The difference between a Class A and a Class B amplifier showed via a 15 W audio amp schematic/demo



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Please read the description/textbox first. In my project to make a 40-60 Watt audio amplifier working on (say) 80 -90 Volts this is the first experiment. Advice: don't make this circuit.

Better good working circuit is here https://youtu.be/cq2fd6zI-mk (5 Watt audio with 3055)
And here https://youtu.be/KSJndhH95M4 (5-8 Watt audio, stereo amp Hi Fi)
Or here (2-3 Watt audio) https://youtu.be/jU6VI2WpGqk
Or here (2 x 5 Watt) https://youtu.be/1id_cNdDLvs

Demo of a class A audio amplifier (taking, say, a constant current when amplifying an audio signal) and a Class B amplifier, where the current through the end transistors driven into the loudspeakers is (directly) related to the audio input level. Say between 0 and 800 Millivolts (AC, consumer electronics), say audio "sound".

Correction to the video (everyone can see it) that there 4 (!) diodes (normal Silicon, like the 1N5400) in the row, generating the voltage drop between the 2 Bases of the Driver transistors.

When I say “a normal Silicon diode” we have to take in account that there is a kind of “spread” in the voltage drop that a Silicon diode can give. It is bound to the physical properties of Silicon (atomic structure, valence bands), though there is a spread of 1/10 of a Volt.

It means that one Silicon diode can have a voltage drop of 0,8 Volt, another one a voltage drop of 0,7 Volt. When we go to the 1/100 Volt range in testing them there is a more precise spread visible.

This is important (1/10 to 1/100 of a Volt) because the setting to the quiescent current in a Class B Amplifier is critical and can depend to this range (1/10 Volt). 1/10 of a Volt is sure for effects.

The voltage drop of a Silicon diode also depends slightly on the applied voltage. That means that, when you want to use this effect (voltage drop) in an audio amplifier that has to work on a higher voltage, that effect has to be taken I account.

The same effects, by the way, apply to the use of Germanium Diodes/a Germanium diode. Its voltage drop can go from 0,2 V to even 0,6-0,7 V, that comes close to Silicon.

Important: these diodes from base to base are responsible for the “complete” voltage drop in the Darlington.

Be it a 3 transistor Darlington that voltage drop is in the range of 3 x 0,7 – 0,8 Volt = 2,1 to 2,4 Volt with a sure margin (for proper functioning of the amplifier) of 1/10 Volt.

Be it a 2 transistor Darlington that voltage drop is in the range of 2 x 0,7 – 0,8 Volt = 1,4 Volt to 1,6 Volt. Again with that margin.

So the alignment of the voltage drop between the bases of the (2 or 3) Driver transistors is very critical when the Audio Amplifier has to work in Class B. This bare & essential circuit shows these properties.

Ideal is this (normal class B audio amplifiers): a small quiescent current in the range of 50 mA (to prevent crossover distortion) combined with a fierce current driving of the end transistors (read: to the loudspeaker) when the audio output has to be high. Thus with the audio input to maximum level with the volume control potentiometer.

By the way: the showed schematic works good on 12-16 Volts and (say) 1,2 Ampère (constant current) thus as a class A Amplifier. It was not (!) tested on thermal runaway on higher supply voltages, also not (!) tested on its frequency range etc. End transistors get very hot in class A (!).

It was in this case (only) used for this demo purpose. But you can have fun with it. And the used transistors are very durable kind of HV transistors, not so much can go wrong, especially when the max. voltage is max. 14 Volts (not higher).

When you have questions about (certain analog) electronic circuits and how to make them: go to my Channel trailer (Radiofun232 on You Tube) and go to the “looking glass”. Type there the keywords that you want/need like: “power supply”, audio amplifier, capacitor tester, oscillator, radio, shortwave, shortwave radio, or any keyword you like to find the essential info & schematics on my YT channel.

My You Tube channel trailer is here: https://youtu.be/xbgQ8T3oqh4 When you search, search always “NEWEST FIRST” to get the right overview. You can also search via the “looking glass” on my Channel trailer via keywords like ”audio”, “radio”, “amplifier”, “filter”, “Shortwave”, “transistor”, “FET”, “oscillator”, “generator”, “switch”, “schmitt trigger” etc. My books about electronics & analog radio technology are available via the website of "LULU”, search for author “Ko Tilman” there.

https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=Ko+Tilman

I keep all my YT videos constant actual, so the original video’s with the most recent information are always on YouTube. Avoid my circuits that are republished, re-arranged, re-edited on other websites, giving not probable re-wiring, etc. Some persons try to find gold via my circuits. I take distance from all these fake claims. Upload 15 March 2022.

By the way: first vid is here https://youtu.be/QITHUqtQ6Ag
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Audio
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