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Douglas Self Sixth Edition
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Old 22nd February 2019, 09:07 AM   #1
FengLiu is offline FengLiu
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Default Douglas Self Sixth Edition

About Chapter 17, Class A Power Amplifier, Figure 17.19. Why is it wrong to connect C5 to the circuit in the simulation? Who can tell the effect of C5?
In fact, I only want to be a Class A, Figure 17.7, it can't be used. It seems that his Class A quiescent current control, I don't understand, who can help me explain.

Last edited by FengLiu; 22nd February 2019 at 09:10 AM.
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Old 22nd February 2019, 07:22 PM   #2
ostripper is offline ostripper  United States
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Post it. (Chapter 17, Class A Power Amplifier, Figure 17.19)
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Old 23rd February 2019, 01:31 AM   #3
FengLiu is offline FengLiu
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Analyze Class A quiescent current control. Thank you! !
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Last edited by FengLiu; 23rd February 2019 at 01:34 AM.
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Old 23rd February 2019, 01:41 AM   #4
FengLiu is offline FengLiu
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If you make a class A according to Figure 17.7, can you? Figure 17.19 is a class A and class AB class B. I only want class A.

Last edited by FengLiu; 23rd February 2019 at 01:45 AM.
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Old 23rd February 2019, 01:42 PM   #5
Bonsai is offline Bonsai  Europe
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You can make class A with fig 17.7. To set the class A current, you must set Vref = Iq * (2*Re). So, for example, if you want say 1 Amp standing current, set Vref to 0.44V

C5 is a boot strap capacitor and gives you more voltage swing.

Last edited by Bonsai; 23rd February 2019 at 01:45 PM.
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Old 24th February 2019, 07:13 AM   #6
bimo is offline bimo  Indonesia
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VREF can use diode 1N4148, it is around 0,6V.
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Old 12th March 2019, 04:38 AM   #7
FengLiu is offline FengLiu
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What is the role of D1-D4 in Figure 17.19? No need?
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Old 12th March 2019, 05:05 AM   #8
nauta is offline nauta  Canada
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hi

The bias circuit in Self's class-A amp is in strong contradiction to the words he wrote regarding bias spreaders in general. He showed that the CFP bias spreader was superior even to his resistor-enhanced single-BJT circuit, but stated he did not want to risk failure of the circuit by adding more parts.

Doctors take a hypacratic oath; maybe engineers take a hypocritical oath?

The diodes across the feedback shunt cap are to protect the cap in case the output fails shorted to either rail. Feedback at DC is unity-gain (1), so the cap would theoretically be subject to the full rail voltage.

Last edited by nauta; 12th March 2019 at 05:08 AM.
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Old 12th March 2019, 08:04 AM   #9
jan.didden is offline jan.didden  Europe
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Why the aggression? What's wrong with showing something is technically better but deciding against it for other reasons? It's something a real designer does all the time.
You may not agree but that doesn't mean you should insult and imply hypocrisy.

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Old 12th March 2019, 10:36 AM   #10
FengLiu is offline FengLiu
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I don't know the actual effect of this circuit? I have drawn the PCB, please correct me. This circuit has many very old triodes. Which new transistors should be used for replacement?
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Last edited by FengLiu; 12th March 2019 at 10:46 AM.
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