In this post I am going to explain power amplifiers and their classification according to their output electrical characters and application. We will be exploring amplifier classes: A, B, AB, C, D, E, F, G, H, I, S and T briefly and yes, that’s a lot of amplifier types that are available in today’s electrical and electronics industry.
Each of the amplifier types described above has one or more special electrical character that makes them well suited for one specific application and that’s why we can find so many types of amplifiers.
Let’s try to understand each class of amplifier one by one starting from class A.
Class A amplifier:
Class A amplifiers are one of the most common types of amplifiers that we can find, this is because of its simple design, high gain, excellent linearity and it usually utilizes one active component for amplification, it can be a BJT, FET, IGBT etc.
Linearity is one of the essential characteristics of any amplifier because the input signal’s wave shape should not get altered by the amplifier circuit except raising the input signal to higher amplitude i.e. increasing the voltage /current of the input signal at the output.
To achieve linearity in the amplifier, the active component (transistor) must operate in the linear region.
A transistor can operate in 3 modes: cut-off, linear and saturation. In cut-off mode the transistor’s output is off, in saturation mode the output of the transistor is equal to Vcc, in linear mode the output of the transistor will increase or decrease proportional to the input signal.
We can set the transistor in linear region / mode by biasing the transistor sufficiently by applying a voltage (above 0.7V) to the base terminal using a voltage divider (R1 and R2). Once we bias it, the transistor is said to be in linear region.
Since the transistor is always ON partially even when no input signal is applied, some current is always passed from collector terminal to R3 or load resistor, this idle current is called Quiescent current and this is wasted as heat.
The load resistor (R3) value is chosen in such a way that when no input is applied the output will be Vcc/2, now the output voltage can swing up and down from the mid-point Vcc/2.
The emitter resistor R4 is for stability of the gain and the capacitor (C3) beside the resistor R4 is called bypass capacitor for neutralizing any AC signal that may pass through the emitter.
Capacitors C1 and C2 are for coupling the input and output respectively and they don’t allow steady DC signal to pass through at input and also at the output.
Since we use a transistor for amplification and a transistor is also an inverted switch, the output waveform will be 180 degree phase shifted.
Advantages of class A amplifier:
- Excellent linearity.
- Good output gain.
- No crossover distortion.
- Low noise distortion at the output.
Disadvantages of class A amplifier:
- The maximum theoretical efficiency of class A amplifiers is around 25%, rest of the energy is wasted as heat and hence no battery operated gadgets prefer them.
- Designing a high power class A amplifier is avoided, because higher the power output higher the losses. But because of its high fidelity output we can find class A amplifiers in high end amplifier markets that cost thousands of dollars.
- Need for filtered power supply to prevent 50 / 60 Hz AC hum at the output.
Class B Amplifier:
The above circuit is an oversimplified version of class B amplifier. Class B overcomes some of the main issues we had with class A type.
Class B amplifier utilizes two complementary transistors (one is NPN and other is PNP) in push-pull configuration as shown above.
When the input signal is +Ve the NPN transistor starts conducting and when the input signal goes –Ve the PNP transistor starts conducting.
So at any given instant only one transistor stays ON. The NPN transistors conduct 180 degree phase of the signal and the PNP transistor conducts another 180 degree phase and recombines at the output.
In class A amplifiers, we had biasing resistors to apply a constant voltage above 0.7V at the base terminal so that the transistor stays in conduction regardless of the input signal.
Here we don’t have to apply any biasing voltage and the input signal will bias the transistors, hence zero quiescent current and better efficiency than class A type.
Class B amplifiers can reach a maximum theoretical efficiency of 78.5%. In the real world we may get up to 50% efficiency, but we made a tradeoff with linearity.
Since a transistor doesn’t start conducting until the base-emitter voltage reaches 0.7V or (-0.7V for PNP) the transistor stays off and any signal between +0.7V and – 0.7V will not be reproduced at the loudspeakers, this is called crossover distortion.
In conclusion class B amplifiers are more efficient than class A type with less linearity than class A.
Class AB Amplifier:
Class AB amplifiers are developed to overcome the issues that class B type had. Class AB amplifiers have good linearity and almost no crossover distortion and this is a good compromise between class A and B amplifier types.
In class AB amplifiers a small bias current is applied to both the transistors using the diodes and resistor, so that when the input signal falls below +/-0.7V the transistor stays conductive and the input is properly replicated at the loudspeaker.
Since we are applying a small bias voltage to make it conductive, there is a small quiescent current (idle current), but the power loss is nowhere close to that of class A type.
