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Comments on Impedance Impedance A thorough discussion of impedance is given below. In summary a signal source can be characterized by an impedance and your input can be characterized by an impedance. These relate current and voltage and so can be used to understand circuit response. In this view we establish one part of the circuit as the source, the first stage in our circuit. The source feeds the second stage at the input. This second stage can then subsequently function as a source for a load or some other input to the next bit of circuitry. One can analyze the impact of current on voltage or voltage on current using impedance. source Input Load etc. In particular transistors have the ability to appear to the source as a large impedance [very little current (power)necessary to establish a voltage] and then serve as a source for a load where the output voltage is maintained even with a large current draw. Out/In impedance allows for a general understanding but many devices will have active components such as transistors that make the actual response more complex. For output/input impedance one can usually put output and input (load) in series and treat it as part of a voltage divider in order to find the voltages and currents. Impedance analysis allows one to maximize power, or voltage transmitted from a source to an input. wiki thoughts In electronics, impedance matching is the practice of designing the input impedance of an electrical load (or the output impedance of its corresponding signal source) to maximize the power transfer or minimize reflections from the load. In the case of a complex source impedance ZS and load impedance ZL, maximum power transfer is obtained when where * indicates the complex conjugate. Plot below shows power lost due to internal resistance of the source and the power transferred to the load as a function of the load resistance. Jaycar Electronics Reference Data Sheet: IMPMATCH.PDF (1) IMPEDANCE MATCHING: A PRIMER From time to time youll come across the term impedance matching in various areas of electronics, and especially in fields like RF and audio engineering. However even in these fields its often misused, probably because many people dont really understand the concepts behind it. In this primer well try to clarify what impedance matching is really all about, why its important in some situations and not important in others. Textbooks usually explain the idea of impedance matching with a very simple example of an electrical generator feeding a resistive load, as shown in Fig.1. Because the generator has an internal resistance of its own (RG) as all real generators do this tends to dissipate some of the generators output power as heat, Fig.1: A generator driving a load resistance RL whenever we connect a load to its output terminals. So and its own internal resistance RG. the full mechanical power fed into the generator cant be drawn from it as electrical power, because some will impedance. And the idea of matching the load and always be wasted in RG. generator resistance became one of matching the source When early electrical engineers were faced with this and load impedance impedance matching. problem, they naturally enough did everything they Now this may sound simple and straightforward, but could to reduce the internal resistance of their its important to remember where the idea came from, generators. However they were inevitably still left with and also realise what exactly is going on when we do SOME internal resistance, because its impossible to match load and generator/source resistances or reduce the resistance to zero unless you run the impedances. True, the POWER transferred to the load generator at a temperature of close to absolute zero will be a maximum; but at the same time, the actual (0Kelvins, or -273 C). ° power being dissipated in the generators internal Once they had minimised the internal resistance, their resistance is EXACTLY THE SAME as that reaching the next step was to see if there was some way that they load! In other words, HALF the total power from the could minimise the amount of power wasted in it, by generator is now being turned into heat inside RG, varying the resistance of the load. And what they because its resistance is now half the total connected discovered is shown in the Fig.2, which plots the amount across the generator. The other half is the load of power transferred into the load RL as its resistance is resistance RL. varied. As you can see, the amount of power reaches a For exactly the same reason, RG and RL will now be peak or MAXIMUM when the load resistance is the acting as a 2:1 voltage divider across the generator same as or matches the generator resistance. It falls so that only HALF the generators output voltage (EG) away for values both higher and lower than this figure, will be appearing across the load. In terms of voltage showing that matching the two is clearly desirable if we transfer, then, matching the impedances isnt particularly want to maximise the power able to reach the load. efficient: it actually gives a 6dB loss. So this is where the idea of matching the resistance of Does this mean that impedance matching really only the load to that of the generator came from. Before applies to generators in power stations? No, and in fact long, it was extended to cover the general situation of it doesnt really apply there either or at least, not any load impedance connected to a source of electrical simply. All it really means is that as you draw more and energy or voltage (EMF), with its own internal source more power from a generator by reducing the load resistance, a point is reached where half the generators output power is being wasted inside it. Obviously with very high power generators its not a good idea to load them even this heavily let alone dropping the RL even further, where even more power is lost inside the generator than reaches the load. (See the blue curve in Fig.2, showing the power lost in the generator.) Most power station generators are loaded with an RL somewhat higher than RG, to waste as little power as possible. So when IS impedance matching a good idea? Glad you asked. Basically its for situations rather different from that in Fig.1, where were stuck with a particular load or cable impedance, and we still want to either maximise the power transferred into the load, or minimise the amount of power reflected back from it into the cable, or both. Fig.2: How the power fed to the load varies as the load RF CABLE MATCHING resistance is varied (red plot), and what happens to the For example in many RF situations, we tend to have power wasted in RG (blue plot). a relatively fixed LOAD impedance say a resonant Minimum reflection is obtained when The concept of impedance matching was originally developed for electrical engineering, but can be applied to any other field where a form of energy (not necessarily electrical) is transferred between a source and a load. An alternative to impedance matching is impedance bridging, where the load impedance is chosen to be much larger than the source impedance and maximizing voltage transfer (rather than power) is the goal. Impedance is the opposition by a system to the flow of energy from a source. If a source with a low impedance is connected to a load with a high impedance the power that can pass through the connection is limited by the higher impedance. This maximum-voltage connection is a common configuration called impedance bridging or voltage bridging, and is widely used in signal processing. In such applications, delivering a high voltage (to minimize signal degradation during transmission or to consume less power by reducing currents) is often more important than maximum power transfer. Horowitz SOME BASIC TRANSISTOR CIRCUITS 2.08 Unity-gain phase splitter 77 an emitter follower will not be dominated by the emitter load resistor, but rather by the impedance looking into the emitter. signal signal 2.08 Unity-gain phase splitter in Sometimes it is useful to generate a signal 1.ov and its inverse, two signals out of phase. That's easy to do - just use an emitter-degenerated amplifier with a gain of -1 (Fig. 2.28). The quiescent collector voltage is set to rather than the usual in order to achieve Figure 2.27. An ac common-emitter amplifier the same result - maximum symmetrical with emitter degeneration. Note that the output terminal is the collector, rather than the emitter. output swing without clipping at either output. The collector can swing from The output impedance is 10kΩ in parallel with the impedance looking into the collector. What is that? toWell, remem- berwhereas that if you the emitter can snip off the collector resis- tor, you're simply looking into a current source. The collector impedance is very large (measured in megohms), and soand the output output impedanceimpedance is just the value of of the collectorthe resistor, 10kΩ swing.It is worthfrom remembering ground that the impedanceto looking into a transistor's collector is high, whereas the impedance looking into the emitter is low (as in the emitter follower). Althoughcommon the output impedance-emitter of a commonamplifier-emitter amplifier will be dominated by the collec- tor load resistor, the output impedance of an emitter follower will not be dominated by the emitter load resistor, but rather by the impedance looking into the emitter.We can easily determine the input and output impedances of the amplifier.
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