#198: Basics of a Vbe Multiplier: what it is, how it works & where it is used
Why Electrical Engineering Stack Exchange recommends it
Electronics Stack Exchange answerers point people to it when explaining how a Vbe multiplier sets output-stage bias and cancels crossover distortion in class AB amplifiers.
Recommended by 3 people across 3 different months, 2015 to 2018.
What it is
A w2aew bench-and-whiteboard explainer on the Vbe multiplier, also called an amplified diode or rubber diode, which sets an adjustable voltage drop without stacking diodes or zeners. It uses a scope as a simple curve tracer with 2N3904/2N3906 transistors and shows typical uses such as biasing a push-pull output stage.
What people said
Recommends blondiepassesby · 2015-07 · Electrical Engineering Stack Exchange · on “BJT Base Collector short circuit”
There is a really good video on this circuit, how it works and its applications: [this video] Here are the lab book notes: [LINK]
Recommends owg60 · 2016-11 · Electrical Engineering Stack Exchange · on “How to design the A-B amplifier? I mean I have understood the working but how do i choose the values?”
Your scope traces look like you have some issues to deal with. This video on Vbe multipliers will help you Vbe multiplier and class B output [this video] . The problem with using diodes to offset the Vbe of the output transistors is that they will never have the transistors and diodes will never have the same current going through them. Although the diode curve is very steep it will has a slight voltage difference with current that you need to take care of.
Recommends owg60 · 2016-11 · Electrical Engineering Stack Exchange · on “Audio Amplifier Repair Explanation”
I think it is too soon to say change the value of the trim pot. If it is not doing it's job, I think you would have distortion at all level, even low levels. Here is a video that may help you to use your scope to see if the pot is doing its job; [this video] This pot corrects something called crossover distortion. It is caused by the upper and lower transistors both being off arround 0V.
Recommends Anthony · 2018-01 · Electrical Engineering Stack Exchange · on “Is there a good topology for splitting a DC power supply from a few LM2576 and LM2577?”
Thanks, this is comprehensive. I wasn't expecting R5, R2 and the V(BE) multiplier. So, a couple of things I gather so far: This is push-pull but with Sziklai multiplying betas. A V(BE) multiplier is like a variable diode giving an adjustable voltage drop between the bases and its purpose in this app is to minimize (null) the region where both of the pairs don't conduct. I found an example in the AofE (figure 2.78 in the new edition) and this w2aew video as well ( [this video] ). I don't think I really understand the I(Q) equation yet. I'm still processing...
Cites G36 · 2017-10 · Electrical Engineering Stack Exchange · on “Power amp Av with current source”
As for design. For good thermal compensation and to be able to "set" the output stage bias current instead of diodes you should use a BJT in "Vbe multiplier" aka "Rubber diode" configuration. [this video] Also you should add some emitter degeneration resistors into your current mirror circuit. They are needed due to Vbe mismatch. Or even in the differential pair. [LINK]