A stereo audio amplifier.

An amplifier is an electronic circuit or device that increases the strength of an electric signal. Amplifiers are frequently referred to as ‘amps’ for short.

Try not to get them confused with amperes, the unit of electric current, which are also referred to as ‘amps’. You may be familiar with amplifiers that are used in acoustics, like guitar or microphone amplifiers.

Instrument amplifiers may be popular, but when we use the term ‘amplifier‘, we are referring to any circuit or system that is designed to increase the strength of an electric signal. The signal could come from an acoustic source, like a microphone, or the pickups on an electric guitar, but it could also have nothing to do with acoustics.

While amplifiers can be standalone devices, they can also be embedded into other, larger, systems. In fact, virtually every electronic device contains an amplifier!

In this series of articles, we’ll cover the basics of amplifiers, and how to think about and classify amplifiers. We’ll supercharge our knowledge of circuits along the way.

There are many different types of amplifiers, from simple amplifiers that are based on single transistors, to complex and precise amplifiers that use many components.

There are so many different types of amplifiers that learning about them can seem daunting. There are also lots of different ways to classify amplifiers into different types based on circuit design and modes of operation.

This makes amplifiers one of the most important topics within the broad field of electronics.

A Brief History of Amplifiers

The first amplifier was invented in 1912, and used a triode vacuum tube (a triode is the vacuum tube predecessor of the transistor). By 1914, the man who invented the triode, Lee De Forest, had invented the first audio amplifier. Audio signal processing was one of the first uses of the amplifier, and the two are still closely coupled today, with the word ‘amplifier’ often bringing to mind one of the many kinds of audio amplifiers around.

When it was first developed, the triode amplifier was unique in its’ ability to use the strength of one signal to control the strength of another. The closest invention at the time was a relay, which is an electrically operated switch. A relay allowed a small signal to turn a stronger signal on or off, but did not allow control of the stronger signal while it was on. The triode therefore presented a great leap forward in technological capability, and also didn’t have mechanical parts that would quickly wear out like the relay.

The triode was originally called an electron relay, but the term ‘amplifier’ emerged in common usage by 1915. It was revolutionary: the first true electronic circuit, it quickly impacted telegraphy and telephony, radio, radar, and computers.

When transistors were invented, they quickly took the place of vacuum tubes in many applications. Vacuum tube computers became transistor-based, allowing the computer revolution predicted by Moore’s Law. Vacuum tube radios became transistor radios, spawning an era of radio-based entertainment and popular music.

If the subject of amplifiers seems daunting or complex, keep in mind that you’re in good company: even De Forest himself had a limited understanding of how his own triode amplifier worked.

Amplifier Gain

The first important topic to understand about amplifiers is gain.

Amplifiers are characterized by their gain, which is the amount of amplification that they provide. Gain is the ratio of the output divided by the input values of voltage, current, or power.

Gain = \frac{Output}{Input}

Gain is a unitless (or dimensionless) quantity. It doesn’t have units of volts, amps, or watts even though the value of gain may be used to characterize a ratio of volts, amps, or watts. In other words, gain is just a number.

Since an amplifier can increase the amounts of either voltage, current, or power, there are therefore three different types of gain: voltage gain, current gain, and power gain.

Voltage Gain

Voltage gain is the ratio of the output voltage divided by the input voltage. An amplifier is used to increase the voltage of the signal, so the voltage gain of an amplifier should always be greater than one (1).

The voltage gain is commonly written as AV :

A_V=\frac{Voltage \, Out}{Voltage \,In}=\frac{V_o}{V_i}

Note that in AV, that ‘A‘ stands for amplification, and ‘V‘ stands for voltage.

Current Gain

Current gain is the ratio of the output current divided by the input current. Like voltage gain, the current gain of an amplifier should always be greater than one (1).

The Current gain is commonly written as AI :


Note that in AI, that ‘A‘ stands for amplification, and ‘I‘ stands for current.

Power Gain

Power gain is the ratio of the output power divided by the input power. As with the others, the power gain of an amplifier should always be greater than one (1).