In class B type I have explained that only one of the transistors stays ON at any given instant, but in class AB type since there is a small bias voltage both the transistors conduct slightly more than 180 degree.
During the crossover both the transistor stays conductive, thus no loss of input signal.
The class AB amplifier can reach efficiency around 50% to 60%.
Class C Amplifier:
The amplifier types that I have explained till now are generally used as audio amplifiers, but the class C type that we are going to discuss now is used only in high frequency applications and it is not a linear amplifier.
Class C type has the most distortion at the output and requires a tuned circuit for practical usage; the tuned circuit is for suppressing distortion at the output.
The tuned load consists of a capacitor and an inductor (LC tank circuit) the values of the components are chosen for operating at a fixed frequency.
Class C amplifiers can reach up to 80% efficiency at radio frequencies.
The conduction angle of class C type is less than 50% or less than 180 degree or in other words when we apply an input with a signal of 360 degree (say, a full sine wave cycle) only less than 180 degree worth of signal passes to the amplifier.
However, when we use a tuned load (LC circuit) the proper shape of the wave is restored.
Since it utilizes a tuned circuit, its output is fixed to a single frequency, thus unwanted frequencies are suppressed. The bandwidth of class C amplifiers is limited by the “quality factor” of the amplifier.
If you are an intermediate to advanced level hobbyist, you may have recognized the LC circuit and transistor configuration is very similar to RF transmitter circuits.
Class D amplifier:
Class D amplifiers are the most preferred amplifier type in battery operated devices simply because of its high efficiency and good amplification.
The class D type is very different from all the amplifier types I have explainedd previously in terms of working and complexity.
A class D amplifier’s working can be explained briefly using the above illustrated block diagram.
Firstly, the input signal is passed in to a comparator that compares the audio signal with a saw tooth shaped wave or triangular wave, the comparator’s output will be square pulses whose pulse width is proportional to audio signal, this is where analog signal is converted into digital.
The pulsed wave is fed to an amplifier stage composed of MOSFETs or BJTs that operates at saturation and cut-off region (transistor either fully ON or fully OFF) and this is where high efficiency of the amplifier prevails.
The previously discussed amplifier types had its transistor operated at a linear region where a lot of energy is dissipated as heat.
Now the amplified square pulses are fed to a LC low pass filter where square pulses are restored to the original analog signal and fed to a loudspeaker. The LC low pass filter stage acts like a digital to analog converter.
Class D amplifiers are also known as switch mode amplifiers.
Class E amplifier:
Class E amplifier is also a switch mode amplifier but used only in high frequency RF applications similar to class C amplifiers. Class E amplifiers are also tuned like class C type to extract high efficiency.
Class F Amplifier:
Class F amplifier is also another high frequency power amplifier and not used in audio applications.
The significance of the class F amplifier is that it uses something called a “harmonic resonator” at the output stage and because of this it can reach efficiency up to 90% practically.
Class G Amplifier:
The Class G amplifier is an upgrade on the class AB amplifier to improve power efficiency. Class G amplifier utilizes multiple power rails with different voltages as its power supply to minimize power loss.
When the input amplitude rises the amplifier switches to a higher voltage power rail and when the input amplitude gets lower it will choose a lower voltage power rail, in this way the power loss is minimized.
Class G amplifiers are more efficient than class AB, but less efficient than class D type.
Class H Amplifier:
Class H amplifier is an enhancement to class G type, instead of using multiple power rails of different voltages, class H amplifier can vary its power rail voltage infinitely within a voltage range.
Thus, when input amplitude rises or fall, the voltage on the power rail follows it and we can achieve a higher efficiency.
Class S amplifier:
The class S amplifier’s operation is similar to that of class D type, but it utilizes something called “delta-sigma '' modulator for analog to digital conversion.
Class T amplifier:
Class T amplifiers possess the efficiency of Class D amplifiers and low distortion levels of class AB type.
Summary
In general, an amplifier can be defined as a circuit designed to boost an applied low power input signal into a high power output signal, as per the specified rating of the components.
Although, the basic function remains the same, amplifiers could be classified into different categories depending on their design and configurations.
Circuits for Amplifying Logic Inputs
You may have come across single transistor amplifiers which are configured to operate and amplify a low signal logic from an input sensing devices such as LDRs, photodiodes, IR devices.
The output from these amplifiers are then used for switching a flip flop or a relay ON/OFF in response to the signals from the sensor devices.
You may have also seen tiny amplifiers which are used for pre-amplifying a music or audio input, or for operating an LED lamp.
All these small amplifiers are categorized as small signal amplifiers.
