Audio Amplifier Basics Explained

We often use term “amplifier” to refer to a circuit that takes input signal and produces an amplified version of it. But as we go deeper into amplifier tutorial, we will discover that not all amplifier circuits are created equal, they are categorized based on their circuit configurations and modes of operation.

In electronics we frequently encounter small signal amplifiers. These handy devices can take a relatively tiny input signal like one from a sensor such as a photo-device and boost it into a much larger output signal. This larger signal can then drive various components like relays, lamps, or loudspeakers.

There is a wide variety of electronic circuits that fall under amplifier category. We have everything from Operational Amplifiers and Small Signal Amplifiers to Large Signal and Power Amplifiers.

Classification of an amplifier hinges on factors like size of signal, whether it is large or small, as well as its physical configuration and how it processes input signal. Actually it is all about relationship between input signal and current flowing through load.

Classification of Amplifiers

We can find more details on types or classifications of amplifiers in following table.

We can think of amplifiers as simple enclosures or modules that contain amplifying components like Bipolar Transistors, Field Effect Transistors, or Operational Amplifiers.

These devices have two input terminals and two output terminals, all sharing a common ground. Thanks to amplification process, output signal we get is much stronger than input signal.

When we talk about an ideal signal amplifier there are three main features we should keep in mind: Input Resistance (RIN), Output Resistance (ROUT), and amplification which we often call Gain (A).

No matter how complex an amplifier circuit might be we can use a generalized amplifier model to clearly show how these three characteristics are connected.

Understanding an Ideal Amplifier Design

So difference between input and output signals in an amplifier is what we call Gain.

It is basically a way for us to measure how much amplifier boosts input signal. For instance if we have an input signal of 1 volt and output is 50 volts, we can say that gain o amplifier is 50.

This tells us that input signal has been increased by 50 times—yes, that is our Gain!

We calculate Gain by comparing output to input. Since it is all about comparison, Gain does not have any units, but in electronics we often represent it with letter “A” for Amplification.

To find gain of an amplifier we simply divide output signal by input signal.

Types of Amplifier Gain

When we think about amplifier gain, we can see it as relationship between output signal and input signal.

There are three main types of amplifier gain that we should know: Voltage Gain (Av), Current Gain (Ai), and Power Gain (Ap).

Each type focuses on a different aspect of signals.

Let us check out some examples of these different gains!

Amplifier Gain Relationship with Input Signal

1. Voltage Amplifier Gain
   Voltage Gain (Av) = Output Voltage / Input Voltage = Vout / Vin
2. Current Amplifier Gain
   Current Gain (Ai) = Output Current / Input Current = Iout / Iin
3. Power Amplifier Gain
   Power Gain (Ap) = Av × Ai

So we should keep in mind that we can figure out power gain by looking at the ratio of output power to the input power.

Also when we are checking out gain of an amplifier, we use the subscripts v, i, and p to indicate what kind of signal gain we’re talking about.

Now the power gain (Ap) or the power level of the amplifier can also be expressed in Decibels (dB). The Bel (B) is a logarithmic unit of measurement (base 10) that doesnt have any dimensions.

Since the Bel is a pretty large scale, we usually prefix it with “deci,” which gives us Decibels—where one decibel is one-tenth (1/10) of a Bel. To calculate the amplifier’s gain in Decibels we can use the following formulas:

Voltage Gain in dB:   a_v  =  20*log(Av)

Current Gain in dB:   a_i  =  20*log(Ai)

Power Gain in dB:   a_p  =  10*log(Ap)

Remember that the DC power gain of an amplifier is ten times the common logarithm of the output-to-input ratio. Meanwhile when we talk about voltage and current gains, those are 20 times the common logarithm of the ratio.

Just a heads-up though: 20dB doesn’t mean we’ve got double the power compared to 10dB because of how the logarithmic scale works.

Also when we see a positive dB value, it means Gain, while a negative dB value indicates Loss in the amplifier.

For example if we have an amplifier gain of +3dB, it means the output signal has “doubled” (x2). On the flip side an amplifier gain of -3dB means the signal has “halved” (x0.5) which shows a loss.

Lastly we call the -3dB point of an amplifier the half-power point. This is -3dB below the maximum output, with 0dB being our peak output value.

