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Figure 7. The output spectra of the HMC920’s V GATE and V DRAIN outputs and maximum allowable noise limits for the ADL8106
Supplement: Power
Figure 8. The output spectra of the HMC920’s V GATE and VDRAIN outputs and maximum allowable noise limits for the ADL8106
Figure 10. The noise figure of the ADL8106 using the HMC920 at the internal negative voltage generator mode and on the external negative voltage generator mode
the VDRAIN and VATE output ports. Figures 7 and 8 show the output spectrum of the VGATE and VDRAIN voltages. Based on the PSMR of ADL8106, traces have also been included in these plots which show the maximum allowable output noise and spurs. The output noise and spurs from the power management circuit must be below these levels to ensure that the amplifier’s performance is not degraded by the power management circuit.
Operating HMC920 with an external negative supply
In the previous example, the HMC920’s internal negative voltage generator is used to
generate the negative gate voltage. An external negative supply can also be used as shown in Figure 9. In this case, the ADP5600 (inverter and negative LDO regulator) is used as the negative supply from which the gate voltage is derived. This result is a slightly lower noise figure and higher gain compared to when the internal negative voltage generator is used. The actual noise performance in this mode still depends on the output noise produced by the external negative voltage generator used. As seen from figures 7 and 8, using HMC920 at external VNEG mode also produces noise spurs that are still below the maximum allowable voltage ripple limit. To utilise this
Figure 9. The ADL8106 and the HMC920 at the external VNEG mode block diagram
Figure 11. The gain of the ADL8106 using the HMC920 at the internal negative voltage generator mode and on the external negative voltage generator mode
mode, the feedback control of the negative voltage generator must be disabled by sorting the VNEGFB pin to the ground. For enhancement type amplifiers (positive gate voltage), both the VNEGFB and VGATEFB pins must be connected to the ground.
Conclusion
Depletion mode GaAs amplifiers are widely used in RF applications because of their wide bandwidth and high dynamic range. But they need a negative bias voltage and must be carefully sequenced. A fixed negative gate voltage can be used to bias the amplifier. This has the benefit that current consumption is dynamic and scales with the RF output level. The circuit presented here uses a fixed drain current and generates low noise drain and gate
32 March 2025 Components in Electronics
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voltages that are safely sequenced and which do not degrade the specified performance of the RF amplifier. It will result in tighter device-to-device performance because every device is operating from the same drain current. However, one downside of this approach is that the drain current is fixed and does not scale with the RF power level. Care and consideration should be taken in deciding on the fixed drain current level, which must be high enough to support the maximum desired output power level but not so high that it results in current wastage. While an external negative supply can be used to override the internal negative voltage generator of the HMC920, the resulting improvement in noise is marginal.
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