Amplifiers and its classification, Physics tutorial

Introduction:

The amplifier receives the signal from some pickup transducer or other input source and gives the larger version of signal to some output device or to another amplifier stage.

Usually, the amplifier or just amp is any device which changes, generally increases, amplitude of signal. Amplifiers can be thought of as simple box or block having amplifying device, like a Transistor, Field Effect Transistor or Op-amp, and that has two input terminals and two output terminals with output signal being greater than that of input signal, being Amplified. The amplifier has three major properties, Input Resistance or Output Resistance or (Rout) and of course gain or (A).

Relationship of input to output of the amplifier-generally expressed as the function of input frequency-is known as transfer function of amplifier, and magnitude of transfer function is termed the gain. In popular use, term generally explains electronic amplifier, in which input signal is generally a voltage or a current. In audio applications, amplifiers drive loudspeakers utilized in PA systems to make human voice louder or play recorded music.

Amplifier Gain:

Gain of the amplifier is ratio of output to input power or amplitude, and is habitually calculated in decibels. (A (dB) = 10log (Pout/Pin))

Then the gain of an amplifier can be said to be relationship which exists between signals calculated at output with signal measured at input. There are three different types of Amplifier Gain (A), Voltage Gain (Av), Current Gain (Ai), and Power Gain (Ap) and examples of these are given below.

1153_Amplifier Gain.jpg

Voltage Amplifier Gain: Voltage gain Av = Output Voltage/Input Voltage=Vout/Vin

Current Amplifier Gain: Current Gain Ai= Output Current/Input Current= Iout/Iin

Power Amplifier Gain Power Gain Ap= Av × Ai

For Power Gain you can also divide power obtained at the output with power obtained at input. Also, subscripts v, i, and p denote the kind of signal gain. To compute gain of amplifier in Decibels or dB, use the following expressions.

Voltage Gain in dB: av = 20 log Av

  • Current Gain in dB: ai = 20 log Ai
  • Power Gain in dB: ap = 10 log Ap

Amplifier Efficiency:

Efficiency is the measure of how much of power source is helpfully applied to amplifier's output. Efficiency of the amplifier refers to ratio of output-signal power compared to total input power. The amplifier has two input power sources: one from signal, and one from power supply. Perfect or ideal amplifier would give efficiency rating of 100% or at least power IN is equal to power OUT. Though, this can never take place as some of its power is lost in form of heat and also, amplifier itself consumes power during amplification process. Then efficiency of amplifier is given as:

Efficiency (η) = (power delivered to load) / (d.c. power taken from supply) = Pout/Pin

Classification of Amplifiers:

Classification of Amplifiers may be categorized according to input (source) they are designed to amplify (like a guitar amplifier, to carry out with electric guitar), device they are intended to drive (like a headphone amplifier), frequency range of signals (RF, Audio, IF, and VHF amplifiers, for instance), whether they invert signal (inverting amplifiers and non-inverting amplifiers), or the kind of device utilized in amplification (valve or tube amplifiers, FET amplifiers, etc.). Related device which emphasizes conversion of signals of one type to another (for instance, a light signal in photons to DC signal in amperes) is transducer, a transformer, or sensor. Though, none of these amplify power.

There are typical maximum efficiencies for different kinds or class of amplifier, with most usually used being:

Class A: Output signal differs for full of the cycle. This needs Q-point to be biased at a level so that at least half the signal swing of output may vary up and down without going to a high enough voltage to be limited by supply voltage level or too low to approach lower supply level, or in this description. The maximum theoretical efficiency of less than 40%.

Class B: A class B circuit gives the output signal varying over one-half the input signal cycle, or for 180° of signal. The dc bias point for class B is thus at 0 V, with output then varying from this bias point for half-cycle. Obviously, output is not a faithful reproduction of input if only one half-cycle is present. Two class B operations-one to give output on positive-output half-cycle and another to give operation on negative-output half-cycle-are essential. Class B operation by itself creates much distorted output signal as reproduction of input happens for only of output signal swing. Maximum theoretical efficiency of about 70%.

