Amplifiers Theory, Block diagram, Gain, Types and Comparison

An amplifier is an electronic device used to boost up the strength of the weak signal. To match the input signal shape with larger amplitude, an amplifier takes the energy from the power supply and controls the output. It can either be a separate piece of equipment or an electrical circuit within another device. In our previous tutorials, we explain oscillators, transistors, semiconductors, diodes in detail. Here we are going to explain Amplifiers, amplifiers work, types etc in detail.

amplifier image

Amplifier Theory:

Amplifiers are one of the most used circuits in electronics, performs a variety of functions in a great way. Without changing other parameters of  the waveform such as frequency or wave shape, an amplifier increases the amplitude of a signal waveform. Have a look at amplifier basics.

(i) Block diagram of Amplifier:

In all practical amplifiers, a number of stages are cascaded to amplify a weak signal to a sufficient level to operate the output device. In these amplifiers, the main function of first few stage is only to amplify voltage and the last stage is designed to drive the output device.

Block diagram of amplifier image

As shown in the block diagram above, in a public address system when a person speaks into a microphone, it converts the sound waves into electrical signals. If we directly fed this signal to the speaker, it will not be in a position to drive the speaker because electrical signal so produced is very low voltage. Due to this reason, the voltage level of the signal is first raised to sufficient level by passing it through a number of stages of voltage amplifier.This amplified voltage signal is then fed to the final stage of the amplifier to deliver the required power to drive the speaker. Finally, after this stage, the loudspeaker converts the electrical signals into sound waves. Therefore, a large number of people will be in the position to hear the speech.

(ii) Amplifier Gain:

It is defined as the ratio of the output voltage or current to the input voltage or current. It can be said to be the relationship that exists between the signal measured at the output to the signal measured at the input. For amplification, it is designated by the symbol A and has no units. Depending upon the quantity being measured, you can calculate amplifier gain in three ways.

(a) Voltage Amplifier Gain: 

It is defined as the ratio of the output voltage to input voltage.Its equation is given as:

Voltage gain (AV) = Vout / Vin

(b) Current Amplifier Gain:

It is defined as the ratio of the output current to input current. Its equation is given as:

Current Gain (Ai) = Iout / Iin

(c) Power Amplifier Gain:

It is defined as the product of the voltage gain and current gain.

Power Gain (Ap) = AV Ai

Note: For Power gain you can divide the power obtained at the output with the power obtained at the input.

Amplifier gain can be calculated in terms of decibels. The expression for calculating gain are given as:

Voltage gain in dB (av) = 20 log AV

Current gain in dB (ai) = 20 log Ai

Power gain in dB (ap) = 10 log Ap

Types of Amplifiers:

As we know, Amplifiers contain the amplifying device such as a transistor, Field Effect transistor, and operational amplifier has two input terminals and two output terminals with the output signal being much greater than that of the input signal. These can be classified into different categories depends on upon the size of the signal i.e. small or large, its physical configuration and how it processes the signal i.e. relationship between the input signal and current flowing  in the load. Following types of amplifier are:

(i) Voltage Amplifier:

voltage amplifier image

These amplifiers are designed to achieve maximum voltage amplification because it raises the voltage level of the signal at the beginning. Voltage gain of the amplifier is given by the expression:

AV = β * (Rc / Rin)

In such amplifier following features are required to be incorporated in such amplifiers for performance well. Following points to be noted are:

  1. High Collector load resistance ( Rc):  The collector load resistance should be relatively high. To obtain this condition we keep the lower collector current i.e. 1 mA.
  2. Higher Value of β: The transistor employed should  have a higher value of β i.e. greater than 100. Hence, the transistor should have the thin base.
  3. R-C coupling: An RC coupling has a smaller size,smaller weight, lower cost and occupies less space, therefore, it is preferred to couple various stages of voltages amplifiers by this coupling method.
  4. Low input resistance ( Rin ): The input resistance of the transistor is quite low.

(ii) Power Amplifier:

Power amplifier image

To handle a larger amount of power to the load and large current, a power amplifier is used to  deliver a larger amount of power to the load. To meet these requirements, following features are to be kept in mind:

  1.  The Larger size of transistor: When heavy current passes through the transistor, more heat is produced at the collector junction. To dissipate the heat produced, a larger size of transistors are required.
  2. Thicker base:  Base of the transistor is thick because the transistor employed in the power amplifier is to handle the larger current.
  3. Low collector load resistance ( Rc): The collector load resistance should be of smaller value because if it has high value there will be more power loss in the resistor.
  4. Transformer Coupling: To transfer maximum power to the output device we have to maintain input impedance equal to the  output impedance. To match the impedance of the loudspeaker to the output impedance of the amplifier, a transformer is employed at the output.

Comparison of Voltage and Power amplifiers:

The comparison  between power amplifier and voltage amplifier on the basis of coupling , collector current, output impedance, input voltage, output power etc is given below in the tabular form.

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