Research On The Theory And Methods For LNA's Dynamic Range Expanding

Dynamic range plays a key role in adaptability of receiving signals with different power levels in a receiver. The larger the dynamic range is, the better anti-interference and anti-fading performance the receiver can achieve. This dissertation firstly investigates the underlying structure of RF front-end at the receiver and, through analyzing the NF (Noise Figure) and IIP3 (Input Third-order Intercept Point), proves that the LNA (Low Noise Amplifier), located at the front of RF front-end, is one of the bottlenecks limiting the dynamic range.Moreover, this dissertation studies the characteristics of the microwave bipolar junction transistor (BJT). It is figured out that as the bias current shifts, the NF and IIP3 vary simultaneously in a common-emitter BJT. Then two novel methods, which can expand both the upper and lower boundaries of the dynamic range are proposed, without losing any sensitivity compared to the other methods.This dissertation makes the following contributions:1. A formula for minimum noise figure based on conveniently measuring parameters is derived from an existing noise model of microwave transistors. A qualitative conclusion that changes of bias current will affect noise figure is made theoretically also. Practical test is conducted on two types of BJT produced by different manufactures. The measurement results are consistent with the analysis aforementioned.2. Based on Volterra Series, the nth-order nonlinear transfer function of the common-emitter BJT amplifier is derived. The curve of IIP3 versus bias current is obtained. Corresponding measurement results prove out the deduction and analysis.3. A method called dynamic bias is proposed to expand the LNA dynamic range. The method is realized by dynamically adjusting the bias current of LNA according to the strength of the signal received, with which a lower NF and a higher IIP3 can be achieved.4. Another method is introduced as well, to expand the LNA dynamic range with the adjustable attenuator. This method consists of two aspects: first, an adjustable attenuator is placed in front of the LNA, which will improve the overall linearity of the circuits; second, the bias current of LNA is reduced to achieve a smaller noise figure.5. Two structures of current mirrors with tunable output current are proposed. These two structures tune the output current by utilizing an analog input voltage and an N-bit digital input signal respectively. These designs can benefit a lot in terms of simplicity, convenience for using, accuracy and so on. This kind of current mirror has been practically used in dynamic bias LNA circuit.Corresponding experiments are carried out to verify the theoretical analysis and the proposed methods in this dissertation. Our methods for expanding the LNA dynamic range have been successfully applied into the RF front-end circuit of TETRA handheld radios which achieve great performance.