224_Amplifier Operating Classes.jpg

Class AB: Amplifier may be biased at dc level above zero-base-current level of class B and above one-half the supply voltage level of class A; this bias condition is class AB. Class AB operation still needs the push-pull connection to attain full output cycle, but dc bias level is generally closer to zero- base-current level for better power efficiency, as explained shortly. For class AB operation, output signal swing takes place between 180° and 360° and is neither class A nor class B operation.  An efficiency rating between that of Class A and Class B

Class C: Output of the class C amplifier is biased for operation at less than 180o of cycle and will function only with tuned (resonant) circuit that gives a full cycle of operation for tuned or resonant frequency. This operating class is thus utilized in special areas of tuned circuits, like radio or communications.

Class D: This operating class is the form of amplifier operation using pulse (digital) signals that are on for short interval and off for longer interval. Using digital methods makes it possible to get the signal which differs over full cycle (using sample-and-hold circuitry) to recreate output from several pieces of input signal. Major benefit of class D operation is that amplifier is on (using power) only for short intervals and overall efficiency can almost be very high.

Class A Amplifiers:

Amplifying devices operating in Class A conduct over whole of the input cycle such that output signal is the exact scaled-up copy of input with no clipping. Class A amplifier (or operational amplifier) is differentiated by output stage (and may be driver) device(s) being biased in Class A; even Class AB and B amplifiers generally have early stages operating in Class A. Class A is usual way of implementing small-signal amplifiers, so term Class A design applied to equipment like preamplifiers (for instance, in recording studios) implies not so much their use of Class A, but that their sound is top quality - good enough to be matched with top quality Class A power amplifiers.

459_Class A Amplifiers.jpg

Class A Operation: The Amplifier operation is where whole input signal waveform is faithfully reproduced at amplifiers output as transistor is perfectly biased within active region, thereby never reaching either of its Cut-off or Saturation regions. This then results in AC input signal being perfectly centered between amplifiers upper and lower signal limits as shown below.

1799_Class A Output Waveform.jpg

Here, Class A amplifier uses same transistor for both halves of output waveform and because of its biasing arrangement always has current flowing through it, even if there is no input signal. The output transistor never turns OFF. This results in class A type of operation being very ineffective as its conversion of DC supply power to AC signal power delivered to load is generally very low. In general, output transistor of a Class A amplifier gets very hot even when there is no input signal present so some form of heat sinking is needed. DC current flowing through output transistor (Ic) when there is no output signal will be equal to current flowing through load.

Advantages of Class A Amplifiers: Class A designs are simpler than other classes; for instance Class AB and B designs need two devices (push-pull output) to manage both halves of waveform, and circuitry to keep quiescent bias optimal during temperature changes; Class A can utilize either single-ended or push-pull and bias is generally less critical. Amplifying element is biased so device is always conducting to some extent, usually implying quiescent (small-signal) collector current (for transistors; drain current for FETs or anode/plate current for vacuum tubes) is close to most linear portion (sometimes called the "sweet spot") of its characteristic curve (known as its transfer characteristic or trans conductance curve), giving the least audio distortion.

The point at which device comes closest to being cut off (and so major change in gain, therefore non-linearity) is not close to zero signal, so problem of crossover distortion related with Class AB and B designs is avoided, even in Class A double-ended stages.

Disadvantage of Class A Amplifiers: They are very inefficient; the theoretical maximum of is reachable with inductive output coupling and only with capacitive coupling, unless Square law output stages are utilized. In power amplifier this not only wastes power and limits battery operation, it may place limitations on output devices which can be utilized, and will increase costs. Inefficiency comes not just from the fact that device is always conducting to some extent (which takes place even with Class AB, yet its efficiency can be close to that of Class B); it is that standing current is roughly half the maximum output current (though this can be less with Square law output stage), together with problem that large part of power supply voltage is developed across output device at low signal levels (as with Classes AB and B, but unlike output stages such as Class D). If high output powers are required from a Class A circuit, power waste (and accompanying heat) will become important.

Series-Fed Class A Amplifier: Simple fixed-bias circuit connection shown in figure given below can be utilized to discuss major features of class A series-fed amplifier. This circuit is not the best to utilize as a large-signal amplifier due to its poor power efficiency. Beta of power transistor is usually less than 100, overall amplifier circuit using power transistor which are capable of managing large power or current while not giving much voltage gain.