Solving an Amplifier Problem #1

Now Let us calculate the Voltage, Current, and Power Gain of our amplifier. We have an input signal of 1.5 mA at 12 mV and an output signal of 12 mA at 1.5 V. Also let’s make sure to express all three gains in decibels (dB).

The Different Amplifier Gains:

Voltage Gain (A_v):

A_v = Output Voltage / Input Voltage = 1.5 / 0.012 = 125

Current Gain (Ai):

A_i = Output Current / Input Current = 12 / 1.5 = 8

Power Gain (Ap):

A_p = A_v * A_i = 125 * 8 = 1000

Amplifier Gains given in Decibels (dB):

Voltage Gain (a_v):

a_v = 20 log A_v = 20 log 125 ≈ 41.94 dB

Current Gain (ai):

a_i = 20 log A_i = 20 log 8 ≈ 18.06 dB

Power Gain (ap):

a_p = 10 log A_p = 10 log 1000 = 30 dB

So the amplifier we are talking about has the Voltage Gain Av and it is at a level of 100. Then there is Current Gain Ai and that one is sitting at 10. Lastly, we have Power Gain Ap and it is a whopping 1,000.

Now when we look at amplifiers in general we can break them down into two main categories based on how much power or voltage gain they provide.

The first category is known as Small Signal Amplifiers and this group includes things like pre-amplifiers and instrumentation amplifiers among others.

These Small Signal Amplifiers are specifically designed to take really tiny signal voltage levels that are only a few micro-volts which is μV that come from sensors or audio signals and make them bigger.

On the flip side we have the second category which is called Large Signal Amplifiers and this includes things like audio power amplifiers or power switching amplifiers.

Large Signal Amplifiers are built to handle much larger input voltage signals or to switch heavy load currents which you would typically see when they are used to drive loudspeakers.

Understanding Small Signal Amplifiers

So when we talk about the Small Signal Amplifier, people often like to call it a “Voltage” amplifier. This is mainly because what these amplifiers do is take a tiny input voltage and turn it into a much bigger output voltage.

Now there are times when you need an amplifier circuit to do something like drive a motor or send signals to a loudspeaker. For these kinds of situations where you really need those high switching currents, that is when you need to use what we call Power Amplifiers.

As the name implies, the main purpose of a “Power Amplifier” which is also referred to as a large signal amplifier, is to provide power to whatever load you are working with.

And just to clarify, power is determined by multiplying the voltage and current that are applied to the load. This means that the power coming out of the amplifier is greater than what was put in.

To put it simply, a power amplifier takes the input signal and boosts its power. This is exactly why you see these types of amplifiers being used in the output stages of audio amplifiers to drive loudspeakers.

Now lets get into how a power amplifier actually works. It operates on a pretty straightforward principle where it takes the DC power that comes from the power supply and converts it into an AC voltage signal that gets delivered to the load.

Even though these amplifiers can provide a significant amount of amplification, it is important to note that their efficiency when converting DC power from the input into AC voltage for output is usually not very good.

If we could dream up the perfect or ideal amplifier, it would have an efficiency rating of 100%, meaning the power going in would be exactly equal to the power coming out.

But in real life this just does not happen because some of that power gets lost as heat and on top of that, the amplifier itself uses some power during the amplification process.

So when we talk about how efficient an amplifier is, we can define its efficiency as follows:

Formula for Calculating Amplifier Efficiency

Efficiency (η) = Power Supplied to the Load / Power Pulled from the Supply = P_OUT / P_IN

Characteristics of an Ideal Amplifier

So based on what we talked about earlier we can now lay out what makes an amplifier ideal especially when it comes to its Gain which is basically the voltage gain.

First off the amplifiers gain A needs to stay the same no matter how much the input signal changes. It should not be influenced by the frequency at all.

This means that signals of every frequency should get amplified by exactly the same amount without any variation.

Next up the amplifier’s gain should not introduce any noise into the output signal. Instead it should actually help in getting rid of any noise that might already be present in the input signal before amplification.

Also really important is that the amplifier’s gain should not be impacted by temperature changes so it has to show good temperature stability.

Lastly we want the gain of the amplifier to remain steady and reliable over long periods of time without any fluctuations.

Understanding Classifications of Audio Amplifiers

So when we talk about figuring out whether an amplifier is a voltage amplifier or a power amplifier we look at how the input signals and output signals behave.

This involves checking out how long the current flows in the output circuit in relation to the input signal. It is all about comparing those characteristics.

