Slide5 15 Power Amplifier

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Transcript of Slide5 15 Power Amplifier

by : Mujahidinhttp://www.iddhien.com [email protected] [email protected]

DefinitionsIn small-signal amplifiers the main factors are amplification linearity gain

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DefinitionsLarge-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

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Amplifier Types Class Class Class Class Class A B AB C D

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Class A AmplifierThe 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.iddhien2008 All Rights Reserved5

Class B AmplifierA Class B amplifier output only conducts for 180 or of the input signal The Q-point (bias level) is at 0V on the load line, so that the AC signal can only swing for of a cycle.

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Class AB AmplifierThis 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.

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Class C and Class D AmplifierClass C The 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 D The Class D output is more like a pulse. It does not resemble the AC sinewave input.

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

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Series-Fed Class A AmplifierThis is similar to the smallsignal 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.

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Series-Fed Class A AmplifierDC Bias Operation

IB =

VCC 0.7V RB

I C = I B VCE = VCC I C RC

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Series-Fed Class A AmplifierIf 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 largest voltage swing will be possible.12

Transistor characteristic showing load line and Qpoint.iddhien2008 All Rights Reserved

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Series-Fed Class A AmplifierAC 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)

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Series-Fed Class A AmplifierInput 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.

Pi(dc) = VCC I CQ

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Series-Fed Class A AmplifierOutput 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.

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Series-Fed Class A AmplifierOutput Power: Series-Fed Class A Amplifier Using rms signals: the ac power delivered to the load (RC) expressed:

Po(ac) = VCE (rms ) I C (rms )

Po(ac) = I C (rms ) RCV 2 C(rms) Po(ac) = RCiddhien2008 All Rights Reserved16

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Series-Fed Class A AmplifierOutput Power: Series-Fed Class A Amplifier Using peak signals: the ac power delivered to the load (RC) expressed:

Po(ac) =

VCE ( p ) I C ( p ) 22

2 VCE ( p) Po(ac) = 2 RC

I ( p) Po(ac) = C RC 2

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Series-Fed Class A AmplifierOutput Power: Series-Fed Class A Amplifier Using peak-to-peak signals: the ac power delivered to the load (RC) expressed:

Po(ac) =

VCE ( p p ) I C ( p p ) 82

Po(ac) =

2 VCE ( p p ) 8 RC

I ( p p) Po(ac) = C RC 8

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

% =

Po(ac) 100 Pi(dc)

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Series-Fed Class A AmplifierHitunglah Input Power, Output Power dan juga effisiensi amplifier disamping. IB (p)=10mA

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Transformer-Coupled Class A Amplifier

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

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Transformer-Coupled Class A AmplifierThe transformer improves the efficiency because of the transformation of voltage and current through the transformer. Voltage Ratio:

V 2 N2 = V 1 N1

Current Ratio:

I 2 N1 = I1 N 222

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Transformer-Coupled Class A AmplifierThe transformer aids in impedance matching to the load. Remember that transformers transform voltage, current, and impedance. Impedance Ratio:RL R1 N 1 2 = = =a RL R 2 N 2 2

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Transformer-Coupled Class A AmplifierOperation of Amplifier Stage DC Load Line As in all Class A amplifiers the Q-point is established close to the midpoint of the DC load line.

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Transformer-Coupled Class A AmplifierOperation of Amplifier StageAC Load Line The saturation point (ICmax) is now at Vcc/RL and the cutoff point is at V2 (the secondary voltage of the transformer). This increases the maximum output swing because the minimum and maximum values of IC and VCE are spread further apart.iddhien2008 All Rights Reserved25

Transformer-Coupled Class A AmplifierSignal Swing and Output AC PowerThe voltage swing: VCE(p-p) = VCEmax - VCEmin The current swing: Ic(p-p) = ICmax - ICmin The AC power:

Po(ac) =

(VCE max VCE min )(IC max IC min ) 8

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Transformer-Coupled Class A AmplifierSignal Swing and Output AC Power For the ideal transformer the voltage delivered to the load

VL = V2 =

N2 V1 N1

IL = I2 =

N1 IC N2

And power delivered to the load

V (rms ) PL = L RL2

PL = I L (rms )RL2

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Transformer-Coupled Class A AmplifierHitunglah 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.

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Transformer-Coupled Class A Amplifier

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Transformer-Coupled Class A AmplifierEffisiensi Power input from the DC source:

Pi(dc) = VCCICQPower dissipated as heat across the transistor:

PQ = Pi(dc) Po(ac)

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Transformer-Coupled Class A AmplifierPada gambar contoh sebelumnya hitunglah dc input power (Pi(dc)), power dissipated by transistor (PQ), dan juga effisiensi rangkaian.

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Class B Amplifier OperationIn 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 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.iddhien2008 All Rights Reserved32

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Class B Amplifier Operation

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Class B Amplifier OperationInput DC Power

Pi (dc) = VCC I dc

Idc =

2

2 Pi (dc) = VCC I ( p ) iddhien2008 All Rights Reserved34

I ( p)

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Class B Amplifier OperationOutput AC Power

VL2 (rms ) Po (ac) = RL VL2 ( p p ) VL2 ( p ) = Po (ac) = 8 RL 2 RL

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Class B Amplifier OperationEfficiency

% =

Po(ac) 100% Pi (dc)

The maximum efficiency of a Class B is 78.5%

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Class B Amplifier OperationPower Dissipated by Output Transistors The power dissipated (as heat) by the output transistor is

P2Q = Pi (dc) Po(ac)Where P2Q is the power dissipated by the two output power transistor. The dissipated power handled by each transistor is

PQ =

P2Q 237

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

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Class B Amplifier OperationTransformer-Coupled Push Pull of Class B Amplifier Circuits

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Class B Amplifier OperationComplementary-Symmetry of Class B Amplifier Circuits

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

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Class B Amplifier OperationQuasi-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.

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