821_Series-fed Class A large Signal Amplifier.jpg

DC Bias Operation:

The dc bias set by Vcc and RB fixes the dc base-bias current at

IB = (VCC - 0.7V)/RB

With the collector current then being

IC = βIBVCC

With the collector-emitter voltage then

VCE = -ICRC

Consider collector characteristic shown below. The ac load is drawn using values of VCC and RC. Intersection of dc bias value of with dc load line then finds operation point (Q- point) for circuit. Quiescent-point values are those computed using above equations. If dc bias collector current is set at one- half the possible signal swing (between 0 and VCC/RC), the largest collector current will be feasible. Additionally, if quiescent collector-emitter is set at one-half supply voltage, largest voltage swing will be possible. With Q-point set at this optimum bias point, power considerations for the circuit given below are determined as described below.

310_Line and Q- Point.jpg

AC Operation:

When the output ac signal is applied to amplifier, output will differ from its dc bias voltage and current, small input signal, will cause best current to differ above and below dc bias point that will then cause collector current (output) to differ from dc bias point set and collector-emitter voltage to differ around its dc bias value. As input signal is made larger, output will differ further around established dc bias point until either current or voltage reaches the limiting condition. For current this limiting condition is either at low end or VCC/RC at high end of its swing. For collector-emitter voltage, limit is either 0v or supply voltage, VCC.

Power Considerations: Power into amplifier is given by supply. With no input signal, dc current drawn is collector bias current, IcQ. Power then drawn from the supply is

Pi(dc) = VCCICQ

Even with the ac signal applied, average current drawn from supply remains the same, so that equation represents input power supplied to class A series-fed amplifier.

Output power:

Output voltage and current varying around bias point give ac power to load. This as power is delivered to load, RC, in circuit. The ac signal, Vi causes base current to differ around dc bias current and collector current around its quiescent level, IQC. The ac input signal result in the ac current and ac voltage signals. Larger the input signal, the larger the output swing, up to maximum set by the circuit. The ac power delivered to load (RC) can be defined in various ways.

Using rms signals: ac power delivered to load (RC) may be defined using:

P0(ac) = VCE(rms)IC(rms)

P0(ac) = I2C(rms)RC

P0(ac) = V2C(rms)RC

Using peak signals: The ac power delivered to load may be defined using:

P0(ac) = VCE(p)IC(p)/2

P0(ac) = I2C(P)

P0(ac) = V2CE(P)/2RC

Using peak-to-peak signals: The ac power delivered to the load may be expressed using

P0(ac) = VCE(p-p)IC(p-p)/8

P0(ac) = (I2c(p-p)/8)RC

P0(ac) = (V2CE(p-p)/8)RC

Efficiency: Efficiency of the amplifier represents amount of ac power delivered (transferred) from dc source. Efficiency of the amplifier is computed using:

%η = P0(ac)/Pi(dc) x 100%

The Transformer-Coupled Class A Amplifier:

A form of class A amplifier having maximum efficiency of utilizes the transformer to couple output signal to load as shown. This is simple circuit form to utilize in presenting some basic concepts. Major reason for poor efficiency of the direct-coupled class-A amplifier is large amount of dc power that resistive load in collector must dissipate. Problem can be solved by using the appropriate transformer for coupling load (say, a speaker) to amplifier stage. As load is not directly connected to collector terminal, dc collector current doesn't pass through it. In the ideal transformer, primary winding resistance is zero. Therefore, dc power loss in load is zero. In practice, though, there is small dc resistance of primary winding that absorbs some power though much less than direct-coupled load. In short, what transformer does is to substitute ac load in place of ohmic or dc load. Secondary load RL when referred to primary become

R'L = RL/K2 = a2RL

Where K = voltage transformation ration = N2/N1 = V2/V1

a = turns ratio N1/N2 = 1/k

1538_Transformer-CoupledAudio Amplifier.jpg

As a is generally made much more than unity or K is much less than unity, RL can be made to look much bigger than what it really is.

In the ideal transformer, there is no primary drop, therefore VCC = VCEQ. Now, all the power supplied by VCC is delivered to transistor. Therefore, overall and collector efficiencies become equal.

ηoverall = P0(ac)/VCCICQ = P0(ac)/VCEQ.ICQ

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