We won’t examine the mathematical derivation of this fact, but it makes basic sense if we consider it from an intuitive perspective. In the context of resistive circuits, maximum power transfer occurs when the input impedance of the load is equal to the output impedance of the source.įigure 3. It turns out that we must take a different approach to input and output impedance when the goal is to maximize the transfer of power, rather than voltage, from source to load. The amplifier’s output voltage (blue) is now only slightly lower in amplitude than the original signal (orange), because the source’s output impedance (10 Ω) is much smaller than the load’s input impedance (100 Ω). If we take the example circuit discussed above and switch the input and output impedances, we obtain the following waveforms:įigure 2. Nevertheless, the source’s output voltage is the same as the load’s received voltage, and the load’s input current is the same as the source’s supplied current. From the perspective of the load circuit, we have received voltage and input current. From the perspective of the source circuit, we have output voltage and supplied current. When your goal is to transfer a voltage signal from one subcircuit to another-i.e., when you want maximum voltage transfer-remember the following: the subcircuit supplying the signal should have low output impedance so that changes in supplied current do not seriously affect the output voltage, and the subcircuit receiving the signal should have high input impedance so that changes in received voltage do not cause large changes in input current.Īn important point to keep in mind when you’re pondering the preceding sentence is that we’re using different words to identify the same quantities because we’re discussing these quantities from two different perspectives. The serious loss of signal amplitude in Figure 1 demonstrates why designers very frequently attempt to reduce output impedance and increase input impedance.
The orange trace is the original waveform, and the blue trace is the output of the amplifier. The sensor has an output impedance of 100 Ω, and the amplifier has an input impedance of 10 Ω.įigure 1. The effect of combining a high-output-impedance voltage source with a low-input-impedance amplifier. The following plot shows the original waveform (i.e., the waveform before it is affected by the source’s output impedance) and the output waveform for a circuit composed of a unity-gain amplifier and a component that generates a sinusoidal signal (let’s say that it’s a sensor of some kind). Our goal is to transfer the original voltage signal from the source to the amplifier, but the signal received by the amplifier will be greatly attenuated-the amplifier draws large amounts of input current, and this current must be supplied by the source, which exhibits large changes in voltage in response to changes in supplied current. Thus, the two subcircuits are working against each other.The low input impedance of the amplifier means that any change in input voltage will create a relatively large change in input current.
The high output impedance of the source means that changes in output current will create relatively large changes in output voltage.Let’s say that we have a voltage source with a high output impedance, and it’s connected to an amplifier circuit with a low input impedance. This means that the output impedance indicates how much the circuit’s output voltage will change for a given change in the amount of current that must be supplied by the output circuitry.Ī simple example will help to reinforce these important concepts. The output impedance of a circuit is the impedance that it presents to the circuit that will receive the output signal. In other words, the input impedance tells us how much the input current will change for a given change in input voltage. The input impedance of a circuit is the impedance that it presents to another circuit that is providing an input signal. In the context of electrical engineering, we can divide a circuit into multiple subcircuits and then perform our analysis based on input impedance and output impedance. Understanding Input Impedance and Output ImpedanceĪ common technique used in engineering is to separate a system into smaller interconnected subsystems. First, though, we need to discuss two parameters that govern the interaction between portions of a circuit.
MAX POWER TRANSEE HOW TO
This page explores the issue of how to design a system that will efficiently transfer power from source to load.
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