Types of Amplifiers
Primarily, amplifier circuits are incorporated for amplifying a music frequency such that the fed small music input is amplified into many folds, normally 100 times to 1000 times and reproduced over a loudspeaker.
Depending on their wattage or power rating, such circuits may have designs ranging from small opamp based small signal amplifiers to large signal amplifiers which are also called power amplifiers.
These amplifiers are technically classified based on their working principles, circuit stages, and the manner in which they may be configured to process the amplification function.
The following table provides us the classification details of amplifiers based on their technical specifications and operating principle:
| Type of Signal | Type of Configuration | Classification | Frequency of Operation |
| Small Signal | Common Emitter | Class A Amplifier | Direct Current (DC) |
| Large Signal | Common Base | Class B Amplifier | Audio Frequency (AF) |
| Common Collector | Class AB Amplifier | Radio Frequencies(RF) | |
| Class C Amplifier | VHF, UHF and SHF Frequencies |
In a basic amplifier design we find that it mostly includes a few stages having networks of bipolar transistors or BJTs, field effect transistors (FETs), or operational amplifiers.
Such amplifier blocks or modules could be seen having a couple of terminals for feeding the input signal, and another pair of terminals at the output for acquiring the amplified signal over a connected loudspeaker.
One of the terminals out of these two is the ground terminals and could be seen as a common line across the input and the output stages.
Three Properties of an Amplifier
The three important properties which an ideal amplifier should have are:
- Input Resistance (Rin)
- Output Resistance (Rout)
- Gain (A) which is the amplification range of the amplifier.
Understanding an Ideal Amplifier Working
The difference in the amplified signal between the output and the input is termed as the gain of the amplifier. It is the magnitude or the amount by which the amplifier is able to amplify the input signal across its output terminals.
Take for example, if an amplifier is rated to process an input signal of 1 volt into an amplified signal of 50 volts, then we would say that the amplifier has a gain of 50, it is as simple as that.
This enhancement of a low input signal to a higher output signal is called the gain of an amplifier. Alternatively, this may be understood as an increase of the input signal by a factor of 50.
Gain RatioThus, the gain of an amplifier is basically ratio of output and input values of the signal levels, or simply the output power divided by the input power, and is attributed by the letter "A" which also signifies the amplification power of the amplifier.
Types of Amplifier GainsThe different types of amplifier gains may be classified as:
- Voltage Gain (Av)
- Current Gain (Ai)
- Power Gain (Ap)
Example Formulas for Calculating Amplifier GainsDepending upon the above 3 types of gains, the formulas for calculating these could be learned from the following examples:
- Voltage Gain (Av) = Output Voltage / Input Voltage = Vout / Vin
- Current Gain (Ai) = Output Current / Input Current = Iout / Iin
- Power Gain (Ap) = Av.x.Ai
For calculating power gain, alternatively you may also use the formula:
Power Gain (Ap) = Output Power / Input Power = Aout / Ain
It would be important to note that the subscript p, v, i used for calculating power are assigned for identifying the specific type of signal gain that's being worked upon.
Expressing Decibels
You will find another method of expressing power gain of an amplifier, which is in Decibels or (dB).
The measure or the quantity Bel(B) is a logarithmic unit (Base 10) that does not have a unit of measurement.
However a Decibel could be too large a unit for practical use, therefore we use the lowered version decibel (dB) for amplifier calculations.
Here are some formulas which can be employed for measuring amplifier gain in decibels:
- Voltage Gain in dB: av = 20*log(Av)
- Current Gain in dB: ai = 20*log(Ai)
- Power Gain in dB: ap = 10*log(Ap)
Some Facts about dB Measurement
It would be important to note that an amplifier's DC power gain is 10 times the common log of its output/input ratio, whereas the gains of current and voltage are 20 times the common log of their ratios.
This implies that because a log scale is involved, a 20dB gain cannot be deemed as twice of 10dB, due to the non-linear measurement characteristic of log scales.
When gain is measured in dB, positive values signify gain of the amplifier while a negative dB value indicates a loss of amplifier's gain.
For example if a +3dB gain is identified it indicates a 2 fold or x2 gain of the particular amplifier output.
Conversely, if the result is -3dB, indicates that the amplifier has a loss of 50% gain or a x0.5 measure of loss in its gain. This is also referred to as half-power point meaning -3dB lower than the maximum achievable power, with respect to 0dB which is the maximum possible output from the amplifier
Calculating Amplifiers
Calculate the voltage, current and power gain of an amplifier with the following specifications:Input signal = 10mV @ 1mAOutput Signal = 1V @ 10mA.Additionally find out the gain of the amplifier using decibel (dB) values.
