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An inductor is a loop of wire that stores energy in the form of a magnetic field. It has uses in oscillators, filters, voltage sources and converters.

Theory checklist:
  • Inductive Reactance
  • Back EMF
  • Q factor
Modes of operation:
  • LC filters and the tank circuit
  • Autotransformer
  • Transformer


A transistor is a semiconductor active device that amplifies or switches and electrical signal. It is used in amplifiers, digital electronics and buffer circuits.

Theory checklist:
  • Gain
  • Bias point
  • Saturation and cutoff
Modes of operation
  • Common base, emitter and collector configurations
  • Logic gates


A diode is a semiconductor device that allow current to flow in only one direction. Depending on the type of diode, it has uses as voltage rectifier, indicator and regulator.

Theory checklist:
  • The P-N junction
  • Forward and reverse bias
Modes of operation:
  • Rectifier
  • Diode as reference voltage
  • The zener voltage regulator
  • The led diode indicator
  • The varicap variable capacitor
  • The photodiode light sensor


Storing charge - the capacitor

To understand how another component in electronic circuits works, imagine the following:

Imagine that we have two 'boxes' to put charged particles separated by a piece of plastic. We fill the top one with positive charges and the bottom one with negative charges, only the positive charges are allowed to move. We know that they will try to come together because of the forces generated by opposite charges, but since they cannot get out of the box, the are just stored there not doing any work.

Now imagine that we connect the two boxes with a pipe through where the charges can move. The positive charges will move to meet with the negatives and be in equilibrium. Now that they are moving, there's work and energy being expended that can be put to use.

The device that accomplishes this is called a capacitor. Basically they are two conductor plates separated by an insulator layer, in effect creating the two boxes mentioned above.

When we connect a voltage source to a capacitor, the capacitor is 'empty', with no charges, then charges from the source will start filling it up. As more and more charges reach the capacitor, they will start exerting a force on the charges trying to come in from the source, so it will start filling slower and slower.

Once the capacitor is filled, no more charges flow from the source to the capacitor. If the voltage source is removed, the charges the capacitor has remain there, waiting for something to allow them to meet with opposite charges to reach equilibrium.

If we connect a resistor across, the potential difference created by the separated charges in the capacitor allow it to function as a voltage source, so these charges start flowing through the resistor. As more charges flow, the capacitor starts emptying, causing less potential difference over time, until it can no longer provide charges and the current flow stops.

One way to picture the charging and discharging of a capacitor is to think of a balloon with two mouths, one connected to an air pump and the other left open simulating the resistor through which charges escape. The pump will inflate the balloon to a certain pressure that will be kept constant by the air that escapes the balloon through the other mouth.

If a sudden increase in pressure from the pump occurs, the balloon will inflate more but the air coming out will remain at about the same level, increasing until the air that comes in is the same air that comes out. If the increase in pressure is short, the balloon will inflate and deflate quickly, and the air coming out would remain almost the same throughout.

Same happens with capacitors, when a sudden spike in voltage occurs, the capacitor stores the charges and the voltage in it rises slowly, outputting about the same current throughout the process. This property gives capacitors most of its uses with direct current circuits.



Resistors are electronic and electric components that oppose the flow of current in a circuit. They are made from relatively poor conductors but that don't stop current from flowing altogether.

There are many kinds of resistor constructions, each suited for many purposes that overlap.

The simplest form is a cylinder of carbon material with two connection leads attached at both sides. The diameter and length of the cylinder, as well as the carbon composition of the filling determine the resistance. In general, a longer cylinder has more resistance than a shorter one, and a thicker one will have less resistance than a thin cylinder.

The apparent counter intuitive nature of a thick resistor having less resistance lies in how current flows in a circuit: it will always look for an easier path, and with a thick resistor with more overlapping paths, current has a higher chance of finding an easier path than in a limited and crowded thin resistor.

Another construction method is to coat a ceramic core with a resistor material and shape it in the form of a spiral by removing some of the material along the edge of the spiral. Since this method effectively increases or decreases the length of the resistor material, resistance can be carefully selected and determined.

High power resistances use that same method but instead of resistor material covering a core, resistive wire is used to allow for better heat handling.

Resistors have a standard color code that reflects the value of the resistance of the component. It consists of four color bands, the first two represent numbers and the third represents the number of zeros to add at the end of such number (more on the color code).

Series and parallel resistors

Series when only two components, in this case resistors, share only one of their connections; It could also be described as connecting one resistor after the other forming a chain.

From the construction characteristics of resistors, we can see that when we connect resistors in series, we are effectively creating a single, longer resistor, so what happens with the total resistance?

Simple, they are added together.

For example, we have a square tube we will fill with water. If we wanted to know the volume, we multiply base times height of the water in it to get the volume of water we put in. We measure separately the volume of a one by one cube of water and another of one by two, and get 1 and 2 respectively. We then fill tube with both, how much volume is the water in the tube?

We only put in 3 units of volume, and if we know that none leaked out of the tube, there can be no less than 3 units. So in effect the volumes add together.

