Introduction

Most electronic devices today work with analog and digital components working together.
Analog and digital describe how information is represented, not whether a component is physical. Analog signals vary continuously over a range; digital systems interpret signals as discrete states. A human-body analogy can be a loose illustration, but the brain is not literally a digital component. Both analog and digital circuits are physical, and they often work together: a sensor produces an analog voltage, an ADC converts it into numbers, and a processor works with those numbers.

Analog circuits use both passive and active components. Passive components dissipate or store energy without providing power gain. Active devices can control power from a supply and provide amplification; they do not create energy.
Passivity and linearity are different properties. Ideal fixed resistors, capacitors and inductors are linear, while a diode is nonlinear despite often being classified as passive. Active devices such as transistors are nonlinear, but circuits with feedback can operate approximately linearly over a specified range. For more about linear and nonlinear behavior, see here!

Analog circuit design is used to design and develop electronic circuits that process continuous-time signals, such as voltage, current, or sound waves, in an analog format. Analog circuit design has a wide range of applications in various industries, including telecommunications, power electronics, audio engineering, biomedical engineering, and instrumentation. Some examples of applications of analog circuit design are:

• Amplification: Analog circuits are used to amplify signals in a wide range of applications, such as audio amplifiers, radio frequency amplifiers, and instrumentation amplifiers.
• Filtering: Analog circuits can be used to filter out unwanted frequencies in signals, such as in audio or radio frequency filters.
• Power management: Analog circuits are used in power management applications, such as voltage regulators, to regulate and stabilize power supplies for electronic devices.
• Sensor interface: Analog circuits are used to interface with sensors that produce analog signals, such as temperature sensors, pressure sensors, and light sensors.
• Data conversion: Analog circuits are used to convert analog signals into digital signals, such as in analog-to-digital converters (ADCs).

Passive components

Passive components do not provide power gain: they dissipate energy or store and return energy supplied by the circuit. Resistors, capacitors and inductors are standard examples; diodes are often classified as passive nonlinear devices, although terminology varies. They control current, store energy and filter signals. The following sections review these components and show their circuit symbols.

Resistors

A resistor is a passive component that opposes current flow. Its circuit symbol is shown below.

It has the effect of reducing the amplitude of the current and voltage signals that flow through it, and is often used to regulate the flow of current in a circuit. Remember when we talked about Ohms law. Be sure to check it that blog out if you haven’t already! It is also used to dissipate energy as heat, or to provide a specific voltage drop in a circuit. We learned about voltage drops here Circuit Theory. 

As shown on the picture above resistors are typically represented in a circuit diagram by a zigzag symbol or by a rectangle, and are available in a wide range of values, ranging from a few ohms to several megaohms - the bigger the value for ohm the higher the resistance. They are constructed from materials that have a high resistance to the flow of electric current, such as carbon or metal alloys, and their resistance value is indicated by colored bands or by the value printed on the component itself - this is also showcased in the picture of the resistor above.

Resistors set operating conditions and limit current to protect components. A short circuit is an unintended low-resistance path; it can draw a dangerously large current, limited by the supply and wiring, and cause heating or damage. It does not mean that current stops. Resistance also occurs in wires and other components. Capacitors and inductors additionally introduce frequency-dependent reactance.

Capacitors

A capacitor is a passive electronic component that is used in electrical circuits to store electrical energy in an electric field. It has the effect of temporarily storing electric charge and can be used to smooth out fluctuations in a circuit, filter signals, or store energy for later use.
It’s kinda like a little storage for energy in circuits, that can be used when needed!

Capacitors are typically represented in a circuit diagram by two parallel plates separated by a gap, and are available in a wide range of values, ranging from a few picofarads to several microfarads, F. The higher value for F the higher the capacitance of the capacitors.

Polarized capacitors must be connected with the polarity and voltage rating specified by their manufacturer. Lead length can help on some through-hole parts, but markings and the datasheet are authoritative. Reverse voltage or overvoltage can cause heating, leakage, rupture or fire; never treat a small capacitor as harmless. The symbol for a polarized capacitor is:

Electrolytic capacitors are a common polarized type; some special electrolytics are designed to be nonpolar.
Nonpolar capacitors can be connected either way around, but their voltage, temperature and ripple-current limits still apply.

