also has a zero rate of change at the end of the half-cycle, thereby ensuring a “soft” turn-on condition. Furthermore, it is shown that the switch current has dropped to zero by the end of the half-cycle, all indicative of class-E operation for the distributed load at 8 and 10 GHz. Similar simulated waveforms were obtained over the entire desired 7 GHz frequency band. The integral of the capacitor (C_ds) current over the half-cycle was also simulated to ensure a zero net current during the switch-off period.

Device Modeling and Circuit Simulation

For accurate and robust nonlinear simulation of switching-mode amplifiers, the device nonlinear model should include the following important parameters:

• Bias dependency of the drain-to-source C_ds(V_ds, V_gs) and gate-to-drain C_gd (V_ds, V_gs) capacitances.
• Bias dependency of the input channel resistance R_I (V_ds, V_gs).
• Bias dependency of the output channel resistance R_ds(V_ds, V_gs).

The device model should also be able to accurately describe the dispersion associated with the drain current, gm and R_ds. Obviously, if a pulsed DC IV technique is used for the model development, this requirement becomes unnecessary. In previous work^7,8 and in current studies, the EEHEMT model available in ICCAP⁹ and ADS¹⁰ was used. It is believed that this is a robust model, suitable for the simulation of class-E amplifiers. The amplifier performance goal was tailored for application in very large (more than 100,000 T/R elements) space-based phased-array radars, requiring simultaneous broadband power (200 mW, 20 dBm minimum) and high PAE over the 7 to 14 GHz band. A 0.3 x 6 x 120 μm PHEMT device, having a gate-drain breakdown voltage greater than 18 V, was found to meet the amplifier’s performance goals.

The details of the amplifier design steps are the same as those described previously.^7,8 Figure 4 shows the final schematic circuit of the monolithic class-E amplifier, depicting the new broadband class-E load topology. Great care was taken in the design of the reactive input matching network to ensure a broadband response under large-signal input drive conditions.

Figure 5 depicts the simulated voltage and current waveforms at the PHEMT output terminals. The waveforms confirm the switching-mode behavior of the PHEMT at 8.5 GHz. Similar waveforms were observed over the 7 to 14 GHz band, thereby confirming the class-E operation of the new distributed broadband load.

Measured Performance

A typical fully fabricated monolithic amplifier chip is shown in Figure 6, while the measured amplifier performance is shown in Figure 7, demonstrating the widest bandwidth for a class-E power amplifier reported so far. As shown, the HPA possesses simultaneous high PAE and P_out over the 7 to 14 GHz bandwidth, when excited by an input drive of 14 dBm. The PHEMT is biased at V_ds = 6 V and V_gs = –0.8 V. The measured maximum and minimum PAE are shown to be greater than 80 and 50 percent, while the measured output power is greater than 24 dBm over the entire 7 GHz bandwidth. Over the sub-band of 8 to 10 GHz, which is of great interest for space-based radar systems, the HPA has a measured maximum and minimum PAE and output power of 75 and 63 percent and 25.8 and 25.0 dBm, respectively.

As can be seen in Figure 8, good agreement between simulated and measured performance has been obtained. Although not shown, similar agreement is also obtained over the 7 to 14 GHz range. Figure 9 shows the measured output power, PAE and gain versus input power at 10 GHz. A maximum PAE greater than 70 percent and an output power of 25 dBm are obtained at P_in = 14 dBm.

The new broadband class-E load also allows the HPA to have a spectrally pure frequency response when operated under linear or compressed power conditions. During extensive AM and PM noise measurements, no bias and/or RF input dependent sub-harmonic oscillations, nor any spurious parametric oscillations were observed. These critical performance attributes are extremely desirable for pulsed linear FM chirp phased-array radars, where the HPAs should avoid introducing any significant amplitude and/or phase distortions.

Figures 10 and 11 illustrate a typical AM and PM noise response of the HPA, respectively. The swept (7 to 12 GHz) AM noise data is obtained at a 10 kHz offset from the carrier over 6 to 14 dBm input RF drive levels. As can be seen, the HPA has a spurious free AM response of less than –130 dBc over the entire frequency and input drive levels. The HPA’s PM noise response shows less than –130 dBc/Hz at 15 kHz offset from 10 GHz for a 14 dBm input drive level. The measured system noise floor is close to –140 dBc/Hz. The HPA’s DC bias for all noise measurements was set to V_ds = 6 V and V_gs = –0.8 V.

Conclusion

In this article, a new broadband distributed class-E load topology is presented, which is suitable for implementation in both hybrid and monolithic technologies. The load allows a broadband class-E performance, showing a nearly frequency independent response over the 7 to 14 GHz band. The monolithic amplifier contains a single GaAs PHEMT of 0.3 x 6 x 120 μm, having a gate-drain breakdown voltage greater than 18 V. The fabrication was performed at the Raytheon foundry using process 62.

The measured PAE and output power over the 7 to 14 GHz band are 82 to 50 percent and 25.8 to 24.0 dBm, respectively, showing simultaneous high PAE and power over the entire 7.0 GHz frequency bandwidth. The new broadband class-E load also allows the HPA to have a spectrally pure, spurious free output response. The observed low AM and PM noise responses are less than –130 dBc at 10 kHz offset from the carrier and less than –130 dBc/Hz at 15 kHz offset from 10 GHz, respectively.

References

1. T. Quach, et al., “Broadband Class-E Power Amplifier for Space Radar Application,” 2001 IEEE GaAs IC Symposium Digest, pp. 209–212.

2. N.O. Sokal, et al., “Class-E – A New Class of High Efficiency Tuned Single-ended Switching Power Amplifier,” IEEE Journal of Solid State Circuits, Vol. 10, No. 6, June 1975, pp. 168–176.

3. F.H. Raab, et al., Solid State Radio Engineering, John Wiley & Sons Inc., Somerset, NJ, 1980.

4. T. Sowlati, et al., “Low Voltage, High Efficiency Class-E GaAs Power Amplifiers for Mobile Communications,” 1994 IEEE GaAs IC Symposium Digest, pp. 171–174.

5. T. Mader, E. Bryerton, M. Markovic, M. Forman and Z. Popovic, “Switched-mode High Efficiency Microwave Power Amplifiers in a Free-space Power Combiner,” IEEE Transactions on Microwave Theory and Techniques, Vol. 46, No. 10, October 1998, pp. 1391–1398.