All three network bias point dependency of temperature is simulated in range from -40 to +60 ☌.įigure 2. Q7, R17, R15 and R5 forms current source, so that base current of Q6 (main transistor) is controlled so that voltage on R5 is constant – that means current also remains constant. Third network is so called active biasing. Second one has addition of emitter resistor, as result simple negative feedback is obtained – if Ic tends to increase, emitter voltage increases, decreasing base-emitter voltage as result Ic decreases and vice versa. The network 1 has grounded emitter and bias point is set by voltage divider R13 – R14 as well as R12. Selected power supply voltage is 8V.įigure 1. Three bias networks are analyzed using LTSPICE. Since it is planned to mount LNA on antenna mast, it is important to have stable bias in wide temperature range. Other important aspect is bias network selection. More voltage “headroom” is needed for voltage regulator dropout and larger output dynamic range. Point is Ic = 25mA and Uc = 2.7V, because lower Uce allows to use lower supply voltage which is nominal 13.8V. It is compromise of gain, noise figure and available supply voltage/current – more collector current results in more noise but higher gain and vice versa. It is very important, because s-parameters thereby AC characteristics depend on DC bias, more accurately particular collector current Ic and collector-emitter voltage Uce. The first design step is choosing of DC bias point and bias network. The final design would use much better (a bit costlier) device like ATF-54143, which has parameters capable to compete with commercial LNAs like Kuhne Electronics ones.

Other reason is its low cost – it costs only a few cents, which makes it perfect for experimentation and prototyping, as this particular design is just experiment to learn basics of design, building, measuring and analyzing LNA devices. One of reasons why this particular device was selected was good documentations, in particular, at has pre-measured s-parameter tables, which includes design frequency of interest. At design frequency of 437MHz these parameters will be better.

5dB and low noise figure of 1dB at 900MHz working frequency. In subsequent design, the selected RF device is general purpose, low noise NPN silicon bipolar transistor AT-41511. Besides gain and noise figure there are other important characteristics of LNA, like bandwidth, third order intercept point (IP3), temperature range. The overall noise factor is dominated by the noise factor of the LNA, if the gain is sufficiently high. Subsequent stages have a diminishing effect on signal-to-noise ratio. An important consequence of this formula is that the overall noise figure of a radio receiver is primarily established by the noise figure of its first amplifying stage.

Use smithchart rf toolbox