Understanding Amplifier Circuits

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:



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 Ratio

Thus, 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 Gains
The different types of amplifier gains may be classified as:
  1. Voltage Gain (Av)
  2. Current Gain (Ai)
  3. Power Gain (Ap)
Example Formulas for Calculating Amplifier Gains

Depending upon the above 3 types of gains, the formulas for calculating these could be learned from the following examples:

  1. Voltage Gain (Av) = Output Voltage / Input Voltage = Vout / Vin
  2. Current Gain (Ai) = Output Current / Input Current = Iout / Iin
  3. 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:

  1. Voltage Gain in dB: av = 20*log(Av) 
  2. Current Gain in dB: ai = 20*log(Ai) 
  3. 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 @ 1mA
Output 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 A100  x 10 = 1000

To get the results in Decibels we apply the corresponding formulas as given below:

av = 20logAv = 20log100 = 40dB
ai = 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 Amplifier

From 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 explained below:

The gain (A) of an ideal amplifier should be constant regardless of a varying input signal.


  1. The gain remains constant regardless of the frequency of the input signal, enabling the output amplification to remain unaffected.
  2. 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.
  3. It remains unaffected by the changes in the ambient temperature or the atmospheric temperature.
  4. 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 negin 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.


Need Help? Please send your queries through Comments for quick replies! And please Bookmark my site :)




Comments

Unknown said…
Hey there Swagatam; Another great post. I had listened to a video explanation in quick reference to what you have explained here in more detail. In addition he talks a bit about the D class amplifiers. Thanks for this - Here is the five minute video for quick reference in under 5 minutes.

https://www.youtube.com/watch?v=rj6h4qYxkCc
Swagatam said…
Thanks friend, I am Glad you liked it!

Contact me for Customized Circuits

Name

Email *

Message *


 Follow on G+  Follow on Facebook   Follow on Tweeter  Follow on G+  Follow on G+

Follow Homemade Circuits