Abstract
This study presents the noise analysis of amplifiers when the input line termination is implemented via an active load. The noise figure (NF) of distributed amplifiers is analysed to determine the effect of the gate and drain terminations. The noise of a transistor configured as a one-port device has been analysed and a lower noise temperature has been achieved as compared with a conventional resistor. In order to improve the noise performance of the overall amplifier, a parallel feedback has been added to the active load, and its effects are analysed in terms of noise. A distributed amplifier has been designed as a test vehicle of the analysis and characterised with a resistor as input line termination and with an active load to demonstrate the improvement in NF at frequencies below 2 GHz. The resulting amplifier exhibits 10.9 dB of small signal gain from 1 to 5 GHz, with a reduction of 0.6 dB in NF below 2 GHz when the input line termination is replaced by an active load.
Highlights
Distributed amplifiers (DAs) are usually designed to provide flat gain response over wide bandwidths
The analysis developed demonstrates that the active load represents an excellent solution to minimize low-frequency noise in a distributed amplifier
The improvement is higher below 2 GHz because the influence of the gate line termination noise temperature is higher at low frequencies, while the noise of the active devices becomes more significant at high frequencies
Summary
Distributed amplifiers (DAs) are usually designed to provide flat gain response over wide bandwidths. Previous works have adopted active loads as line terminations in order to improve the noise performance [4]-[6], only simulation results have been presented and the influence of the active load in the overall amplifier, in terms of noise, is not completely explained. This work provides an extra overview over the noise reduction in a distributed amplifier when using feedback-active loads as line terminations and a mathematical analysis is proposed to explain such noise reduction phenomena. A small-signal analysis of a feedback transistor including noise sources demonstrates the resulting lower noise temperature of such arrangement, as compared to a conventional resistive load. The frequency band of the amplifier spans from 1 to 5 GHz. fabrication and measured results are presented for both amplifiers, demonstrating the improvement achieved in terms of noise performance when an active terminating load is used. The noise temperature of an active load considered as a one port device and implemented with a feedback transistor, is obtained
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