Now remember from our Common Emitter Transistor tutorial that in order for a transistor to work properly in what we call its “Active Region,” it needs something known as “Base Biasing.”

This little Base Bias voltage that we add to the input signal is super important because it helps the transistor to faithfully reproduce the entire input waveform at its output without losing any part of the signal.

But here is where it gets interesting. if we decide to change where that Base Bias voltage is placed, we can actually make the amplifier work in different ways instead of just reproducing the full waveform.

By introducing this Base Bias voltage into the amplifier, we can explore various operating ranges and modes of operation, which are grouped together based on their classification. These different ways of operating are what we call Amplifier Classes.

When it comes to audio power amplifiers, they get sorted into different classes based on how their circuits are set up and how they operate.

You will see them labeled with letters like class “A,” class “B,” class “C,” class “AB,” and so on. Each of these amplifier classes has its own unique characteristics, ranging from outputs that are almost linear but not very efficient to outputs that are non-linear but super are efficient.

So no single type of amplifier operation is really “better” or “worse” than any other type. The kind of operation you are dealing with really depends on how the amplifying circuit is set up.

Each type of amplifier has its own typical maximum conversion efficiency and here are the most common ones you will come across:

Class A Amplifier

This one is known for being not very efficient at all usually under 40 percent. But on the bright side it does a fantastic job at reproducing signals and keeping things linear.

Class B Amplifier

Now when we talk about Class B amplifiers, they are actually twice as efficient as Class A ones.

They can reach a maximum theoretical efficiency of around 70 percent because the amplifying device only works for half of the input signal. This means they use power more effectively.

Class AB Amplifier

Moving on to Class AB amplifiers, these sit in between Class A and Class B when it comes to efficiency.

However they do not quite match up to Class A amplifiers in terms of how well they reproduce signals.

Class C Amplifier

Finally we have Class C amplifiers which are the champs when it comes to efficiency. But they have a lot of distortion because they only amplify a tiny part of the input signal.

This means that the output signal looks very little like what went in. In fact Class C amplifiers are known for having the baddest signal reproduction out there.

Class A Amplifier Working

So let’s understand the fundamental setup of a class-A amplifier because it really gives us a solid starting point when we are looking at amplifier circuits.

When we talk about how a Class A Amplifier works we are referring to a situation where the whole input signal waveform is accurately and faithfully reproduced at the output terminal of the amplifier.

This happens because the transistor is ideally biased so that it stays within its active region.

What this means in simpler terms is that the switching transistor is never pushed into either its cut-off region or its saturation region.

Because of this careful operation the AC input signal ends up being perfectly balanced or centered between the upper and lower limits of the amplifier’s signal range as illustrated below.

Output Waveform of Class A Amplifier

So in a Class-A amplifier setup, it is pretty interesting because it uses the same switching transistor for both parts of the output waveform. Whats cool about this setup is that it has this central biasing thing going on.

This means that the output transistor always has this steady DC biasing current which we refer to as ICQ, flowing through it.

This happens even when there is no input signal at all. Basically the output transistor never really turns “OFF” and just hangs out in a constant idle state.

Now because of how it operates, the Class-A amplifier is not super efficient. It does not do a great job of converting the DC power from the supply into the AC signal power that actually gets sent to the load.

In fact this conversion is usually pretty low.

Another thing to keep in mind is that because of that centered biasing point I mentioned earlier, the output transistor in a Class-A amplifier can get really hot even when there is not any input signal.

So you definitely need some kind of heat sink to help manage that heat. The DC biasing current that flows through the collector of the transistor which we call ICQ, is actually equal to the current going through the collector load.

This is why Class-A amplifiers are considered quite inefficient, a lot of that DC power just ends up turning into heat instead of useful signal power.

Class-B Amplifier Working

So this thing called the Class-A amplifier uses just one single transistor to handle all the output power.

But then we have the Class-B amplifier which does things a bit differently.

Instead of relying on just one transistor, it actually uses two complementary transistors.

These can either be an NPN and a PNP type or even an NMOS and a PMOS type. The cool part is that these two transistors work together to amplify each half of the output waveform.

What happens here is pretty interesting. One of the transistors takes care of amplifying just one half of the signal waveform while the other transistor steps in to amplify the opposite half.

Because of this setup, each transistor only gets to work during half of the time for the signal waveform.