Capacitors use a dielectric such as ceramic, plastic film or an oxide layer; tantalum capacitors use tantalum oxide as the dielectric. Read the capacitance, voltage rating and other limits from the marking or datasheet, not physical size alone.

Capacitors are used in many electronic circuits, especially in applications such as power supply filtering, signal coupling, and energy storage. They are also used in AC/DC power conversion and in applications such as radio frequency filtering, timing, and signal smoothing.
The frequency-dependent opposition of capacitors and inductorsis described using impedance. Impedance combines resistance and reactance and includes phase information; it is not simply ordinary resistance.

Inductors

An inductor is a passive electronic component that is used in electrical circuits to store energy in an electromagnetic field. It has the effect of temporarily storing magnetic energy and can be used to filter signals, store energy for later use, or generate magnetic fields.

Inductors are typically represented in a circuit diagram by a coil symbol and are available in a wide range of values, ranging from a few microhenries to several henries.

They are constructed from a coil of wire, such as copper, wound around a core made of materials with high magnetic permeability, such as iron or ferrite. The inductance value of an inductor is determined by a few things. The number of turns in the coil, the cross-sectional area of the coil, and the type of core material used are all considered for the inductance value.

Inductors play a crucial role in many electronic circuits, especially in applications such as filtering, energy storage, and the generation of magnetic fields. They are also used in applications such as power supplies, and in the design of transformers, motors, and generators. And like capacitors, these are used for wireless power transfer!

Diodes

A diode is a passive electronic component that is used in electrical circuits to allow electric current to flow in only one direction. Like a one way street! It acts as a one-way valve for electric current, and is often used to convert alternating current (AC) to direct current (DC), to protect other components in a circuit from damage due to reverse voltage, or to rectify signals.

The diode symbol identifies an anode and a cathode, with a bar marking the cathode. Do not confuse it with the triangle used for an op-amp. Types include rectifier diodes, Zener diodes and LEDs. In forward operation the anode is at a higher potential than the cathode; the required connection depends on the circuit. 

Diodes use semiconductor materials such as silicon or germanium. Forward current rises continuously with forward voltage; there is no perfect threshold below which current is exactly zero. The quoted forward voltage is specified at a particular current and temperature. In reverse bias, the cathode is more positive than the anode and a small leakage current normally flows. Exceeding the reverse-voltage rating can cause breakdown and damage; Zener diodes are specifically designed for controlled breakdown with current limiting.  

Diodes are used in applications such as power rectification, voltage regulation, and signal detection. And as mentioned earlier, they are also used in applications such as rectifying AC to DC - this is called a diode rectifier and most often consists of four diodes working together. This rectifier converts an AC-signal to a DC-signal and a basic example of the circuit and input and output is shown below, see figure 1. The red part is the symbol for an AC-generator and R_L are often used to showcase the load in a circuit.

Figure 1: Illustrates the basic circuit of the full bridge rectifer.

A full bridge converts AC into a unidirectional but pulsating output. A smoothing capacitor charges near the peaks and supplies the load between them, reducing ripple. Whether a capacitor is present must be determined from the schematic rather than inferred solely from a stylized waveform. Figure 2 shows the added capacitor.

Figure 2: Illustrates the circuit of the full bridge rectifer that includes a capacitor to smoothing the output.

Visit this site to learn more about this rectifier.

Diodes are also used for generating square waves, and providing protection to other components in a circuit from voltage spikes or reverse voltage. They can act as the protectors of the other components but also bring other skills to the table as well!

Active Components

Active components use energy from a supply to control current or provide signal gain; they do not create energy. Transistors, op-amps and regulator ICs are examples. They can amplify signals, switch current and support signal generation, processing and power control. An active circuit can contain several components working together but a bridge rectifier built only from diodes is not thereby an active amplifier. 

