Why is output impedance of eletronic circuit kept low?

Answer 1

In the general sense of the question one can in a general term say that where it applies to output drives, that it improves the control the drive have over the actuator it controls and in other applications it "might" improve energy efficiency and output stability depending on the application and available options.

Just to establish an understanding to the general theory that might assist in a wider range of applications. The load impedance is the impedances seen from the source, the output impedance is the impedance seen from the load. All what this means is that, if the load would have acted as a source of energy, the output impedance is the load impedance the drive circuit will impose on such source.

In my experience in electronics, this is best demonstrated in the field of high fidelity audio power amplifiers. The reason I mention high fidelity is because even for most general purpose power amplifier application for all practical purpose it is just not that important.

The amplifier has the function to provide energy to the loudspeaker where the loudspeaker will respond to that electrical energy by converting it to mechanical energy. In the process the loudspeaker has many mechanical characteristics that influence its behaviour. Some of these characteristics might be due to the weight of the cone, voice coil or the elasticity of the suspension components. This however leads to the actual behaviour of the loud speaker to deviate from the control signal (audio) resulting in unwanted audio characteristics where the most prominent one in this context is known as a damping factor.

Damping can be demonstrated in simple terms by securing the tip of a ruler the edge of a table, the mechanical nature of the structure will cause it to continue to oscillate for a while after you gave the other free hanging tip a slight knock. The oscillation will gradually loose momentum and discharge its mechanical energy into the surrounding structure and so on. This happens to a practical loudspeaker as well after an electric pulse is introduced to its voice-coil, it will continue to oscillate for a short period of time after each change in state. But, as this voice-coil continues to oscillate through the magnetic pole-ring it will generate electrical energy that is effectively fed back to the amplifier. The output impedance of the amplifier will act as a load to this energy being generated by the voice-coil. If the amplifier has a low output impedance, then it will become more difficult for this voice coil to freely move as the current caused by the low output impedance will setup a magnetic force that apposes its direction of movement resulting in the position of the voice-coil to be better secured to the status of the control signal.

This however leads to the fact that the lower the output impedance of an amplifier is, the more control the amplifier has to the absolute movement, rate of change and position of the voice-coil resulting in more precise audio reproduction. However some manufactures claim ridiculously high damping factor for high fidelity audio amplifiers. This however have little use to even the audiophiles as the resistance of the voice-coil as well as the loud speaker cable resistance will limit the positive effect of the damping factor has beyond the approximate factor of 50. The damping factor is purely the ratio between source resistance and load resistance and therefore (loudspeaker +cable) impedance divided by output impedance is the damping factor. Just to reiterate that high damping factor is a direct result of low output impedance and therefore to keep the output impedance low is a good thing, but at some point one must bare in mind that the design for low output impedance should not go overboard for the application it applies too. One must bare in mind the load resistance ext.

In some cases an application might enjoy the advantages of other specifications and therefore design considerations should never only value output impedance. For example Class B amplifiers are capable of excellent low output impedances while they are known for poor energy efficiency in comparison of Class D amplifiers which are more energy efficient, and often better precision at low cost, But they lack that nice low output Independence. Where a portable music center might benefit from the Class D amplifier for practical reasons, a top-end high fidelity system might prefer the Class B amplifier, slightly modified, for its simplistic first principal operation and low output impedance that will result in an excellent damping factor on the conditions that the correct loudspeakers and cables are used.

As with audio there may be other applications in industry and science where low output impedance for much the same reasons will be a design requirement in precision control of actuators. In high power applications even more critical to ensure proper supply or prevention of unnecessary internal energy losses or heating. However some application considerations need to be scrutinized where the demand for low output impedance becomes unreasonable. As many factors such as cabling, temperature and many uncontrollable factors can result in deviations and unrealistic expectations and therefore, must enjoy a fair consideration.