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CHAPTER 5 - Amplifier Design 200 5 Amplifier Design 5.1 Introduction – The transistor at Radio Frequency In this section we will be looking at the most common methods employed to perform high- frequency amplifier design. Modern amplifier design is almost invariably carried out with the aid of simulation tools which employ specific models for the active devices at the heart of the amplifier. The accuracy of the simulation results is chiefly governed by the accuracy of such models. These may be obtained in a number of ways but they can all be grouped into two main categories: linear and non-linear. Linear models may be employed under small signal conditions, when the amplitude of the signals involved is such that the inherent non-linearities of the active device may be neglected. These include linear electrical models such as the hybrid-π (section 5.1.1) and S-parameter models (section 3.7). Non-linear models may also be of circuital nature but they would include non-linear elements such as diodes for instance. Other non-linear models are based on the actual physics of the devices and are considerably more complex but often more accurate than electrical models. Other characterisation methods exist but are beyond the scope of this chapter. Although this chapter provides some concise reference material in written form, the majority of the teaching is delegated to the video tutorials listed in section 5.5. 5.1.1 Linear Models for BJTs Let us first take a look at the small-signal 35 model of a BJT in common emitter configuration. This type of model is usually referred to as hybrid-π Notice how this electrical model comprises exclusively of linear elements. From this model it is apparent that amplifier design at high frequency is a task that would be very difficult to accomplish without a thorough understanding of impedance matching concepts and of the elements which are utilised to perform it. Figure 5.1-1 Small-signal, hybrid-π model of a BJT First and foremost this model reminds us of the fact that the bipolar junction transistor is a current amplifier. We have a small current flowing from the base through to the emitter terminal ( ) and this gets amplified by a factor of thereby allowing a current equal to to flow 35 Small-signal modelling is a common analysis technique in electrical engineering which is used to approximate the behaviour of nonlinear devices with linear equations. This linearization is performed about the DC bias point of the device and can be accurate for small excursions about this point. ' bb r e b r ' c b r ' ce r e C c C ' B I ' B I B E E C C E B Conquer Radio Frequency 200 www.cadence.com/go/awr

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