What is output impedance

Let's look at an example of connecting a battery to a resistor. When we see a 9V battery, we often think it should output 9V from its terminals.

It truly does, but only when it's not sourcing any current. It turns out that there is some internal impedance inside the battery that prevents the output voltage from remaining at 9V once current starts to flow.

Here is an example of a 9V battery's I-V curve current-voltage relationship. Now let's connect load to this source. When we plot the I-V characteristics of the load with the source, the intersection of the two lines is the operating point.

Here we can see that once the load is connected to the source, the output voltage is no longer 9V; rather it is given by the operating point. The example drawn above shows the 9V battery having an output resistance of 1. This shows us that even though the 9V battery outputs 9V when there's no current flowing, once we apply the load, the output voltage drops down to 6V to account for the increased current flow.

What would have happened if the load resistance were very high? Think about how the change in slope of the I-V curve for the load would affect the operating point.

After observing the previous example, you can see that loading a source where the input impedance of the load is similar in magnitude to the output impedance of the source causes the output voltage to drop. How can you load a source such that you maintain the output voltage?

Nowadays, as a general rule, high input and low output impedances are the norm, even if it does not lead to an impedance match. However, we will see in the next section that in some cases, impedance matching can be more suitable. Basically, we can distinguish three scenarios of connection. The first one, is when a source is connected to an amplifier, this is what is shown in Figure 2.

The second case is when the amplifier is connected to a transducer. A transducer is the final stage of the circuit, it is the element that converts the electric signal into sound and movement for example, examples of transducers are loudspeakers and motors. The configuration of this connection is the same as presented in Figure 2 where the source would be the amplifier and the load the transducer. In modern electronics, this type of architecture is very common to realize multiple operations and amplifications to the signals.

In the input stage, where a power supply source R S is connected to an amplifier R L , a maximum transferred power is not necessary since the amplifier can itself re amplify the signal.

Usually, a signal loss of -6 dB between the source and the first amplifier commonly known as preamplifier is acceptable, such a loss is achieved when an impedance match is realized. In the case of the cascade configuration presented in Figure 4 , two functioning modes can be distinguished and treated differently :.

For the final stage, where a last amplifier supplies a transducer lets say a loudspeaker , the output impedance of the amplifier must be lower than the internal loudspeaker resistance. Again for the same reasons, the power is transferred more efficiently to the transducer if the amplifier has a low output impedance.

In this case, most of the power can be used by the transducer. However, the global resistance should not be too high to avoid a low power magnitude. The input and output impedances values are fully given by the architecture of the amplifiers.

We can list some of the architectures that are available in order to modify the input or output impedances :. This tutorial has first of all defined what exactly the input and output impedances are. We have seen that they represent the total resistances of the amplifier at the input terminals and at the biased output terminals.

Since they do not represent any physical resistance they cannot be removed, but as a consequence of the amplifier architecture, their value can be adjusted.

Let's say you want to connect your well-running pipe system, with plenty of smooth water flow, to another pipe system. If you just weld a small pipe in there, it'll put massive pressure on the system and possibly burst the pipe. Conversely, if you weld a bigger pipe in, you'll get just a trickle of water when you turn on the faucet. In electronics, this is either reflected by, for example, muddy sound or no sound at all from speakers, or whatever you plugged into the system overloading.

This is why high-end audio systems often include an amplifier; they need the boost in energy to properly match impedance. Unless you're custom-engineering your own circuits, the heavy lifting has already been done for you. Any device where output impedance is relevant, such as an amplifier or a set of speakers, will have the output impedance and input impedance as part of the overall specifications of the device.

You can easily find these online or in the user's manual. In most cases, cheaper devices, like earbuds , will have lower impedance than expensive closed-cup headphones. For example, if you have an audio player, a cable, and a set of speakers, the output impedance of the player should match the input impedance of the cable, and the output impedance of the cable should match the input impedance of the speakers. Actively scan device characteristics for identification.

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