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5.1 Introduction – The transistor at Radio Frequency 201 between collector and emitter terminals (red ovals Figure 5.1-1). The resistance represents the input resistance of the transistor and it is the resistance which occurs at the base-emitter junction of the forward biased transistor. Typical values range around 1000 Ω. The output resistance between the collector and emitter terminals is usually rather high in value. Since this resistance appears in parallel with the load and with the current generator supplying the load, we want it to be as high as possible. This way the current through it would be minimised and we would maximise the load current and hence the power delivered to our load. This resistance is also responsible for the Early effect i.e. the IV curves sloping up in the active region of the transistor as shown in Figure 5.1-2. Figure 5.1-2 I-V Characteristic for a BJT The other important elements are and which allow feedback from the output to the input to take place. The most significant of these two is the feedback capacitance , which is formed at the reverse-biased collector-to-base junction of the transistor. As the frequency of operation of the transistor increases, the impedance of decreases and thus has a much greater effect on the transistor operation. A typical value for this component might be 3 pF. The feedback resistance , which is the resistance appearing from the base to the collector of the transistor, is a very large (> 5 MΩ). Since the impedance of a parallel can at most be equal to the lowest of the two, as the frequency increases the impedance of the parallel will be dominated by . Because thruough this capacitance current can flow back to the input from our current generator instead of going to the load, may act as to decease the gain. The other elements in this model are: - Base spreading resistance ( ) which is an inevitable resistance that occurs at the junction between the base contact and the semiconductor material that composes the base. Its value is usually of the order of tens of ohms. - Emitter diffusion capacitance ( ) which is the sum of the emitter diffusion capacitance and the emitter junction capacitance, both of which are associated with the physics of the semi- conductor junction itself. I B2 I B3 I B4 I B5 I B1 I C V CE The slopes of these lines are due to ce r Conquer Radio Frequency 201 www.cadence.com/go/awr