Now the volume can be thought as the resistance, put two resistors into a single line and their resistances add up. No math involved, although there's a math proof of this derived from ohm's law.

Parallel is when two or more components share both of their connections together.

What happens with the resistance in parallel circuits? It happens something similar as having a thicker resistor, but not for the same reasons.

Imagine a circuit with one voltage source and two resistors in parallel, both resistors draw current from the source. From the point of view of the source, providing more current to the circuit is the same as providing current to a lower valued resistance, following I = V/R. To know exactly how much resistance the source 'sees' we have to do some math.

It = V/R1 + V/R2 : where It is the total current supplied by the source, R1 and R2 the respective resistances.
V/Rt = V/R1 + V/R2 : We replace It with V/Rt, since we want to know the total resistance the source 'sees'
1/Rt = 1/R1 + 1/R2 : Divide both sides by V

From this last formula we see that the inverse of the resistance is what's added thogether. The formula can be further worked to result in a simple, easy to remember formula.

1/Rt = (R1+R2)/(R1*R2)
Rt = (R1*R2)/(R1+R2)

Note that this only works for two resistors.

Voltage Follower Circuit

Two examples of the most common types of Voltage followers (buffers). You can find some theory behind them in our amplifier gain and buffer amplifier pages.

Transistor voltage follower:

This first circuit is a very simple one transistor voltage follower. Consist of two biasing resistors, and one other resistor at the emitter to acquire the output voltage from.

How it works:

The first to resistors connected to the transistor's base are forming a voltage divider, in order to set a biasing point for the transistor to work in our desired range. Then the transistor, our gain component for the circuit which in this case is only used as a gateway to isolate two circuit stages.

The resistor in the emitter is used to create a voltage from the current passing from the transistor; Without it we can't get any voltage as our output would be effectively shorted to ground (0 volts).

The capacitors that are displayed in the schematic are optional, but very useful to prevent a wrong operation of the circuit, specially in audio or high frequency uses. they stop any DC voltage to move or otherwise disrupt the bias point of the transistor, thus causing undesired operation. If you build this circuit only with dc remove the capacitors, as they will prevent the circuit from functioning under those conditions.

Transistor voltage follower

Op Amp Voltage Follower:

This circuit's operation is far more predictable and stable than the transistor version, and also requires less external components.

How it works:

Works as described above, no external elements to explain. This circuit uses feedback to maintain the voltage output the same as the input. Note that this schematic does not display power, ground and other connections for the op amp, these vary widely among manufacturers and op amps so refer to your op amp's datasheet for pinouts and power connections.

Opamp voltage follower

Buffer amplifier

A buffer amplifier, or simply a buffer, is an electronic amplifier that is designed to have an amplifier gain of 1. Buffers are used in Impedance matching, the benefit of which is to maximize energy transfer between circuits or systems.

There are two main kinds of buffer circuits, Voltage buffers and Current buffers. The purposes of each is to isolate the mentioned characteristic to avoid loading the input circuit or source from the output stage.

Another name by which buffer amplifiers are known as is a voltage follower. The name is given because of the characteristic of the amplifier to output a signal of the same amplitude as the input (given the unity gain [gain of 1 or 0dB] ).

Examples of Buffer amplifiers:

The examples are too many to mention in this page, the most common being the transistor voltage follower and op amp version of it. The exact characteristics, formulas and construction instructions can be found on the specific component's page.

Amplifier Gain

This is one the main characteristics to determine when designing or choosing an amplifier. This is a measure of the increase (or decrease in case of negative gain) the amplitude of the input signal.


There are a few different ways to represent amplifier gain. One of the more common way among beginners and hobbyists, specially for DC or small signals, is to describe gain as the ratio of input vs output amplitude:

Gain = Vout/Vin, Where both input and output are either voltage or current (amperage).

Another way to represent amplifier gain is using a logarithmic decibel scale (dB). This representation is calculated using the ratio of input/output powers using the formula:

Gain = 10log(Pout/Pin)

Utility of gain:

There are countless applications and uses for amplifiers, since in the electronics world most signals we get from sensors or transmission lines is very small. There are also other times when not the amplitude of the signal is required but its power to transform into useful work, like when powering a motor, transmitting a radio signal and displaying an image on a screen.

How to calculate:

The specifics of how much gain can an amplifier have depend heavily on the components or circuits used, as well as the topology (configuration) of the amplifier. You can have a better understanding of the formulas used for each component and configuration by going to the specific page.

Welcome To Electronic Circuits For Beginners!

All circuits included here are recommended to be assembled in printed circuit boards. Printed circuit boards, or PCB's increase the circuit reliability and mechanical stability.

Circuits quick links:


Simple FM Transmitter


Led Flasher Circuit


Quadrocopters for beginners


Voltage follower circuit


Beginners Audio Amplifier


Led chaser circuit


Tone generator circuit


H bridge circuit


Simple power supply


Beginner Electronics mini course index


All circuits include parts list and complete How-it-works for beginners and hobbyists to easily understand.