Introduction

Most electronic devices today use both analog and digital components. This blog focuses on the analog side. So what is considered analog and what is considered digital?

Analog and digital describe how information is represented, not whether a component is physical. Analog signals vary continuously over a range, while digital systems interpret signals as discrete states. The human body can be a loose analogy, but the brain is not literally a digital component.
Both analog and digital circuits are physical. They often work together: a sensor produces an analog voltage, an ADC converts it into numbers, and a processor handles the numbers.

Analog circuits use both passive and active components. Passive components dissipate or store energy without power gain. Active devices can control energy from a supply and amplify signals; they do not create energy. Passivity and linearity are different properties. Passive components can have linear or nonlinear behavior. Ideal fixed resistors, capacitors and inductors are linear, while a diode is nonlinear. Active components can form circuits with approximately linear behaviorwith suitable bias and feedback, even though the component itself is nonlinear.

Analog circuit design develops electronic circuits that process continuous-time signals, such as voltage, current or sound waves, in analog form. It has applications in telecommunications, power electronics, audio engineering, biomedical engineering and instrumentation. Examples include:

• Amplification: Analog circuits amplify signals in applications such as audio, radio-frequency and instrumentation amplifiers
• Filtering: Analog circuits filter unwanted signal frequencies, as in audio or radio-frequency filters
• Power management: Analog circuits are used in power-management applications such as voltage regulatorsto regulate and stabilize supplies for electronic devices
• Sensor interfaces: Analog circuits interface with sensors producing analog signals, such as temperature, pressure and light sensors
• Data conversion: Analog circuits convert analog signals into digital signals, as in analog-to-digital converters (ADCs)

 

Passive components

Passive components do not provide power gain: they dissipate energy or store and return energy from the circuit. Resistors, capacitors and inductors are standard examples. Diodes are often classified as passive nonlinear devices, although terminology varies. These components control current, store energy and filter signals. We review them below.
The symbol for each component is shown at the beginning of its section.

Resistors

A resistor is a passive electronic component used in electrical circuits to oppose electric current.

A resistor can reduce the amplitude of current and voltage signals flowing through or appearing across it, and is often used to control circuit current. Our blog on Ohm's lawexplains this. Take a look if you have not already!
Resistors are also used to dissipate energy as heat or provide a specific voltage drop in a circuit. We learned about voltage drops here: Circuit Theory. 

As shown above, resistors are usually represented by a zigzag or rectangle in a circuit diagram and come in values from a few ohms to several megaohms. A higher value in ohms means higher resistance. They use resistive materials such as carbon (graphite film) or metal film deposited on a porcelain cylinder or tube and their resistance is indicated by colored bands or a value printed on the component.

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

 

Capacitors

A capacitor is a passive component used to store electrical energy in an electric field. It temporarily stores electric charge and can smooth fluctuations in a circuit, filter signals or store energy for later use. It can therefore store energy already present in the circuit and return it when needed.

Capacitors are usually represented by two parallel plates and are available in a wide range of values, measured in farads, F. A higher capacitance means a greater amount of energy can be stored.

Polarized capacitors have a positive terminal, called in some types the anode , and a negative terminal, the cathode . Always follow the component markings and datasheet; lead length is not a universal rule. Incorrect polarity or excessive voltage can cause heating, leakage, rupture or fire. Even small capacitors must not be treated as harmless. The symbol for a polarized capacitor is:
 

Electrolytic capacitors are a common polarized type; some special electrolytics are nonpolar. Polarized types must be connected correctly. Nonpolar capacitors can be connected either way, but 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. Read capacitance and ratings from the markings or datasheet, not size alone. Applications include supply filtering, signal coupling, energy storage, AC/DC conversion, timing and signal smoothing. Impedance describes a circuit's frequency-dependent relationship between voltage and current, including resistance, reactance and phase. It is not simply ordinary resistance. So what is an inductor?

 

Inductors

An inductor is a passive component used to store energy in an electromagnetic field. It temporarily stores magnetic energy and can filter signals, store energy for later use or generate magnetic fields. Inductors come in values from a few microhenries to several henries, H.

They are made from a coil of wire, such as copper, wound around a core of high magnetic permeability, such as iron or ferrite. Inductance depends on the number of turns, the coil's cross-sectional area and the core material.

Inductors play an important role in many electronic circuits. They are used in power supplies and in the design of transformers, motors and generators. Like capacitors, they also feature in wireless technology such as wireless power transfer!

Diodes

A diode mainly conducts current in one direction and is used for rectification and protection against reverse polarity, among other things. Its symbol identifies anode and cathode, with a bar at the cathode; do not confuse it with the op-amp triangle. Types include rectifier diodes, Zener diodes and LEDs. In forward operation the anode is more positive than the cathode; the right connection depends on the circuit.

They use semiconductor materials such as silicon or germanium. The forward voltage drop is the voltage drop at a specified current and temperature. Forward current increases continuously with voltage; there is no perfect threshold with exactly zero current below it. A small leakage current normally flows in reverse bias. Exceeding the rated reverse voltage can cause breakdown and damage. Zener diodes are designed for controlled breakdown with current limiting.

Diodes are used in power rectification, voltage regulation and signal detection. As mentioned, converting AC to DC is called rectification (rectifier) and often uses four diodes working together. This rectifier converts an AC signal to a DC signal; a basic example is shown below in figure 1. The red part is the AC generator symbol, and R_L is often used to denote the load in a circuit.