Transistors

The symbol for a BJT, bipolar junction transistor
C stands for collector
B stands for base
E stands for emitter

A transistor is an active electronic component that is used in electrical circuits to amplify or switch electrical signals. It has the effect of controlling the flow of current in a circuit and can be used to amplify weak signals, switch high-power signals, or perform digital operations.

Transistors are available in a wide range of types, including bipolar junction transistors (BJTs) and field-effect transistors (FETs). They are constructed from semiconductor materials, such as silicon or germanium, and are typically represented in a circuit diagram by a symbol that indicates the type of transistor and the configuration in which it is being used.

Bipolar Junction Transistors (BJTs) and Field-Effect Transistors (FETs) are two different types of transistors that are used in electronic circuits. While both types of transistors perform similar functions, such as amplification and switching, there are several key differences between BJTs and FETs.
We will focus more on BJTs as we wish to display the overall function of transistors in this blog.
But here are some quick similarities and differences between the two.

• A BJT requires base current, while a FET gate has high DC input impedance; actual input impedance depends on the circuit and frequency.
  There is no universal rule that BJTs have lower gain or lower amplification efficiency than FETs. Gain depends on the device, bias and circuit.
• Both BJTs and FETs have polarity and voltage limits. FET terminals are not freely interchangeable, and MOSFET gate-oxide limits must be respected.
• Voltage capability depends on the particular device rating; high-voltage devices exist in both families.
• Switching speed depends on device structure, drive circuitry and operating conditions, not simply whether the transistor is a BJT or FET.

Both BJTs and FETs have their own advantages and disadvantages, and the choice of which type of transistor to use in a particular application depends on a number of factors, including the required operating voltage, gain, input impedance, speed, and cost.

BJT

In a bipolar junction transistor (BJT), the flow of current is controlled by the application of a small voltage to the base terminal. This voltage controls the flow of current between the emitter and collector terminals, allowing the transistor to act as an amplifier or switch.

For a BJT in its forward-active region, collector current depends approximately exponentially on base-emitter voltage, and base current must also be supplied. This relationship does not apply without qualification in saturation or breakdown; collector-emitter voltage and device ratings matter. Collector current is denoted I_c.

A BJT can control current through a load, but the load placement and bias network depend on the chosen circuit topology. Provide appropriate base-current limiting and keep voltage, current and dissipation within the device ratings. The valve analogy is useful only as an illustration of control.

There are other types of transistors as well, such as the Junction Field-Effect Transistors (JFETs) and Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs). These two different types of field-effect transistors both perform similar functions, such as amplification and switching, but there are some key differences between JFETs and MOSFETs, which are similar to the differences between BJT and FET. But just be aware that there are many different types of transistors all for different applications!

Transistors are used to perform a wide range of functions, such as amplification, switching, digital logic (which we will cover in these blogs: digital circuits and microcontroller), and power control.

Operational amplifiers (Op-Amps)

An operational amplifier (op-amp) is a type of integrated circuit that is used as a high-gain amplifier in electrical circuits. Its main objective is amplifying small voltage signals to produce a larger output voltage and can be used for a wide range of applications, including amplification, filtering, and signal conversion.

Op-amps are typically represented in a circuit diagram by a triangle symbol with multiple input and output terminals, and are available in a wide range of types, including single-supply op-amps, dual-supply op-amps, and rail-to-rail op-amps. Through this blog we will get a basic understanding of the op-amp, so the symbol of the op-amp above, with two inputs and one output is sufficient at the moment.

Op-amps may use bipolar, FET or mixed internal circuitry. Their gain, noise and other properties depend on the design.

Op-amps are used to perform a wide range of functions, such as amplification, filtering, signal conversion, and control. They are widely used in applications such as amplifiers, filters, comparators, and control circuits, and are an essential component in many analog and mixed-signal circuits. There are many different types of op-amps but as with the transistors, they all serve the same main function but for different applications.

The op-amp consists of somewhat complex circuits. So let's continue with a simple introduction of how op-amps work. We will won't discuss the circuits found within an op-amp but we will concentrate on their main functions.

