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What is the purpose of diodes D1 and D2 in the 8 Ohm amplifier circuit below?

8 Ohm amplifier circuit

Visit ZL2PD No IC Audio Amplifier for more information of this circuit.

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    \$\begingroup\$ Way too broad... \$\endgroup\$ – pipe Oct 30 '17 at 15:52
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    \$\begingroup\$ Do you have any background about amplifiers (biasing, quiescent, amplifier classes etc)? Don't misunderstand, a little background is needed for you to understand. \$\endgroup\$ – Rohat Kılıç Oct 30 '17 at 15:53
  • \$\begingroup\$ Read a little bit about basic transistor configurations and push-pull amplifier stages. That is what the design is, a common emitter pre-amplifier stage, followed by a push-pull driven current amplifier. \$\endgroup\$ – Bart Oct 30 '17 at 15:55
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    \$\begingroup\$ Many books and websites discuss how such a class AB audio amplifier works so go study that. What makes you think that anyone would take the trouble to explain everything just for your personal benefit? Especially when it has already been written down. Your task is to find those explanations. \$\endgroup\$ – Bimpelrekkie Oct 30 '17 at 15:58
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    \$\begingroup\$ This is a very very standard configuration, there are many descriptions of how it works available on the interwebs. Put 'AB Amplifier' into google, and start reading tutorials from the top of the page, the top 3 hits are all tutorials. \$\endgroup\$ – Neil_UK Oct 30 '17 at 16:45
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In short, those diodes model the forward voltage drop of the BE diodes of Q2 and Q3, so the voltage at the base of Q2 is always two diode voltage drops higher than that on the base of Q3. This is needed for taking over conduction on Q2 as soon Q3 closes and vice versa.

Without those diodes, there would be a gap in the input signal from -0.7V to +0.7V in which none of the transistors would be conducting. Replacing the diodes by a simple resistor has the disadvantage that correction voltage depended on the current then. The diodes have an exponential I/U dependency, so if the current is high enough, the voltage drop on them is nearly constant.

Another measure you could take is mounting the diodes on the same heatsink as the transistors, so temperature difference doesn't change the biasing. But then, better biasing circuits exist und should be used.

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Those diodes are forward biased by R4 and what they do is help bias the top transistor's base a little more positive to minimize the crossover distortion (the distortion when the audio voltage is crossing zero). Very common technique.

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