Solution:
Applying the formulas learned above, we can evaluate the different types of gains associated with the amplifier as per input output specifications in hand:
Voltage Gain (Av) = Output Voltage / Input Voltage = Vout / Vin = 1 / 0.01 = 100
Current Gain (Ai) = Output Current / Input Current = Iout / Iin = 10 / 1 = 10
Power Gain (Ap) = Av. x Ai = 100 x 10 = 1000
To get the results in Decibels we apply the corresponding formulas as given below:
av = 20logAv = 20log100 = 40dBai = 20logAi = 20log10 = 20dB
ap = 10log Ap = 10log1000 = 30dB
Amplifier Subdivisions
Small Signal Amplifiers: With respect to the power and voltage gain specs of an amplifier, it becomes possible for us to sub divide them a couple of diverse categories.
The first type is referred to as the small signal amplifier. These small signal amplifiers are generally utilized in preamplifier stages, instrumentation amps etc.
These types of amplifiers are created for handling minute signal levels at their inputs, within the range of some micro volts, such as from sensor devices or small audio signals inputs.
Large Signal Amplifiers: The second type of amplifiers are named as large signal amplifiers, and as the name implies these are employed in power amplifier applications for achieving huge amplification ranges.
In these amplifiers the input signal is relatively larger in magnitude so that they could be substantially amplified for reproducing and driving them into powerful loudspeakers.
How Power Amplifiers Work
Since small signal amplifiers are designed to process small input voltages, these are referred to as small signal amplifiers.
However when an amplifier is required to work with high switching current applications at their outputs, like operating a motor or operating sub-woofers, a power amplifier becomes inevitable.
Most popularly, power amplifiers are employed as audio amplifiers for driving large loudspeakers and for achieving huge music level amplifications and volume outputs.
Power amplifier require external DC power for their working, and this DC power is utilized for achieving the intended high power amplification at their output.
The DC power is usually derived through high current high voltage power supplies through transformers or SMPS based units.
Although, power amplifiers are able to boost the lower input signal into high output signals, the procedure is actually not very efficient.
It is because in the process a substantial amount of DC power is wasted in the form of heat dissipation.
We know that an ideal amplifier would produce an output almost equal to the power consumed, resulting in an efficiency of 100%.
However, practically this looks quite remote and may not be feasible, due to inherent DC power losses from the power devices in the form of heat.
Efficiency of an AmplifierFrom the above considerations, we can express efficiency of an amplifier as:
Efficiency = Amplifier Power output / Amplifier DC consumption = Pout / Pin
Ideal Amplifier
With reference to the above discussion, it may be possible for us to outline regarding the main characteristics of an ideal amplifier. They are specifically as I have explained below:
The gain (A) of an ideal amplifier should be constant regardless of a varying input signal.
- The gain remains constant regardless of the frequency of the input signal, enabling the output amplification to remain unaffected.
- Amplifier's output is free from any kind of noise during the amplification process, on the contrary, it incorporates a noise reduction feature cancelling any possible noise introduced through the input source.
- It remains unaffected by the changes in the ambient temperature or the atmospheric temperature.
- Long time usage has minimal or no effect on the performance of the amplifier, and it stays consistent.
Electronic Amplifier Classification
Whether it's a voltage amplifier or a power amplifier, these are classified based on their input and output signal characteristics.
This is done by analyzing flow of current with respect to the input signal signal and the time required for it to reach the output.
Based on their circuit configuration, power amplifiers can be categorized in an alphabetical order. They are assigned with different operational classes such as:
Class "A"
Class "B"
Class "C"
Class "AB" and so on.
These may have properties ranging from almost linear output response but rather low efficiency to a non-linear output response with high efficiency.
None of these classes of amplifiers can be distinguished as poorer or better than each other, since each have its own specific application area depending on the requirement.
You may find optimal conversion efficiencies for each of these, and their popularity can be identified in the following order:
Class "A" Amplifiers: Efficiency is lower typically less than 40%, but may show improved linear signal output.
Class "B" Amplifiers: Efficiency rate may be twice that of class A, practically around 70%, due to the fact that only the active devices of the amplifier consume power, causing only 50% usage of power.
Class "AB"Amplifiers: Amplifiers in this category have efficiency level somewhere between that of class A and class B, but the signal reproduction is poorer compared to class A.
Class "C" Amplifiers: These are considered to be exceptionally efficient in terms of power consumption, but the signal reproduction is worst with plenty of distortion, causing very poor replication of the input signal characteristics.