Figure 1: Illustrates a full bridge rectifier with its input and output signals.

A bridge rectifier 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, not solely from a stylized waveform. Figure 2 shows the added capacitor.

Figure 2: Illustrates a full bridge rectifierwith a capacitor to smooth the output signal.

Diodes are also used to generate square-wave signals and protect other circuit components.

 

Active components

Active components use energy from a supply to control current or provide signal gain; they do not create energy. Examples include transistors, op-amps and voltage regulators. They can amplify signals, switch current and support signal generation, processing and power control. An active circuit can contain several components, but a full bridge rectifierbuilt only from diodes does not thereby become an active amplifier.

 

 Transistor

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 used to amplify or switch electrical signals. It controls circuit current and can amplify weak signals, switch high-power signals or perform digital operations. Types include bipolar junction transistors (BJTs) and field-effect transistors (FETs). They use semiconductor materials such as silicon or germanium and are represented by a symbol indicating transistor type and configuration.

BJTs and FETs are two types of transistor used in electronic circuits. They perform similar functions but have important differences. We focus mainly on BJTs to illustrate the overall operation of transistors. Here are some similarities and differences.

•  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 general rule that BJTs have lower gain or efficiency than FETs; it depends on device, bias and design.
• Both BJTs and FETs have polarity and voltage limits. FET terminals cannot be freely interchanged, and MOSFET gate-oxide limits must be respected.
• Voltage capability is determined by 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 it is a BJT or FET.

Both BJTs and FETs have advantages and disadvantages. Selection depends on required operating voltage, gain, input impedance, speed and cost.

 

BJT

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 does not apply without qualification in saturation or breakdown; collector-emitter voltage and ratings matter. Collector current is denoted I_c.

 

A BJT can control current through a load, but load placement and the bias network depend on circuit topology. Provide appropriate base-current limiting and respect voltage, current and dissipation limits. The valve analogy is only an illustration of control.

Other transistor types include junction field-effect transistors (JFETs) and metal-oxide-semiconductor field-effect transistors (MOSFETs). They have similar functions but important differences, as with BJTs and FETs. There are many types for different applications! Transistors perform amplification, switching, digital logic (covered in our blogs on digital circuits and microcontrollers ) and power control.

 

Operational amplifiers (op-amps)

An operational amplifier is an integrated circuit used as an amplifier. Its main purpose is to amplify small voltage signals into a larger output voltage, with applications in amplification, filtering and signal conversion.

Op-amps are normally shown as a triangle with inverting and non-inverting inputs and one output. Supply terminals must also be connected correctly even if omitted from a simplified symbol. There are single-supply, dual-supply and rail-to-rail types. Internal circuitry may use BJTs, FETs or both; gain, noise and other properties depend on the design.

Op-amps are used for amplification, filtering, signal conversion and control. Suitability for comparator use must be assessed separately. Here we focus on main functions rather than internal circuits. (+) and (−) at signal inputs mean non-inverting and inverting input, not positive and negative supply. Within the valid operating range, an open-loop equation applies to output V_out. V_out has an equation describing this gain:

A in the expression above is the gain, which is unlimited for an ideal op-amp. An ideal op-amp is a theoretical model with certain ideal characteristics. No real device achieves all of them, but they help us understand behavior and analyze op-amp circuits.

The equation above says that voltage V_out equals the 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 (−). High open-loop gain means even a small input difference can drive a real output into saturation. The output is limited by supply and output swing. Negative feedback is normally used to set a useful closed-loop gain.
We will cover 2 basic principles for an ideal op-amp that help explain how and why it works.

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; the inputs are not physically shorted.

The approximation does not apply to an open-loop comparator, saturated output or unstable feedback. A voltage difference between ideal inputs does not force current into them: ideal input impedance is infinite. In a real circuit, check common-mode range, output swing and stability. This leads to the second principle.

Second principle
The second principle says that no current flows through the input terminals. There is no input current into V_in (−) or 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 analysis using Kirchhoff's laws in the external feedback network. The model does not account for every real-world error.

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 measured difference between the inputs. Include offset and drift in the error budget when accuracy matters.

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

Slew rate
Slew rate is the maximum rate of output-voltage change, typically expressed in V/µs. It is a slope, not a rise or fall time. Internal currents and capacitances limit this rate. Large or fast signals can become slew-rate limited even if small-signal bandwidth seems adequate. Choose according to both amplitude and frequency.

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 different limits. Exceeding either can cause attenuation or distortion. Check both, along with stability and load requirements.

That is enough for an initial understanding of op-amps! They appear throughout electronic circuits, and with this foundation they become easier to work with and extremely useful.

Voltage regulator

A voltage regulator keeps output near a target within specified input, load and temperature limits. Some linear regulators have three pins for input, output and ground, but other pin counts and external-component requirements exist. Protection features vary; regulation does not guarantee protection from every overvoltage or undervoltage.

Voltage regulators come in many types, including linear, switching and Zener-diode regulators. They combine 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 (pulse-width modulation) or other control methods to stabilize output voltage. 

Voltage regulators are widely used in power supplies, voltage-reference circuits and voltage-protection circuits. 

We now better understand what some analog components consist of and how they affect circuit voltages and currents. It can quickly become complex, and there is still much to learn! This introduction to analog circuit design gives us a foundation to build on and expand our understanding of electronics.