Push-pull BJT output biasing - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: July 09, 2022, 06:42:49 am
content: I'm learning how BJT push pull amps are designed, and I found a technique like this used for biasing:[ Attachment Invalid Or Does Not Exist ]I found this circuit on another forum, it works fine in LTC sim.as I understand it, it makes a divider with the 1.5k and 470 ohm resistor, to form a sort-of custom Vf drop diode.and this is done for better linearity than a diode yes?why not just take regular diodes and put a 22u cap across them?although I suppose that would still be the same amount of components

author: pqass
author_rank: Super Contributor
timestamp: July 09, 2022, 07:22:48 am
content: Quote from: ELS122 on July 09, 2022, 06:42:49 amIt's called a Vbe multiplier (aka "rubber diode") and is used to vary the voltage drop to just turn on both transistors until there is no crossover distortion. That is, to precisely dial-in a minimal current through both transistors when no input signal is present. You can use two diodes+parallel cap but there may be a slight gap where both transistors are off or too much current if diode Vf and transistor Vbe don't match.See these amp designs for the application of either techniques.https://sound-au.com/project3a.htmhttps://sound-au.com/project113.htmSee this video for how a Vbe multiplier works and its application at 9:55:

author: CaptDon
author_rank: Super Contributor
timestamp: July 11, 2022, 01:15:17 pm
content: In that layout that transistor and pair of resistors acts more like a constant current control and is usually placed very close to the output transistors physically so they are all at a similar temperature. The capacitor helps provide a low impedance signal drive path to the base of the upper transistor. These minimal parts count circuits are rarely used above a couple of watts of output. They work but not with the stability required when you get into the tens, hundreds or thousands of watts.

author: CaptDon
author_rank: Super Contributor
timestamp: July 22, 2022, 12:25:55 pm
content: There was an op-amp years ago but sadly they went obsolete. I don't exactly recall the number, something like LM540? They were usually in the 8 lead (or was it 10?) TO-5 style can. They had nearly everything you needed to drive a push-pull pair of output transistors and I believe they could operate from voltages considered insane for op-amps at the time, perhaps +/- 40vdc or 80vdc total. They were very common around 1975 or so but no pin compatible replacement ever came along. What came next were the 10 lead black epoxy lumps good up to around 100 watts RMS (with zero headroom so about 25 watts average output). I retro-fitted a bunch of old guitar amps with those Toshiba, Sanyo and Hitachi 'STK' modules. Even ECG/NTE sold the replacements (ECG1350 ECG1128 if I remember correctly) and were readily available and not very costly. Our band has a Kawai keyboard amp that uses the 100 watt module. I have a couple spares as our keyboard player seems to blow up the amp about once every 2 years.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 22, 2022, 12:46:29 pm
content: The transistor based biasing of the output stage works reasonably good. With parallel use of transistors for higher power and for short circuit protection one needs emitter resistors anyway. The thermal design is not easy but managable to get a good enough / stable enough bias.There are a few examples with an additional loop to control the biasing with an OP-ap circuit. However this is not so simple, as sensing of the biasing current is only possible if there is no active current from the side in question and there can be dynamic loads, that can draw current even at zero voltage. So an OP-amp alone is not sufficient. It is more a nonlinear circuit with sampling and similar. The simple temperature dependent voltage shift is not that bad in comparison. Once set up correctly there is nothing wrong with it and the small BJSTs , diodes and resistors are usually cheap.For the LTP in the actual amplifier input, the use of discrete transistors if for several reasons: many audio amplifiers use a supply considerably higher than the usual +-15 V supply for OP-amps. As simple 2N3904 pair is also quite fast to the usual audio op amps. Audio amplifiers usually have a gain of some 20-30 dB this would be part of an OP-amps GBW. It takes a little more effort, but discrete transistors can get lower noise / better lineartiy than most OP-amps. Many linear audio amplifiers designs, especially those one finds in the web are old - more like 1980s based, when OP-amps were not as good as they are today. Moden amplifiers now often use class D amplifier chips. There are also ready made class AB hifi ampliers of good quality for some 50 W, which is normally sufficient for home use. However a circuit like the typical circuit around an LM3886 is not really interestring for DIY.

author: MrAl
author_rank: Super Contributor
timestamp: July 23, 2022, 03:26:08 am
content: Hello,You could most likely do the bias control with low cost op amps. If the power rail is too high, you could use the op amp and transistors to operate at higher voltages. The idea in theory would be to create two current controlled current sources that measure the emitter currents and adjust the bias to stay at least at some lower limit. This requires two emitter resistors and the output is taken at the junction of the two resistors.This kind of circuit would be linear, but would be controlling the bias current in a non linear way because of the way the output transistor behave.Also interesting although not sure how practical, you could use a microcontroller to measure the emitter currents and adjust the two biases as needed. Would be interesting although some care would be required to prevent noise injection.Most higher power modern designs now would be done with class D because they are the most efficient. If you analyze a push pull output amplifier you would be surprised to see how much power is wasted. It's because the output is sinusoidal and the power rail is a constant DC voltage. That means that any time the output is not getting the full voltage of the power rail, there is a loss in that transistor. We could go over that math but it's a lot of power wasted that's for sure.Class D transistors are either fully 'on' or fully 'off' and that's similar to how power converters work. If sine power converters had to use linear amplifiers to output their sinusoids, we'd all be in trouble. Efficiency would be in the dumps :-)