Amplifier

DEFINITION

An amplifier is a device that increase the voltage, current or power of an input signal with the aid of transistor by furnishing the additional power from a separate dc source.

 

COMMON EMITTER AMPLIFIER

The circuit arrangement of single stage common emitter amplifier. A practical amplifier uses different circuitry components to achieve faithful amplification. The functions of this amplifier circuit is discussed as:

1. Biasaing / Stabilization Circuit
2. Coupling / Blocking Capacitor (C_in/C_C­)
3. Bypass Capacitor (C_E)

a. Biasaing / Stabilization Circuit: The resistances R₁, R₂ and R_E form the voltage divider biasing circuit for the CE amplifier. It basically sets the proper operating point for the CE amplifier. It basically sets the proper operating point for the CE amplifier.

b. Coupling / Blocking Capacitor (C_in/C_C­): Its main function is to couple the signal to the base of the transistors. It blocks any dc component present in signal and passes ac signal only for amplification.

c. Bypass Capacitor (C_E): It is connected in parallel with the emitter resisitance R_E to provide a low reactance path to the amplified ac signal. It is not connected then the amplified ac signal passing through R_E will cause a voltage drop across it. As a result, the output voltage get reduced and thus reducing the gain of the amplifier.
 

 

COMMON COLLECTOR AMPLIFIER

The common –collector circuit where the dc-bias is provided by R₁ R₂ and R_E. When a signal is applied via to the base of the transistor, V_B increase and then decrease as the signal goes from positive and negative respectively. The voltage across the emitter resistance R_E is V_E­ which is given as,

V_E = V_B-V­_BE

        Where, V_BE is a constant.

Hence, the emitter voltage V_E will vary in the same manner as the base voltage V_B. Thus, the output voltage(V_E) closely follows the input voltage(V_B), both in phase and magnitude. Hence, its  other name is Emitter Followers. This configuration has high input impedance and hence it can be used in interfacing voltage sources with relatively high output resistance. Also, it has low output impedance, it can drive large current into load. This property is known as buffering. The use of emitter follower between source and load isolates low impedances loads from high impedance sources. This is known as impedance buffering. This configuration is also known as Unity Gain Follower due to its unity gain.   

 

COMMON BASE AMPLIFIER

 The common-base amplifier circuit. The signal source is coupled to the emitter of the transistor through the coupling capacitor C_C1.   The load resistance R_L is connected to the collector of the transistor through the output coupling capacitor C_C2. When input signal is   applied at the emitter, the positive going pulse of input source increase the emitter voltage. As base voltage is constant, the forward   bias of emitter base junction reduces. This reduces I_B, resulting in reduction of I_C and the voltage drop across R_C. Since, the output   voltage, V₀ = V_CC ­– I_CR_C­, the reduction in I_C results in an increase in V_o. Thus, the positive half cycle of the input produces the   positive half cycle of the output and vice versa. Therefore, there is no phase shift between input and output in common base   amplifier.

 

 

 

COMMON SOURCE JFET AMPLIFIER

The circuit of a common source N-channel JFET amplifier. It is similar to a common emitter amplifier. Hence the resistor R₁ and R₂ (called a voltage divider) are used to bias the field-effected transistor. The capacitor (C₁) and(C₂) are used to couple the a.c input voltage source and the output voltage respectively, these are known as coupling capacitor.

The capacitor (c_s) keeps the source of the FET effectively at a.c ground and is known as bypass capacitor.

This produces variations in the drain current. As the gate-to-source voltage increase. As a result of this , the voltage drop across the resistor (R_D) also increase. This cause the drain voltage to decrease.

It means that the positive half cycle of the input voltage produces the negative half cycle of the output voltage . in other words, the output voltage (at the drain) is 180^O out of phase with the input voltage (at the gate).

 

AMPLIFIER DISTORTION

 Distortion in amplifier basically implies the variation in the waveform received at the output with respect to the applied input. Amplifier   distortion can take on many forms such as amplitude, frequency and phase distortion due to clipping. The unwanted alternation   generated during amplification is known as distortion.

