VSWR Protection of Power Amplifiers Using BST Components
Fig. 1: Load-pull simulation results: output impedance chart with contours for the transistors maximum gate current and dissipated power.
Fig. 2: Designed PA with BST varactor detuning for VSWR protection.
Fig. 3: Measured gain, PAE, and dissipated power versus input power for nominal and maximum varactor bias voltages.
Fig. 4: 3D plot of dissipated power as a function of VSWR at the load and varactor bias voltage.
High output voltage-standing-wave-ratio (VSWR) conditions are critical for most solid state power devices used for commercial applications. Gallium-nitride (GaN) high-electron-mobility-transistors (HEMTs) benefit from high breakdown voltage and consequently a high ruggedness against output VSWR. Nevertheless certain high VSWR ratios may cause degradation or even damage due to high voltage or thermal stress. The resulting electrical and thermal stress may induce electromigration, non-reversible structural disorders or even destroy the transistor completely.
Previous research has shown that load mismatch conditions lead to an active load modulation at the output of a power amplifier (PA). This and the power that is reflected back into the transistor can cause excessive power dissipation (PDISS), critical peak voltages and high gate currents (IG). The graph in Fig. 1 shows the load impedance boundaries for maintaining a safe operating area. The red line indicates PDISS maximum rating, while the blue one marks the IG limit. The graph is scaled to a nominal 50 Ω external load.
Several approaches can be used to protect solid state power amplifiers. For example, high output voltages can be simply short circuited using clamping diodes. Or, if high VSWR ratios are detected, gain or output power can be reduced by either controlling a driver amplifier or by adjusting the gain of the output stage. This work focuses on final stage gain control using a tunable ceramic capacitor at the input of the PA. For this purpose, a barium-strontium-titanate (BST) varactor is introduced to detune the input impedance of the PA. BST is a ferroelectric material which changes its dielectric constant (εr) with applied electrical field. Therefore the capacitance of a BST varactor can be changed electrically, which enables detuning of the transistor's input impedance and a reduction of the PA power gain. Consequently, a critical VSWR situation is prevented by reducing the amount of reflected power that could cause excessive power dissipation and lead to high voltage levels. Fig. 2 shows the fabricated PA with BST VSWR protection.
The results are plotted in Fig. 3. The curves show that the transducer gain GT at 20 V is 5 dB lower than at 5 V. Since the transducer gain includes losses in the matching network, the graph indicates that the change in varactor capacitance causes an input loss increase of 5 dB. The dissipated power is affected by the reduction of GT. At the nominal operating input power of 35 dBm the dissipated power can be reduced from 56 W to 43 W at no VSWR.
In order to prove the protection effect, load-pull measurements were performed generating various mismatch conditions. The dissipated power as function of VSWR and BST control voltage is plotted in Fig. 4. The measurements prove the concept of VSWR protection by reduction of the transducer gain. Up to a VSWR of 30:1 the power dissipation could be kept below the maximum rating, without even using the full span of tunability.
Publication
J. Ferretti, S. Preis, W. Heinrich, O. Bengtsson "VSWR Protection of Power Amplifiers Using BST Components," Presented at the German Microwave Conference GeMiC 2016.