Slide5 15 Power Amplifier

by : Mujahidin

http://www.iddhien.comiddhien@gmail.commujahidin@iddhien.com

Definitions

In small-signal amplifiers the main factors are • amplification • linearity • gain

Definitions

Large-signal or power amplifiers function primarily to provide sufficient power to drive the output device. These amplifier circuits will handle large voltage signals and high current levels. The main factors are • efficiency• maximum power capability• impedance matching to the output device

Amplifier Types

• Class A• Class B• Class AB• Class C• Class D

Class A Amplifier

The output of a Class A amplifier conducts for the full 360° of the cycle

The Q-point (bias level) must be biased towards the middle of the load line so that the AC signal can swing a full cycle. Remember that the DC load line indicates the maximum and minimum limits set by the DC power supply.

Class B Amplifier

A Class B amplifier output only conducts for 180° or ½of the input signal

The Q-point (bias level) is at 0V on the load line, so that the AC signal can only swing for ½ of a cycle.

Class AB Amplifier

This amplifier is in between the Class A and Class B. The Q-point (bias level) is above the Class B but below the Class A.

The output conducts between 180° and 360° of the AC input signal.

Class C and Class D Amplifier

Class CThe output of the Class C conducts for less than 180°of the AC cycle. The Q-point (bias level) is at cutoff, the output signal is very small.

Class DThe Class D output is more like a pulse. It does not resemble the AC sinewave input.

Amplifier Efficiency

Efficiency refers to the ratio of output to input power. The lower the amount of conduction of the amplifier the higher the efficiency.

Series-Fed Class A Amplifier

This is similar to the small-signal amplifier except that it will handle higher voltages. The transistor used is a high power transistor.This circuit is not the best to use as a large-signal amplifier because of its poor power efficiency. The Beta is generally less than 100.

DC Bias Operation

CCCCCE

RIVVIIR

If the DC bias collector current is set at one-half the possible signal swing (between 0 and Vcc/Rc), the largest collector current swing will be possible. If the quiscent collector-emitter voltage is set on one half the supply voltage the largestvoltage swing will be possible.

Transistor characteristic showing load line and Q-

point.

AC Bias Operation

A small input signal will cause the output voltage to swing to a maximum of Vcc and a minimum of 0V. The current can also swing from 0 mA to ICSAT (Vcc/RC)

Input Power: Series-Fed Class A Amplifier

The power into the amplifier is from the DC supply. With no input signal, the DC current drawn is the collector bias current, ICQ.

CQCCi IV(dc)P =

Output Power: Series-Fed Class A Amplifier

The ac signal Vi causes the base current to vary arround the dc bias current and the collector current arround its quiscent level ICQ.The ac input signal result in ac current and ac voltage signals.

Using rms signals: the ac power delivered to the load (RC) expressed:

R(rms)V(ac)P

)()()( rmsIrmsVacPo CCE=

CC RrmsIacPo )()( 2=

Using peak signals: the ac power delivered to the load (RC) expressed:

( ) ( )

2pCpCE

(ac)P =

CCo RpI(ac)P2)(2

CEoRpV(ac)P

Using peak-to-peak signals: the ac power delivered to the load (RC) expressed:

( ) ( )

8ppCppCE

(ac)P −−=

CCo RppI(ac)P8

)(2 −=

ppV(ac)P8

)(2 −=

Efficiency

The maximum efficiency is at maximum output and current swings. It is 25% for a Class A amplifier Maximum Efficiency

100×=Pi(dc)Po(ac)%η

Hitunglah Input Power, Output Power dan juga effisiensi amplifier disamping.IB (p)=10mA

Transformer-CoupledClass A Amplifier

This circuit uses a transformer to couple to the load. This improves the efficiency of the Class A to 50%.

The transformer improves the efficiency because of the transformation of voltage and current through the transformer.

•Voltage Ratio:

•Current Ratio:

The transformer aids in impedance matching to the load. Remember that transformers transform voltage, current, and impedance.

•Impedance Ratio: 22

⎠⎞

⎜⎝⎛==

DC Load LineAs in all Class A amplifiers the Q-point is established close to the midpoint of the DC load line.

Operation of Amplifier Stage

AC Load LineThe saturation point (ICmax) is now at Vcc/R′L and the cutoff point is at V2 (the secondary voltage of the transformer).This increases the maximum output swing because the minimum andmaximum values of IC and VCE are spread further apart.

Operation of Amplifier Stage

The voltage swing: VCE(p-p) = VCEmax - VCEmin

The current swing: Ic(p-p) = ICmax - ICmin

The AC power:

Signal Swing and Output AC Power

8minmaxminmax )I)(IV(V(ac)P CCCECE

o−−

For the ideal transformer the voltage delivered to the load

Signal Swing and Output AC Power

CLL INNIIV

22 ==⇔==

And power delivered to the load

( ) ( ) LLLL

LL RrmsIP

RrmsVP 2

Hitunglah ac power yang diterima speaker 8 Ohm pada penguat class A disamping jika diketahui nilai IB = 6 mA, dan signal input Vi akan mengakibatkan peak base current swing sebesar 4 mA.

EffisiensiPower input from the DC source:

CQCCi IV(dc)P =

Power dissipated as heat across the transistor:

(ac)P(dc)PP oiQ −=

Pada gambar contoh sebelumnya hitunglah dc input power (Pi(dc)), power dissipated by transistor (PQ), dan juga effisiensi rangkaian.

Class B AmplifierOperation

In Class B the dc bias leaves the transistor biased just off. The AC signal turns the transistor on. This is essentially no bias.The transistor only conducts when it is turned on by ½ of the AC cycle.

In order to get a full AC cycle out of a Class B amplifier, you need two transistors.One is an npn transistor that provides the negative half of the AC cycle and the other is a pnp transistor that provides the positive half.

Input DC Power

dcCCi IVdcP =)(

( )pIIdcπ2

( )⎟⎠⎞

⎜⎝⎛= pIVdcP CCi π2)(

Output AC Power

rmsVacP )()(2

pVRppVacP

Efficiency

%100)()(% ×=

dcPiacPoη

The maximum efficiency of a Class B is 78.5%

Power Dissipated by Output Transistors

)()(2 acPodcPiP Q −=

The power dissipated (as heat) by the output transistor is

Where P2Q is the power dissipated by the two output power transistor.The dissipated power handled by each transistor is

Problem

For a class B amplifier providing a 20V peak signal to a 16Ω load (speaker) and a power supply of Vcc = 30V, determine the input power,output power and circuit efficiency.

Transformer-Coupled Push Pull of Class B Amplifier Circuits

Complementary-Symmetry of Class B Amplifier Circuits

Disadvantage: need for two separate voltage supplies, Resulting the crossover distortion in the output signal.

Quasi-Complementary Push-Pull of Class B Amplifier Circuits

Calculate the input power, output power, and power handled by each output transistor, and the circuit efficiency for input of 12V rms.

Slide5 15 Power Amplifier

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