 Amplitude distortion: Amplitude distortion occurs due to following reasons-

(a) Amplitude Distortion Due to Incorrect Biasing
(b) Amplitude Distortion Due to Clipping

 

 

(a) Amplitude Distortion Due to Incorrect Biasing                                    (b)Amplitude Distortion Due to Clipping

 

 

Frequency distortion: Frequency Distortion Due to Harmonics

 

 

(c) Phase distortion: Phase Distortion Due to Delay

 

CLASS A AMPLIFIER

 

• The power amplifier in which collector current flows during the whole cycle of the input signal are known as class A power amplifiers.
• In these amplifiers, biasing R₁, R₂, R_E is so adjusted that the Q-point lies in middle of the load line and the collector current flows during the whole cycle of the input signal.
• As a result, the output shape is almost same the input, i.e., distortion is minimum. No part of the signal is cut off.
• However, these amplifier suffer from the disadvantage of very low efficiency which is about 35%.

 

Power amplifier efficiency


Where:
η%  – is the efficiency of the amplifier.
Pout  – is the amplifiers output power delivered to the load.
Pdc  – is the DC power taken from the supply.

For a power amplifier it is very important that the amplifiers power supply is well designed to provide the maximum available continuous power to the output signal.

Transformer-Coupled Amplifier Circuit

Class A amplifier configuration can be calculated as follows.

The r.m.s. Collector voltage is given as:

The r.m.s. Collector current is given as:

The r.m.s. Power delivered to the load (Pac) is therefore given as:

The average power drawn from the supply (Pdc) is given by:

and therefore the efficiency of a Transformer-coupled Class A amplifier is given as:

 

CLASS B AMPLIFIER:
 The power amplifiers in which the operating point is so adjusted that the collector current, flows only during   the positive half cycle of the input signal are known as Class- B amplifiers.
 For class-B operation of the amplifier, the biasing circuit is so adjusted that the operating point Q lies at the   collector cut-off voltage i.e., zero collector current I_C =0.
 During positive half-cycle of the signal, the input circuit is forward biased and hence collector current flows.   But, during negative half cycle of the signal, the input circuit is reverse biased and hence no collector current   flows.
 In class-B amplifier, since the negative half-cycle of the signal is cut-off, it suffers from a severe distortion.   However, the points in favour of this amplifier are:
 1. Higher power output
 2. Higher collector efficiency (50 to 60%)
 Such amplifier are mostly used for power amplification in push-pull arrangement.  
 

 

CROSSOVER DISTORTION IN AMPLIFIERS

Class B pull-push amplifier produces amplifier called crossover distortion. The  output of push-pull amplifier. The reader should carefully note the distorted signal which is caused due to the clipping action occurring in between the half  cycles.

It is no longer an exact sine wave. Since the clipping occurs between the time when one transistor become OFF, and the other transistor become ON, w call it a called ‘cross-over’ distortion.

This distortion is caused by non-linearity of the input characteristics of the transistors.  

 

 

TRANSISTOR BIASING

• Transistor biasing is defined as the proper flow of zero signal collector current and the maintenance of proper collector emitter voltage during the passage  of the signal.
• Transistor can operate in three regions namely cut off, active and saturation region. In order to operate the transistor the transistor in the desired region we have to apply external dc voltages of correct polarity and magnitude to the two junction of the transistor.
• The basic purpose of transistor biasing is to keep the base-emitter junction properly forward biased and collector-base junction reverse biased during the application of signal.

 

FREQUENCY RESPONSE

• The dependence on signal frequency of the output-input ratio of an amplifier or the other device.
• Frequency response is the quantitative measure of the output spectrum of a system of a system or device in response to a stimulus, and is used to characterize the dynamics of the system.
• The ability of a device (such as an audio amplifier) to handle the frequencies applied to it.

CLASS AB AMPLIFIER

• The power amplifier in which the operating point is so adjusted that the collector current flows for more than half cycle but less than full cycle of the input  signal are known as class-AB amplifiers.
• For  class-AB operation of the amplifier, the biasing circuit is so adjusted that the operating point Q lies near the cut-off voltage.
• During a small portion of the negative half cycle and for complete positive  half cycle of the signal, the input circuit is forward biased and hence collector flows. The input circuit is reverse biased and hence no collector current flows during this period.

Comparison of the Different Amplifier Classes

Then the amplifier classes are always defined as follows:

Class A: – The amplifiers single output transistor conducts for the full 360o of the cycle of the input waveform.

Class B: – The amplifiers two output transistors only conduct for one-half, that is, 180o of the input waveform.

Class AB: – The amplifiers two output transistors conduct somewhere between 180o and 360o of the input waveform.