# changing circuit of class A output stage to a class B amplifier

Discussion in 'Homework Help' started by fantabulous68, Feb 5, 2010.

1. ### fantabulous68 Thread Starter Active Member

Nov 3, 2007
51
4
The {attached document} is a diagram of a class A output stage. Could some one please tell me how to CHANGE THE CIRCUIT to make it a class B amplifier....

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2. ### Audioguru New Member

Dec 20, 2007
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The circuit is a class-AB amplifier, not class-A. You do not want class-B because it has crossover distortion.

3. ### fantabulous68 Thread Starter Active Member

Nov 3, 2007
51
4
could someone please help me:

1. Give the circuit diagram for a class 'A' output stage
2. Describe how one could change the circuit to make it a class 'B' amplifier

Apr 5, 2008
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5. ### hobbyist Distinguished Member

Aug 10, 2008
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First what do you ubderstand about classes of amplifiers.

1. A class "A" amplifier is biased at cutoff, (TRUE or FALSE) ??
2. A class "B" amplifier conducts only half the time of a input excursion. (TRUE or FALSE) ??

In answering these questions, will determine to what extent the people on this forum can help you, with..

6. ### fantabulous68 Thread Starter Active Member

Nov 3, 2007
51
4
I know that in a class A amplifier:
current must run in both bjts at all times

Each device conducts 180 degrees --> 360 degrees of cycle

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I know that in a class B amplifier:
current must ONLY ever run in 1 bjt at any instant

Each device conducts only 180 degrees

BJTs CANT BE CLASS B AMPS.

to be class B---->NO quiescent current(Iq)
NO Iq---->VBE=0,

if VBE=0, it shorts the bases of the 2 BJTs that are connected together-------->hence no current in both bjts

and as Audioguru said..........class B is much more distorted than class A

Am i RIGHT?

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Last edited: Feb 6, 2010
7. ### hobbyist Distinguished Member

Aug 10, 2008
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I could not open your attachment,

I was basing my questions on a single transistor stage.

I don't know what you drew up but your explanation about a Class "A" your using the word they, so I'm assuming this drawing has 2 transistors in totem pole like a class "B" config.

A class "A" uses only one transistor, and is biased 'on' at the center of it's load line, so it conducts the full input excursion, 360 degress of input signal, and has an inverted output.

A class "B" could use a single transistor, where it is 'not' biased into conduction, and relies solely on a input signal, to bring it into conduction. So it conducts half the input wave form, 180 deg.

So to change a class "A" amplifier to a class "B" is to remove the base bias resistors. For a single transistor stage, that is.

8. ### Audioguru New Member

Dec 20, 2007
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I show simple transistor amplifiers with class-A, class-B and class-AB:

• ###### transistor amplifier classes.PNG
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9. ### fantabulous68 Thread Starter Active Member

Nov 3, 2007
51
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Wow thanks Audioguru, a picture is worth a 1000 words.
Would love to know the source from which you got the above schematic to broaden my knowledge....???
Thanks everyone for your assistance....

10. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Audioguru drew these diagrams by himself.
And of course you can get class A amplifier with push pull output stage.

11. ### Audioguru New Member

Dec 20, 2007
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I simply used my knowledge of transistor amplifier classes and manipulated your sketch in Microsoft Paint program to show them.

12. ### fantabulous68 Thread Starter Active Member

Nov 3, 2007
51
4
HeheheOk, thanx again

13. ### fantabulous68 Thread Starter Active Member

Nov 3, 2007
51
4
Q1} would the attached picture be a class A output stage? and if i wanted to change the picture to a class b amplifier, i should replace the collector resistor and bjt by a pair of bjts?

Q2}On positive swings the voltage across the collector resistor reduces because the collector voltage gets closer to the supply rail?
VRC=VCC-VCollector-VPeak

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14. ### Audioguru New Member

Dec 20, 2007
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No.
It might melt. The collector resistor would need to be 1.7 ohms for a 700mA peak output current then at idle it would dissipate 9.4W and the transistor would also dissipate 9.4W. usually a constant current source is used instead of a collector resistor so it doesn't dissipate so much heat.

Maybe.
Usually a class-AB circuit is used in an audio amplifier.

Yes.
Then the current in the resistor must be 700mA. The at idle the current will be much more.

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15. ### fantabulous68 Thread Starter Active Member

Nov 3, 2007
51
4
Q1) Define the conditions that would cause the amplifier to be on the limit of Class A.

Using a voltage follower for the output stage:
Its when 1 bjt is saturated and has 0V across it whilst the other bjt is just about to run out of current????

conditions:
1) IQ=delta
2) peak load current= 2 IQ

ARE there any MORE conditions to be on the LIMIT?

Q2) Given that the stage is running on the limit of Class A, calculate the maximum input power and hence determine the efficiency at:
1. Full load
2. 5% output voltage

1. At full load Pin=(VCC^2)/RL #### Pout=(VCC^2)/2RL #### efficiency=Pout/Pin=50%

2. Help with this pls....

• ###### BasicPwrAmpOPStage1.jpg
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Last edited: Feb 23, 2010