| 摘要 |
<p>A monolithic amplifier having a common-gate input stage with a device transconductance which is higher than required for input match, and a load impedance presented to the common-gate stage which is not conjugate matched. Common-gate circuits are commonly thought to have poorer noise figures and poorer stability than their common-source counterparts. However, in the past, common-gate microwave circuits have relied on the widely known fact that if a device having 20 mS transconductance is used in a 50 ohm circuit with its output conjugately matched, very wideband performance is obtained with excellent input match. Noise figure in this case is necessarily poorer than with the conventional common-source configuration. The present invention teaches a common-gate configuration using an FET with higher transconductance and a higher output load impedance. Over narrower bandwidths, excellent input match is thus obtained with noise figures at least as good as those obtained with the common-source approach, which cannot provide good input match. This combination of noise figure and input match is achieved in a compact monolithic structure. In order to provide low and exactly repeatable common lead inductance (gate lead inductance) and low feedback parasitics in a common-gate low noise amplifier, a GaAs FET connects the gate electrode 20 to ground at various points along its width by means of an air bridge crossover structure 14, 22, 23, 24. This structure crosses over the input (source) lines 16 with very low capacitance. Since the gate lead inductance is low in this design, and because in monolithic form this inductance does not vary as is the case for a device grounded using bond wires, common-gate circuit stability is assured. This device preferably uses the well-known pi-gate configuration to provide low drain-gate parasitic capacitance and equal phasing to all parts of the device.</p> |
