The rapid proliferation of wireless communications necessitates software defined radios (SDRs). In an SDR receiver, a multiband LNA is the first active element, which should provide good impedance matching, adequate gain, low noise figure (NF) and low power consumption. Multiband LNAs for SDRs include Concurrent LNAs, Non-Concurrent LNAs and Reconfigurable LNAs. Existing LNA designs for SDRs have poor stopband rejection ratio, gain imbalance, low interference rejection, increased NF, degraded linearity and reduced operational bandwidth.

To fill these research gaps, firstly we propose a step-wise impedance scaling method to design concurrent multiband matching networks for LNAs. Subsequently, we design a dual band LNA (1.1GHz and 2.4GHz) that substantially reduces the in-band interference, stopband rejection ratio and gain imbalance between bands.

Thereafter, we propose the design of a continuously tunable LNA using a transformer-based input matching network for dynamic frequency tuning. The designed LNA has a wide tuning range (2.2 GHz$\sim$2.8 GHz) and the input impedance is dynamically tuned by controlling the bias voltage of the tuning transistor.

Next, we propose the design of a multimode LNA with a tunable input matching network that provides a wide tuning and operating range (0.9 GHz$\sim$2.5 GHz). By varying the biasing condition of the varactors, the designed LNA operates in six different modes. The LNA achieves a high gain, low noise, high interference rejection along with linear and stable operation in all modes.

Finally, we propose the design of a reconfigurable LNA with the continuous tuning of operational bandwidth and frequency in two separate switchable bands. The LNA implements a PIN diode in the input matching stage to reconfigure between the low band (0.2$\sim\sim$3.2 GHz). A varactor diode is added to the output load network of the LNA. The reverse bias voltage of varactor diode is varied to achieve continuous bandwidth tuning within each band.

This research proposes the design of multiband concurrent and reconfigurable LNAs for SDRs to achieve (i) wide tunable/switchable operational bandwidth with continuous tuning, (ii) high interference rejection, (iii) good linearity-gain trade-off, (iv) wide operating range, (v) improved stop band rejection ratio. Starting from mathematical derivation and modelling, we perform extensive simulation studies, and then fabricate the designed LNAs as microwave integrated circuits on printed circuit boards to obtain measurement results. Finally, we compare the analytical results, simulation results with measurement results in terms of gain, noise, linearity, and impedance matching. For future work, switchable LNAs together with field programmable grate arrays (FPGAs) can be investigated.