Amplifier

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

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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Fig. 12.6: Transformer-coupled audio power amplifier



A transformer improves the efficiency because it is able to transform the voltage, current, and impedance

Voltage Ratio

Current Ratio

Impedance Ratio

Transformer Action Review

1

2

1

2NN

VV

2

1

1

2NN

II

22

2

1

2

1

L

L aNN

RR

RR

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Fig. 12.6: Transformer-coupled audio power amplifier



Example 12.2

Calculate the effective resistance seen looking into the primary of a 15:1 transformer connected to an 8Ω load.

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Example 12.3

What transformer turns ratio is required to match a 16Ω speaker load so that the effective load resistance seen at the primary is 10kΩ.

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Transformer-Coupled Class A Amplifier DC Load Line

As in all class A amplifiers the Q-point is established close to the midpoint of the DC load line.

AC Load Line

The saturation point (ICsat) is at Vcc/RL and Icq . While 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.

22

Fig. 12.8: Load lines for class A transformer-coupled amplifier



Transformer-Coupled Class A Amplifier

The voltage swing:

8)I)(IV(V

P CminCmaxCEminCEmaxo(ac)

Signal Swing and Output AC Power

The AC power:

The current swing:

minCEmaxCE)pp(CE VVV

minmax CC II

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Some of your friends asked what’s the different between this Icswing and Icswing = Vce/RL’If you refer to Electronic Devices And Circuit TheoryR. Boylestad reference book, both also know as Icswing . To avoid confusion, Ics = Vce/RL’ Icswing = Icmax – Icmin



Example 12.4

Calculate the ac power delivered to the 8Ω speaker for the circuit of Fig. 12.10. The circuit component values result in a dc base current of 6mA, and the input signal (Vi) results in a peak base current swing of 4mA.

24

Fig. 12.10: Transformer-coupled class A amplifier for Example 12.4



1) Draw DC load line

2) Draw AC load line. How?

i)Find RL’ii) Find the Ics = Vce/RL’ (current swing)

iii) Find ∆Ic/Icsat = Iceq+IcsDraw the line from ∆Ic intercept the operating point until Vce line downward

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2

1

2

1

L

L aNN

RR

RR

Saturation region

Cutoff region

3) Find your Vce& Ic max and min. How?i) Icmax :At first I thought

the first IB line that intercept with the AC load line but after refer to several reference book, its actually the next IB line that intercept with the AC load line Why? Because the first line is in saturation region and the transistor CANNOT operate that far into saturation region. Draw the horizontal line from that point

ii) Icmin: Draw the line from the last point that intercept with your IB.

iii) The same method applied to your VCE.

This is what we called as GRAPHICAL method. To get the values directly from graph.



Transformer-Coupled Class A Amplifier Efficiency

Power input from the DC source:

CQCCi(dc) IVP

o(ac)i(dc)Q PPP

2

CEminCEmax

CEminCEmaxVVVV

50%η

Maximum efficiency:

Power dissipated as heat across the transistor:

Note: The larger the input and output signal, the lower the heat dissipation.

Note: The larger VCEmax and smaller VCEmin, the closer the efficiency approaches the theoretical maximum of 50%.

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Example 12.5

For the circuit of Fig. 12.10 and results of Example 12.4, calculate the dc input power, power dissipated by the transistor, and efficiency of the circuit for the input signal of Example 12.4.

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Example 12.6

Calculate the efficiency of a transformer-coupled class A amplifier for a supply of 12V and outputs of:

a) V(p) = 12Vb) V(p) = 6Vc) V(p) = 2V

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Summary

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Example 12.1: = 6.5 % (Series Class A)

Example 12.5: = 35.71% (Transformer-Coupled Class A)



Class A Amplifier Summary

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• Q-point – set at the middle of the load line

• Conduction angle - 360

• Maximum output swing = Vcc/2

• Maximum efficiency for series class A amplifier = 25%

• Maximum efficiency for transformer-coupled class A amplifier = 50%

• DC loadline – DC analysis, ICQ, VCEQ, Ic(sat) and VCE(cutoff)

• AC loadline – AC analysis, slope =

• Output (ac) current swing and output (ac) voltage swing

• Calculation and graphical method

• Pi(dc), Po(ac) , % and PQ

LR '1

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