Connect the supply pins correctly. The (+) and (−) signal inputs are the non-inverting and inverting inputs, not positive and negative supply pins. Within its valid operating range, the open-loop relation for output V_out. V_out is:
 

A is here the amplification value (the gain value), and for an ideal op-amp this value has no limit. What is an ideal op-amp? An ideal op-amp is a theoretical model of an op-amp that has certain ideal characteristics. In practice, no op-amp can achieve all of these ideal characteristics, but they are useful in understanding the behavior and analyzing the performance of op-amp circuits.

The equation says that output V_out equals open-loop gain multiplied by the difference between V_in (+) and V_in (−). The differential input is the difference between V_in (+) and V_in (−). A large open-loop gain means that even a small differential input can drive a real output into saturation. The output cannot exceed its supply and output-swing limits. Negative feedback is normally used to set a useful closed-loop gain.

We are going to go through 2 basic principles for an ideal op-amp that will help us understand how and why it functions.

First principle
For an ideal op-amp operating linearly with stable negative feedback, the voltages V_in (-) and V_in (+) are equal in the ideal limit.
This virtual-short approximation follows from infinite open-loop gain with finite output and negative feedback. It does not mean the inputs are physically connected.
The approximation does not apply to an open-loop comparator, a saturated output or an unstable feedback circuit. A voltage difference between ideal inputs does not force current into them: their ideal input impedance is infinite.
For a real op-amp, high but finite gain can make the differential input small in a correctly designed linear feedback circuit. Check common-mode range, output swing and stability before using this approximation.
This takes us to principle two.

Second principle
The second principle says: There's no current into the inputs. So theres no input current into V_in (-) or in V_in (+). Each ideal input current is zero independently; current does not flow into one input and out of the other. Together with the conditional virtual-short approximation, this simplifies circuit analysis by applying Kirchhoff’s laws to the external feedback network.
Okay, so now we have covered the two first principles, which are the most important ones. If thats all we need then great. But as we are dealing with an ideal op-amp this doesn't take into account a lot of the real life issues that can occur.

Offset Voltage
In practical use V_in (-) and V_in (+) may require a small compensating differential voltage to obtain the intended zero output. Input offset voltage models this input-referred error; it is not simply any observed difference between the input voltages. Include offset and its drift in the error budget when accuracy matters.

Input Bias Current
Real inputs draw bias currents. These currents create voltage errors across source and feedback resistances. Their significance depends on the amplifier, resistance values and required accuracy; they cannot always be ignored.

Slew Rate
Slew rate is the maximum rate of change of output voltage, typically expressed in V/µs. It is a slope, not a rise or fall time.
Internal charging currents and capacitances limit how quickly the output can change. Large or fast signals may become slew-rate limited even when small-signal bandwidth appears sufficient. Select a device suitable for the required signal amplitude and frequency.
 

Bandwidth
Another practical limitation is small-signal bandwidth.
Closed-loop gain and frequency response depend on the op-amp’s gain-bandwidth behavior and feedback network. Small-signal bandwidth and large-signal slew rate are distinct limits; exceeding either can distort or attenuate the signal. Check both, together with stability and load requirements.

Okay, this will do for the understanding of Op-amps! Op-amps are all over the place in circuits and once we have this foundational understanding of the op-amp they become easier to work with and they will certainly become incredibly helpful.

Voltage regulators

A voltage regulator maintains a target output within specified input, load and thermal limits. Some familiar linear regulators have three pins, but regulators can have other pin counts and require external components. Protection features vary; regulation does not guarantee protection against every overvoltage or undervoltage condition.

Voltage regulators are available in a wide range of types, including linear regulators, switching regulators, and Zener diode regulators. They are constructed from a combination of transistors, diodes, and other components.

Linear regulators control a pass element and dissipate the input-output voltage difference as heat. Switching regulators transfer energy through switched components and often use pulse-width modulation or other control schemes to regulate output voltage.

Voltage regulators are widely used in applications such as power supplies, voltage reference circuits, and voltage protection circuits.

Okay, now we have a better understanding of what some analog components consist of and how they affect the voltages and currents in a circuit. You can imagine that it quickly can get somewhat complex, and that there still is much to learn! But with this introduction to analog circuit design, we are getting smarter and we can build upon that and broaden our understanding of electronics!