Equivalent circuit device models are critical for the accurate design and modelling of RF components including transistors, diodes, resistors, capacitors and inductors. This chapter will begin with the bipolar transistor starting with the basic T and then the π model at low frequencies and then show how this can be extended for use at high frequencies. These models should be as simple as possible to enable a clear understanding of the operation of the circuit and allow easy analysis. They should then be extendible to include the parasitic components to enable accurate optimisation | Fundamentals of RF Circuit Design with Low Noise Oscillators. Jeremy Everard Copyright 2001 John Wiley Sons Ltd ISBNs 0-471-49793-2 Hardback 0-470-84175-3 Electronic 1 Transistor and Component Models at Low and High Frequencies Introduction Equivalent circuit device models are critical for the accurate design and modelling of RF components including transistors diodes resistors capacitors and inductors. This chapter will begin with the bipolar transistor starting with the basic T and then the n model at low frequencies and then show how this can be extended for use at high frequencies. These models should be as simple as possible to enable a clear understanding of the operation of the circuit and allow easy analysis. They should then be extendible to include the parasitic components to enable accurate optimisation. Note that knowledge of both the T and n models enables regular switching between them for easier circuit manipulation. It also offers improved insight. As an example S21 for a bipolar transistor with an fT of 5 GHz will be calculated and compared with the data sheet values at quiescent currents of 1 and 10mA. The effect of incorporating additional components such as the base spreading resistance and the emitter contact resistance will be shown demonstrating accuracies of a few per cent. The harmonic and third order intermodulation distortion will then be derived for common emitter and differential amplifiers showing the removal of even order terms during differential operation. The chapter will then describe FETs diode detectors varactor diodes and passive components illustrating the effects of parisitics in chip components. 2 Fundamentals of RF Circuit Design It should be noted that this chapter will use certain parameter definitions which will be explained as we progress. The full definitions will be shown in Chapter 2. Techniques for equivalent circuit component extraction are also included in Chapter 2. Transistor Models at Low Frequencies .
