# Need Help to Determine Quiescent Current in Class AB Output Stage

I am studying class AB amplifier output stage and having Vbe multiplier as biasing to address cross-over distortion. I am able to get good results with no Cross-Over distortion in output. However I am not able to find equation for quiescent collector current of NPN transistor in output stage due to my limited knowledge of circuit analysis. Please help me analyze how the Ic_Q is 19.4 mA, and what equations are involved that are governing this collector current of NPN output stage transistor for the given circuit.

• 19.4 mA is too high for the puny BC. I remember the days of good ol' 3055 required some 5-10 mA. Also, you're using a complementary pair, you should check both sides. Oct 20 '20 at 18:02
• Right now I am not concerned about if this current is 19mA or 19uA. All I am concerned about is what is that equation that is driving this 19mA into collector given this circuit configuration. I could have settled for my sweet sounding output stage which it is (for my application) but I just can not settle their and want an analytical reasoning for this 19mA value at collector...
– Amit
Oct 20 '20 at 18:16

However I am not able to find equation for quiescent collector current of NPN transistor in output stage due to my limited knowledge of circuit analysis.

There isn't really a recommended value. For some who require extremely low distortion, the collector current will be significantly higher and for the rest of us mortals with an "adjuster" (aka small screwdriver) we'll combine R3, R18 and R9 into a pot and adjust it so that it sounds about right.

• Hi Andy, thanks for replying. Yes, I could have settled by just adjusting the pots which I did for 1 week, but just could not settle with that I really dont have any clue on what is that circuit analysis equation that is responsible for driving this 19mA current given the above circuit configuration. Greatly appreciate if you have more directions from circuit analysis stand point. Thanks.
– Amit
Oct 20 '20 at 18:19
• It's a marginal thing and given that transistors are so flaky on repeatability it's just not worth analysing because any analysis simply won't reflect all the practical nuances associated with our living reality. It's just not worth any effort - a bit like trying to predict the average length of a banana grown in a particular field in a particular country having an average rainfall of x and an average daylight length of y. It's not important - you'd measure them when pulling em off the tree. Oct 20 '20 at 19:37
• Thanks Andy. I appreciate the unpredictability of transistors. However, given the topic and subject of circuit analysis, I am looking forward to at least a governing equation, whether or not it produces results as per the LTSpice value. Currently, I am more interested in the model or some system of equations.
– Amit
Oct 21 '20 at 8:34
• The Ebers Moll equation is a good place to start then @Amit Oct 21 '20 at 11:47
• Hi Andy, I have got this good article while searching for this question. There are few more like me hunting for perfect equations. I hope you find it useful. Thanks. uweb.engr.arizona.edu/~brew/ece304spr07/Pdf/VBE%20Design.pdf
– Amit
Oct 21 '20 at 15:36