How Class A Amplifiers Work:
Class A amplifiers have a ideally biased transistors within the active region which it makes it possible for the input signal to be accurately amplified at the output.
Due to this perfect biasing feature, the transistor are never allowed to drift towards their cut off or over saturation regions, resulting in the signal amplification being correctly optimized and centered between the specified upper and the lower limitations of the signal, as shown in the following image:
In class A configuration, identical sets of transistors are applied across two halves of the output waveform.
And depending upon the kind of biasing it employs, the output power transistors are always rendered in the switched ON position, regardless of whether the input signal is applied or not.
Because of this, class A amplifiers get an extremely poor efficiency in terms of power consumption, since the actual delivery of power to the output gets hampered due to excess wastage through device dissipation.
With the above explained situation, class amplifiers can be seen always having over heated output power transistors even in the absence of an input signal.
Even while there's no input signal, the DC (Ic) from the power supply is allowed to flow through the power transistors, that may be equal to the current flowing through the loudspeaker when input signal was present. This gives rise to a continuous "hot" transistors and wastage of power.
Class B Amplifier Operation
In contrast to class A amplifier configuration which depend on single power transistors, class B uses a pair of complementary BJTs across each half sections of the circuit.
These could be in the form of NPN/PNP, or N-channel mosfet/P-channel mosfet).
Here, one of the transistors is allowed to conduct in response to the one half waveform cycle of the input signal, while the other transistor handles the other half cycle of the waveform.
This ensures that each transistor in the pair conducts for half of the time within the active region and half of the time in the cut-off region, thus allowing only 50% involvement in the amplification the signal.
Unlike class A amplifiers, In class B amplifiers the power transistors are not biased with a direct DC, instead the configuration ensures that they conduct only while the input signal goes higher than the base emitter voltage, which could be around 0.6V for silicon BJTs.
This implies that, when there's no input signal, the BJTs remain shut off and the output current is zero.
And due to this only 50% of the input signal is allowed to enter the output at any instance enabling a much better efficiency rate for these amplifiers. The result can be witnessed in the following diagram:
Since there's no direct involvement of DC for biasing the power transistors in class B amplifiers, in order to initiate the conduction in response to the each half +/- waveform cycles, it becomes imperative for their base/emitter Vbe to acquire a higher potential than 0.6V (standard base biasing value for BJTs)
Due to the above fact, it implies that the while the output waveform is below the 0.6V mark, it cannot be amplified and reproduced.
This gives rise to a distorted region for the output waveform, just during the period when one of the BJTs becomes switched OFF and waits for the other to switch back ON.
This results in a small section of the waveform being subjected to minor distortion during the cross over period or the transition period near the zero crossing, exactly when the changeover from one transistor to the other occurs across complementary pairs.
Class AB Amplifier Operation
The class AB amplifier is built using a blend f characteristics from class A and Class B circuit designs, hence the name Class AB.
Although Class AB design also works with a pair of complementary BJTs, the output stage ensures that the biasing of the power BJTs are controlled close the the cut-off threshold, in the absence of an input signal.
In this situation, as soon as an input signal is sensed, the transistors begin operating normally in their active region thus inhibiting any possibility of a cross over distortion, which is normally prevalent in Class B configurations.
However, there could be a slight amount of collector current conducting across the BJTs, the amount may be considered negligible compared to Class A designs.
Class AB type of amplifier exhibit a much improved efficiency rate and a linear response as opposed to the Class A counterpart.
Class AB Amplifier Output Waveform
Amplifier Class is an important parameter which is depending on how the transistors are biased through the amplitude of the input signal, for implementing the amplification process.
It relies upon how much of the magnitude of the input signal waveform is utilized for the transistors to conduct, and also the efficiency factor, which is determined by the amount of power actually used for delivering the output and/or wasted through dissipation.
With regards to these factors we can finally create a comparison report showing the differences between the various classes of amplifiers, as given in the following table.
Then we can make a comparison between the most common types of amplifier classifications in the following table.
Power Amplifier Classes
Final Thoughts
If an amplifier is not designed correctly, like for example a class A amplifier design, may demand substantial heatsinking on the power devices, along with cooling fans for the operations.
Such designs will also need a larger power supply inputs for compensating the huge amounts of power wasted in heat.
All such drawbacks can render such amplifiers very inefficient which in turn could cause a gradual deterioration of the devices and eventually failures.
Therefore, it may be advisable to go for a Class B amplifier designed with higher efficiency of around 70% as opposed to 40% of a Class A amplifier.
Said that, Class A amplifier may promise a more linear response with its amplification and a wider frequency response, although this comes with a price of substantial power wastage.