Top Ten Most Common Circuits

Focused engineer assembling circuit board
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Electronic products are often complex masses of circuits, but as you peel back the layers of any complex electronic product, common circuits, subsystems and modules are repeatedly found. These common circuits are simpler circuits that are much easier to design, work with, and test. This article discusses the top ten of the more common circuits used in electronics. 

1. Resistive Divider

One of the most common circuits used in electronics is the humble resistive divider. The resistive divider is a great way to drop the voltage of a signal to a desired range. Resistive dividers offer the benefits of low cost, ease of design, few components and they take up little space on a board. However, resistive dividers can significantly load down a signal which can change the signal significantly. In many applications, this impact is minimal and acceptable, but designers should be aware of the impact that a resistive divider can have on a circuit.

2. OpAmps

OpAmps are also very useful in buffering a signal while boosting or dividing the input signal. This comes in very handy when a signal needs to monitored without being impacted by the circuit doing the monitoring. Also the boost and divider options allow for a better range of sensing or control.

3. Level Shifter

Todays electronics is full of chips that require different voltages to operate. Low power processors often operate on 3.3 or 1.8v while many sensors run on 5 volts. Interfacing these different voltages on the same system requires that signals either be dropped or boosted to the required voltage level for each individual chip. One solution is to use the FET based level shifting circuit discussed in the Philips AN97055 App Note or a dedicated level shifting chip. Level shifting chips are the easiest to implement and require few external components, but they all have their quirks and compatibility issues with different communication methods.

4. Filter Capacitors

All electronics are susceptible to electronic noise that can cause unexpected, chaotic behavior or completely halt the operation of electronics. Adding a filter capacitor to the power inputs of a chip can help to eliminate noise in the system and is recommended on all microchips (see the chips datasheet for the best capacitors to use). Also caps can be used to filter the input of signals to lower the noise on the signal line.

5. On/Off Switch

Controlling the power to systems and subsystems is a common need in electronics. There are several ways to achieve this effect including using a transistor or a relay. Optically isolated relays are one of the most effective and simplest ways to implement such an on/off switch to a sub-circuit.

6. Voltage References

When precision measurements are required, a known voltage reference is often needed. Voltage references come in a few flavors and form factors and for much less precise applications even a resistive voltage divider can provide a suitable reference.

7. Voltage Supplies

Every circuit needs the right voltage to operate, but many circuits need multiple voltages for every chip to work. Stepping down a higher voltage to a lower voltage is a relatively simple matter using a voltage reference for very low power applications, or voltage regulators or dc-dc converters can be used for more demanding applications. When higher voltages are needed from a low voltage source, a dc-dc step up converter can be used to generate many common voltages as well as adjustable or programmable voltage levels.

8. Current Source

Voltages are relatively simple to work with within a circuit, but for some applications a steady fixed current is required such as for a thermistor based temperature sensor or controlling the output power of a laser diode or LED. Current sources are easily made from simple BJT or MOSFET transistors, and a few additional low cost components. High power versions of current sources require additional components and demand a greater design complexity to accurately and reliably control the current.

9. Microcontroller

Nearly every electronic product made today has a microcontroller at its heart. While not a simple circuit module, microcontrollers provide a programmable platform to build any number of products. Low power microcontrollers (typically 8-bit) run many items from your microwave to your electric tooth brush. More capable microcontrollers are used to balance the performance of your car's engine by managing the fuel to air ratio in the combustion chamber while handling a number of other tasks simultaneously.

10. ESD Protection

An often forgotten aspect of an electronic product is the inclusion of ESD and voltage protection. When devices are used in the real world they can be subjected to incredibly high voltages which can cause operational errors and even damage the chips (think of ESD as miniature lightning bolts attacking a microchip). While ESD and transient voltage protection microchips are available, basic protection can be provided by simple zener diodes placed at critical junctions in the electronics, typically on critical signal runs and where signals enter or exit a circuit to the outside world.