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CHAPTER 5 - Amplifier Design 204 cause the output impedance to decrease with frequency. However, although and do have an effect on the output impedance of the device, there is another, more subtle, mechanism that also affects it. Let us assume that the transistor is in operation and that some of the collector signal is being fed back to the base through . Some of the feedback voltage will appear across , causing current to flow through this resistor. The BJT amplifies this current by a factor of β thus increasing the collector current. Since impedance is the ratio of voltage and current, this increase in collector current appears as a decrease in collector impedance. and of course also act to reduce the output impedance of the transistor through a decrease in their capacitive reactances, but the mechanism that we have just described also plays an important role. In addition, the impedance connected at the input terminals will also affect the output impedance of the transistor. This is because if we increase , more of the feedback current will flow through thereby increasing the collector current further and therefore decreasing . 5.1.4 Feedback Characteristics The feedback components of the transistor equivalent circuit are and as shown in Figure 5.1-1. Of the two, is the most important since its value changes with frequency. The resistance is very large and constant and hence does not contribute significantly to the feedback characteristics of the device. As the frequency of operation of the transistor increases, the reactance of decreases and hence more of the collector signal is able to be fed back to the base. At low frequencies, the feedback is usually not much of a problem because has a low reactance and the phase difference between the feedback and input signals is large enough for the onset of oscillations to be avoided. However as frequency increases the lower value of reactance for , coupled with the reduction in phase difference between input and feedback signal makes oscillations more likely (video 5.3). Another problem associated with BJTs is the fact that the collector circuitry is not truly isolated from the base circuitry. This means that any change in the load resistance of the collector circuitry directly affects the input impedance of the transistor. Or, similarly, any change in the source resistance in the base circuitry directly affects the output impedance of the transistor. As we will see in video 5.5, this problem is apparent when we try to match input and output independently. We will see how, if we first match the transistor's input impedance to the source and then match the load to the transistor's output impedance, the output matching network will cause the transistor's input impedance to change from its original value. Therefore, the input matching network is no longer valid and must be redesigned. Once you redesign the input matching network however, this impedance change will reflect through to the collector causing an output impedance change which invalidates the output matching network. Therefore, if you totally ignore the feedback components in the transistor's equivalent circuit when designing impedance matching networks, you will not obtain a perfect match for the transistor. Nevertheless, if is small, the match at both the input and the output might be tolerable in many cases. We will also see however that there are ways to circumvent this problem, most notably a technique called simultaneous conjugate match, which are illustrated in section 5.3.3.2 and 5.3.3.3. Conquer Radio Frequency 204 www.cadence.com/go/awr