This means that each one spends half its time in the active region where it is actually amplifying and the other half in the cut-off region, where it is not doing anything at all. So each transistor is only amplifying 50 percent of the input signal.

Now heres another key difference with Class-B operation: it does not have a direct DC bias voltage like you would find in a Class-A amplifier.

Instead what happens is that a transistor will only start conducting when the input signal goes above a certain threshold known as the base-emitter voltage or VBE.

For silicon transistors, this threshold is roughly around 0.7 volts. This means that if there is no input signal at all, you are going to get zero output from the amplifier.

Since only half of the input signal gets sent out through the amplifier’s output, this setup actually makes the amplifier more efficient compared to that earlier Class-A configuration we talked about.

Output Waveform of Class B Amplifier

Understanding Class-B Amplifiers and Crossover Distortion

In a Class-B amplifier setup, there is no direct current voltage that is used to set the bias for the transistors.

Because of this, for the output transistors to actually start working and conduct during each half of the waveform, whether it is the positive half or the negative half, they need the base-emitter voltage VBE to be higher than 0.7 volts.

This specific voltage drop is what a regular bipolar transistor needs in order to begin conducting.

Now here, the lower section of the output waveform that falls below this 0.7-volt mark is not going to be reproduced accurately.

So what happens is that you end up with some distortion in that part of the output waveform.

This occurs because when one transistor switches off, it has to wait for the other transistor to turn back on again which only happens once VBE goes above 0.7 volts.

So there is this small section at the point where the output waveform crosses zero voltage that ends up being distorted.

This kind of distortion is what we refer to as Crossover Distortion and we will look deeper into this topic later on in this section.

Working of Class AB Amplifier

Now let us talk about the Class-AB Amplifier which is kind of like a middle ground between the Class-A and Class-B amplifier setups we discussed earlier.

In a Class-AB amplifier you still have two complementary transistors working together in the output stage but interestingly a tiny bit of biasing voltage is applied to the Base of each of these transistors.

This little voltage nudges them close to their cut-off point when there is no input signal coming in.

Now when an input signal does show up, it makes the transistors spring into action and operate normally within their active region.

This is really great because it helps to get rid of that pesky crossover distortion that you always find in Class-B configurations.

Even when there is no input signal, a small amount of biasing Collector current ICQ flows through the transistor.

However this current is usually much less than what you would see in a Class-A amplifier setup.

What this means in practical terms is that each transistor stays “ON” for just a little over half of the cycle of the input waveform.

Thanks to this slight biasing in the Class-AB amplifier configuration, we end up with improvements in both efficiency and linearity compared to what you would get with a pure Class-A configuration.

Output Waveform of the Class AB Amplifier

So when you are putting together amplifier circuits it becomes so much important to think about the class of operation of the amplifier.

This is because it plays a crucial role in figuring out how much biasing you will need, for the transistors to function properly and it also helps to establish the maximum size of the input signal that the amplifier can handle.

Now when we talk about classifying amplifiers we are looking at how much of the input signal actually gets the output transistor, to do its thing.

This classification is also key in understanding, how efficient the amplifier is and how much power the switching transistor uses up and lets off as heat that we do not want.

To make things clearer we can take a look at a comparison of the most common types of amplifier classifications which I will lay out in a table for you.

Table Showing Classification of Power Amplifiers

When it comes to amplifiers that are not designed very well, especially those Class “A” types, they tend to need bigger power transistors.

This means you might have to spend more money on heat sinks that are larger than usual and you might even need cooling fans to keep everything from overheating.

On top of that you could find yourself needing a bigger power supply just to handle all the extra power that gets wasted by the amplifier.

You see when power is turned into heat by transistors or resistors or really any other component in the circuit, it makes the whole electronic setup less efficient and can lead to the device breaking down way sooner than it should.

Now you might be wondering why anyone would even want to use a Class A amplifier when its efficiency is less than 40 percent especially when there are Class B amplifiers out there that boast an efficiency rating of over 70 percent.

The thing is, Class A amplifiers provide a much more linear output.

This means that they maintain Linearity across a wider range of frequencies even though they do tend to gobble up a lot of DC power.

In this Introduction to the Amplifier tutorial we have taken a look at the various types of amplifier circuits out there, each one coming with its own set of pros and cons.

Sources:

IN-AMP BASICS

Amplifier Classes