# Need help to determine quiescent current in a class AB output stage

I am studying class AB amplifier output stages and having Vbe multiplier for biasing to address cross-over distortion.

I am able to get good results with no cross-over distortion in the output. However, I am not able to find the equation for the quiescent collector current of the NPN transistor in the 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 the collector current of the 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. Commented Oct 20, 2020 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
Commented Oct 20, 2020 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
Commented Oct 20, 2020 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. Commented Oct 20, 2020 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
Commented Oct 21, 2020 at 8:34
• The Ebers Moll equation is a good place to start then @Amit Commented Oct 21, 2020 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
Commented Oct 21, 2020 at 15:36

Since each transistor has a range of base-emitter voltage then you need to be able to adjust the voltage at the base of Q5 for the amount of idle current.

Since the small transistors have a maximum allowed current of only 100mA peak then the amplifier produces a maximum peak power of only 0.04W into 4 ohms.