Power gain is commonly written as either AP or G :

A_P=G=\frac{Power\, Out}{Power\,In}=\frac{P_o}{P_i}

Note that in AP, that ‘A‘ stands for amplification, and ‘P‘ stands for power. The alternate designation, ‘G‘, stands for gain.

Inverting Amplifiers

Some amplifiers are designed to amplify as well as invert a signal; such circuits are called inverting amplifiers.

An inverting amplifier will accept a positive signal and output a negative signal, or take a negative signal and output a positive signal.

The gain of an inverting amplifier is therefore expressed as a negative number.

For example, an inverting voltage amplifier with voltage gain of AV = -5 will both invert and amplify at the same time. If we provide this amplifier with a steady DC signal of 1.5V, this amplifier will output -7.5V:

V_o=V_i \times A_V = (1.5V)(-5)=-7.5V

Other important quantities are input impedance and output impedance.

Amplifier Impedance

Impedance is opposition to the flow of electric current in a circuit. In DC circuits, impedance is equivalent to electrical resistance. Resistors contribute to impedance but capacitors and inductors do not. In DC circuits, we often approximate capacitors as functioning as open circuits, and inductors as short circuits.

Things are a little more complex in AC circuits. Capacitors and inductors both allow AC to pass but reduce the flow of current via capacitive and inductive reactance. The total impedance in an AC circuit includes terms for resistance as well as capacitive and inductive reactance.

There are two types of impedance that are associated with amplifiers: input impedance and output impedance.

Input Impedance

Input impedance is the opposition to current flow experienced between the input terminals. Every component of the amplifier contributes to this impedance, but it is also dependent on the frequency of the input signal as well as the amplifier gain.

Input impedance is modeled as an impedance between (i.e. in parallel with) the input terminals.

Output Impedance

Output impedance is opposition to current flow that results in a voltage drop prior to output measurement. It therefore results in an apparent reduction of the output signal.

Output impedance is modeled as an impedance in series with the output.

Limitations of Amplifiers

An ideal amplifier would have no effect other that increasing the signal. They would operate perfectly across all frequencies, with zero distortion or noise of any kind. In reality, there are several limiting factors that dictate amplifier performance in a given setting.

These limiting factors are compliance, distortion, frequency response, and noise.

As with many other types of electronic circuit, amplifiers are carefully selected or designed for specific applications because there is no such thing as an ideal amplifier for every situation.

Amplifier Compliance and Distortion

Compliance and distortion are related to each other.

Compliance is the maximum signal strength that an amplifier is rated for.

Distortion is what happens when the signal strength exceeds the compliance. When distortion occurs, the signal waveform becomes distorted; its shape is changed. A common example is clipping, which is when the top of the peak is removed, leaving the waveform flat whenever the signal strength exceeds the compliance.

Total Harmonic Distortion (THD)

Total harmonic distortion (THD) is a measurement of distortion used to categorize different amplifiers. A pure sine wave, called the ‘fundamental‘, is input to the amplifier. The signal passes through the amplifier and the output is recorded. A filter is used on the output to remove the fundamental, leaving only the distortion.

The THD is found by calculating the percent of the magnitude of the distortion divided by the fundamental.

Amplifier Frequency Response

Real-life amplifiers aren’t as effective at all frequencies. Many that work at low frequencies produce a poor or even attenuated output at high frequencies.

The following image shows a generic frequency response for an AC amplifier (note that the output is zero at a frequency of zero hertz (0 Hz), indicating that the amplifier will not work for DC signals.

The range of frequencies over which an amplifier works well is called the midband. The midband is defined by two ‘corner’ frequencies f1 and f2, which are the points at which the output drops off. f1 is the lower frequency limit, and f2 is the upper frequency limit.

Amplifiers that operate on DC and low frequencies are called low frequency amplifiers.

Most amplifiers are designed for specific applications and feature frequency bands suitable for their application.

Many amplifiers block DC by using capacitors or transformers. It is common to see capacitors at both the input and output, providing an easy way to tell if an amplifier circuit is designed for AC.


Noise, like distortion, is an unwanted part of the output signal. Unlike distortion, noise is not dependent on the strength of the input signal.

