Low Noise Amplifiers (LNAs) are the very first active components in a receiver chain, directly following the antenna. Their primary purpose is to amplify the extremely weak RF signals captured by the antenna without adding significant noise of their own. The performance of an LNA largely dictates the overall sensitivity of the entire receiver system.

Design considerations for LNAs:
1. Noise Matching (Most Critical):
- Concept: This is the paramount consideration for an LNA. To achieve the lowest possible noise figure, the LNA's input impedance must be matched to the optimum noise impedance of the active device (transistor), not necessarily to the source impedance for maximum power transfer (e.g., 50 Ohms). These two matching conditions (for minimum noise and maximum power) are often different.
- Trade-off: There's usually a trade-off between achieving minimum noise figure and achieving maximum power gain. Often, the design aims for a compromise: a low noise figure (close to the device's minimum noise figure, Fmin) with acceptable gain, rather than strictly maximum gain.
- Tools: Noise circles on the Smith Chart (discussed in Section 5.4) are used to visualize and design for noise matching.

1. Power Gain (Gp):
2. Concept: While noise is paramount, the LNA still needs to provide sufficient gain to raise the signal level above the noise floor of subsequent stages. A typical LNA gain might be 15-25 dB.
3. Impact on overall NF: As shown by Friis' formula, the gain of the first stage (LNA) significantly desensitizes the overall receiver to noise contributed by later stages. A higher LNA gain means the noise from later stages has less impact on the overall system NF.
4. Trade-off: Too much gain in the LNA can lead to instability or force the LNA into compression at moderate input signal levels, which is undesirable.

1. Stability:
2. Concept: An amplifier is stable if it does not oscillate (produce unwanted output signals on its own) under any source and load impedance conditions. RF amplifiers, especially those with high gain, are prone to instability due to parasitic feedback paths.
3. Unconditional Stability: An amplifier is unconditionally stable if it will not oscillate for any passive source and load impedance. This is often a design goal for LNAs.
4. Conditional Stability: An amplifier is conditionally stable if it will oscillate only for specific combinations of source and load impedances. In this case, the design must ensure that the actual operating source and load impedances fall within the stable region.

1. Linearity:
2. Concept: While LNAs handle very small input signals, they can still be driven into non-linearity by strong interfering signals (blockers) that are outside the desired band. Non-linearity can cause intermodulation products that fall into the desired band, effectively raising the noise floor or corrupting the signal.
3. Importance: A good LNA should have sufficient linearity (e.g., a reasonable IIP3) to handle strong out-of-band signals without generating in-band interference.

1. Input/Output Matching:
2. Concept: After noise matching the input, the output of the LNA needs to be matched to the input of the next stage (often 50 Ohms) for efficient power transfer.
3. Tools: Constant gain circles and stability circles are used.

Design examples of common LNA configurations:
LNAs are typically built using Field-Effect Transistors (FETs), especially MOSFETs or HEMTs (High Electron Mobility Transistors), due to their inherently low noise characteristics at high frequencies. Bipolar Junction Transistors (BJTs) can also be used. Here, we'll look at two common single-stage configurations.

• Common Source (or Common Emitter for BJT) LNA:
• Description: This is a very common configuration for LNAs due to its high gain. The input signal is applied to the gate (or base), and the output is taken from the drain (or collector). The source (or emitter) is typically grounded or bypassed to ground for RF.
• Advantages: Provides significant voltage and power gain. Relatively easy to implement input and output matching networks. Offers good isolation between input and output if the reverse isolation (S12) of the transistor is low.
• Noise Matching: The input matching network is crucial. It transforms the source impedance (e.g., 50 Ohms from the antenna) to the optimum noise impedance (Zopt) of the transistor. This network often uses inductors and capacitors.