1 WATT 17.7 GHz – 32 GHz Linear Power Amplifier

1.0  Introduction 

This application note provides application information and performance data on the use of TC915 linear amplifiers in multiple chip combined configurations to increase output power. Configurations of two (2x) and four (4x) MMICs were assembled and tested to demonstrate feasibility and measure performance. 

2.0 Description 

The TC915 (MMIC) is a 17.7 GHz to 32 GHz high performance linear power amplifier. Each MMIC provides greater than 1/4 watt output power at 1 dB gain compression (P–1). The MMIC was designed as a linear power amplifier for use in microwave and millimeter wave transmitters. It is possible to power combine these amplifiers to increase the output power level. The results indicate > 1/2 watt for the 2x configuration, and > 1 watt for the 4x configuration. This is the P –1 power output into a 50 Ω load. The demonstration circuits use high performance thin film microcircuit networks to power divide and equally drive the MMIC inputs. The same microcircuits are used to combine the output power of each MMIC. The circuits are a variation of the Wilkinson divider (Wilkinson, 1960; Mercer, 1996). When properly designed, these networks have remarkably low loss while maintaining a 50 Ω impedance at their input and output terminals. The networks used in the demonstration application were designed in a 5 mil sapphire thin film process usable to 50 GHz. Since the TC915 has integrated coupling capacitors and operates to 33 GHz, the combiner thin film circuits should perform adequately using a 5 mil alumina thin film process without the need for capacitors.1 Figure 1 illustrates the physical layout of the 4x amplifier configuration. Details of the 2x amplifier configuration are not illustrated. Biasing and bypassing of the combined amplifiers is covered in specific sections of this application note, as well as bonding and die attach. The demonstration circuits were assembled in a specially designed, low loss, instrument grade microcircuit package. The RF connectors are 2.4 mm usable to 50 GHz. All measurements and performance data includes package and combiner losses. 

3.0 Performance 

Four 4x amplifiers were evaluated. All four amplifiers exhibited similar performance within a few tenths of one dB. Most measurements were performed on all four amplifiers. Typical performance is provided in this section. Only one 2x amplifier was evaluated. A summary of the RF performance measurements on the combined amplifiers is included in this section of the application note. The performance data includes: S–parameter sweeps, power and gain measurements at four temperatures, IMD measurements, and some load pull results. 

3.1 S–Parameters 

S–parameters were measured on the two– port amplifier assembly. Figure 2 gives test results for the packaged 4x amplifier assembly. Note the input and output return loss (S11 and S22): both are similar because of the Wilkinson network on both input and output. S21 or the SS–gain is about 2 dB low which is attributed to connectors + connector launches + power divider + power combiner losses (see section 3.4 Load Pull). 

3.2 Power 

Figure 3 illustrates the P–1 performance of the 1x, 2x, and 4x amplifier configurations at four frequencies. The TC915 is designed to operate as a linear amplifier up to 1 dB of gain compression (P–1). At P–1 the single amplifier provides better than 24 dBm (> 1/4 watt) output power; the 2x amplifier increases the output power level 3 dB to > 27 dBm (1/2 watt); and the 4x amplifier increases power by 6 dB to > 30 dBm (1 watt). Figure 4 illustrates the gain and power saturation characteristics of the 4x amplifier at four frequencies. The gain of the power–combined amplifier is the gain of the single MMIC less the resistive losses of the package, power divider, and power combiner. 

3.3 Power vs. temperature 

Gain and power performance of the 4x amplifier were measured at –25 °C, 0 °C, 25 °C and 75 °C. Figure 5 illustrates the amplifier performance at these four temperatures. Thermal coefficients of gain and P–1 were calculated for temperature 25 °C to 75 °C. The temperature coefficient of gain is –0.05 to –0.06 dB/°C and the coefficient of P–1 is –0.01 dB/°C. 

3.4 Load Pull 

One 4x amplifier was tested at 29 GHz on a load–pull system. This system adjusts the output match or impedance while monitoring output power. The results indicate that the output impedance of the amplifier package is somewhat capacitive, probably due to imperfect connector transitions. This capacitive loss (3/4 dB) could be reduced by milling out a small air cavity under the connector launch–to–substrate area. The load–pull system cannot adequately check the MMIC output to the combiner match directly. 

3.5 Intermodulation Distortion (IMD) 

Two–tone measurements were performed at 23, 29 and 31 GHz. Two tones were combined and calibrated as an input signal to the 4x amplifiers. The input tones were of equal amplitude and stepped in two dB increments from –15 dBm to +15 dBm. Output power and third order intermodulation products were measured. The third order intercept (TOI) was calculated. The results indicate similar performance at all three test frequencies. The calculated intercept varied from 36 dBm to 40 dBm, depending on input level. Figure 6 illustrates typical third order products at 29 GHz, with 1, 10 and 200 MHz input signal spacing. 

4.0 Driving the Power Amplifier 

Driving the 4x amplifier to P–1 (30 dBm) requires about 17 dBm input drive power. TheTC906 provides 18 to 19 dBm at P–1 and is the “designated driver” for the TC915. The gain of the TC915 is 24 dB typical at 29 GHz. If the TC915 is used to drive the 4x amplifier, then the input drive power would be about –6 dBm. In cases where there may be excessive losses between the driver and the power amplifier, such as a filter network, the TC916 may be selected. The TC916 provides 22 dBm at P–1. This selection provides 5 dB more drive power in the linear range than the TC906. The TC916 provides 7 dB of gain which means that the driver input power would be 10 to 11 dBm.