A great example of noise is the ‘hum’ that microphones pick up from the ambient environment. Since the microphone will pick up any audio signal it receives and transmit the signal to the amplifier, this noise can be difficult to completely remove. However the noise doesn’t scale with the strength of the input signal. So if one sings loudly, the noise will not be noticeable. In a crowded karaoke environment, this may require one to sing significantly louder than the recording artist needed to sing in the original studio recording. Recording studios use noise reduction and cancellation strategies to allow singers to record high quality vocals even at low volumes.

Noise generally occurs along a wide range of frequencies.

Amplifier Classifications

Active Device Amplifiers

An active electronic device can control the flow of electric current. All amplifiers use at least one active device, and the type of device used is also be used to classify different types of amplifiers.

The two most common active devices used in amplifiers are transistors and vacuum tubes.

A familiar example is that of guitar amplifiers. Most amplifiers today are solid-state, meaning that they use transistors instead of vacuum tubes. However, many guitar aficionados prefer the distortion effects of vacuum tubes and will even seek out different types of tubes to try in their amplifiers. Tubes are less predictable, and therefore more unique, than transistors. The tubes, as well as circuit design, play a role in the audio qualities of an amplifier, and many players prefer the uniqueness or the specific sound qualities that they obtain from tube amplifiers.

Number of Stages

Amplifiers can be classified based on how many amplification stages they have.

Single Stage

A single stage amplifier has only one amplification stage and includes single transistor amplifiers. These are the first types of amplifiers that we will encounter in the next few lessons.


Any amplifier that contains more than one stage is called a multi-stage amplifier. Adding more stages can improve an amplifier’s characteristics for a specific application. Most amplifiers in commercial devices are multi-stage.

Amplification Type

We saw earlier that amplifiers can boost the strength of voltage, current, or power.

Amplifiers can therefore be classified as voltage amplifiers, current amplifiers, and power amplifiers.

Input Signal Strength

Amplifiers are commonly classified based on the strength of the input signal. Small signal amplifiers operate at low inpu levels, and large signal amplifiers operate at high input levels.

Transistor Orientation Classification

Transistor amplifiers can be classified based on which of the three terminals are common.

For bipolar junction transistors (BJTs), the three categories are common base, common emitter, and common collector.

Power Amplifier Class

A power amplifier is designed to increase the power available to a load. Power amplifiers are used in radio frequency (RF) as well as audio applications.

Power amplifiers have their own system for classification, using a series of classes. The classes are based on how long the active element conducts during each input cycle.

Class A

Class A amplifiers have active elements that conduct all (100%) of the time, and are said to have a conduction angle of 360°. Class A designs can be simpler than other classes, but are inefficient; the maximum theoretical efficiency for a Class A amplifier is 25%.

Class B

Class B amplifiers conduct half (50%) of the time, and have a conduction angle of 180°. Proper operation requires the use of two active devices (i.e. two transistors or tubes), with each acting on half of the waveform. The output of the two are then combined.

Class B amplifiers are more complex but significantly more efficient than Class A amplifiers. The maximum theoretical efficiency of a Class B amplifier is 78.5%.

The main problem with Class B amplifiers is that they suffer from crossover distortion. At the point when one device is about turn off and the other is about to turn on (called ‘the joins’), distortion can occur due to the momentary ‘gap’ in signal.

Class AB

Class AB is a compromise between Class A and Class B. Two active devices are used, like Class B. Each device conducts more than half the time, so that there is sufficient overlap that no signal distortion occurs. The conduction angle of Class AB amplifiers is between Class A and Class B.

Efficiency is also midway between Class A and B amplifiers.

Class AB amplifiers are a popular choice for audio applications.

Class C

Class C amplifiers feature a single active device that is used less than half of the time. Conduction angle is therefore less than 180°.

Class C amplifiers have high distortion but very high efficiency (up to 80%). They are commonly used in radio frequency (RF) applications.

Class D

Class D amplifiers use pulse width modulation (PWM). PWM chops up the signal into small, discrete pulses and allows for a very high efficiency; the tradeoff is complexity and cost.