Hi Folks,

I made a short video outlining my troubles getting this circuit going. Any tips would be awesome!!

http://youtu.be/CdUqb_PwcHU

Thanks,

Joe

Troubleshooting Class AB Darlington Audio Power Amp

 

What is the bias current through the diodes?
What type of diodes are they?
Are you absolutely positive that everything is wired correctly including the transistor pinouts and/or interchanging the NPN and PNP (?) since it really sounds like something is miswired.

 

Thanks for the reply!

I'll have another look at the wiring tonight. I think I'll spread it out on the breadboard so I can make it easier to wire and to show the connections. The diodes are 1N4001. I'll measure the current through the diodes aswell. I would assume it would be the same as the current through R1? i.e. (Vcc-2.8)/R1? I suppose aslong as I measure a 2.8 drop across all four diodes I can assume that the current through them is efficient.

Am I right about setting Vq to 1/2Vcc. If so, how is this done?

 

Diode current is equal to
Id = (Vcc - 4VD)/(R1 + R2) if we ignore Darlington base currents.
And Vq = 1/2Vcc only if R1 = R2

 

Awesome! Thanks! Can't wait to take another stab at this circuit tonight!

If everything is working correctly what would the standby current draw be. I assume it would be a lot less than the 5 amps I'm drawing currently. Or would it be equal to the current through R1, D1-4, R2 plus yet base currents?

 

To be honest you should never try to build this theoretical circuit in real life.
This circuit is a disaster waiting to happen. Firs add some emitter resistor.
Re1 and Re2 = 0.1Ω ... 1Ω. Next replace D1 with a potentiometer 1KΩ and set the standby current in range of 0.1A.

 

Please be aware that the TIP 142 / 7 incorporate internal resistors and do not conform to your diagram.

http://www.st.com/web/en/resource/technical/document/datasheet/CD00000914.pdf

 

............................. Firs add some emitter resistor.
Re1 and Re2 = 0.1Ω ... 1Ω. Next replace D1 with a potentiometer 1KΩ and set the standby current in range of 0.1A.

Adding a pot is a good idea but you want to retain the diode. You need four diodes to help provide temperature compensation for the four transistor base-emitter junctions. In practice these diodes should be thermally glued to the transistor cases (two on each case) to improve this compensation effect.