ECE3400 Lecture 27: BJT Push-Pull Amplifiers and VBE Multipliers (Analog Electronics, Georgia Tech) 

Thank you for your kind words!

You are welcome!

Thanks!

@@Lantertronics it so amazing to see how someone can master a subject, I'm still at the stage where this stuff is so overwelming and feels like magic. Maybe one day the math will make sense haha. Thanks for making your videos they are quite inspirational

True!

They don't exist in a refined video form -- maybe in the future if/when I get time, so I wanted to leave space for them in the numbering sequence.

These are Class AB amplifiers, but they differ in their depth of Class A operation. There is a myth that Class A operation provides better sound quality. In fact, deep Class A operation results in higher power losses, and on top of that, it sometimes degrades the performance of power transistors, as their gain drops at higher currents. Deep Class A is sometimes used in Class AB amplifiers, in which transistors in the power amplifier output operate in a common-emitter arrangement, that is, the signal is received from the collectors or drains. These are rail-to-rail amplifiers, which actually work better at higher resting currents. This is because these amplifiers are less resistant to load, and raising the resting current partially offsets this problem. Amplifier outputs with transistors in a common emitter arrangement are also often found in high-speed ICs that have rail-to-rail output. Manufacturers of such circuits, however, state that achieving the full voltage range of operation is possible only with very small loads - for example, 10 k ohms. If you increase the load, for example, to 1 k ohms, the circuit will work, but the allowable amplitude of the output signal will be greatly reduced. In conclusion, we can say that sometimes this increased resting current is a marketing gimmick, and sometimes it is necessary to achieve the intended output parameters of the amplifier.

@@GluonToo I agree

Check out Marshall's writeup on the Low TIM amplifier: leachlegacy.ece.gatech.edu/papers/lowtim/feb76feb77articles.pdf

The multipliers you are referring to in the THAT and SSI chips are very different than the VBE multiplier discussed here.

@@Lantertronics thank you, but: Connect to the campus VPN and then retry

@@Lantertronics Thanks. i have introduced it in to the simulator. at first it clipped massively to the power rails, then i realized i put the power on the wrong side of the circuit, now it doesn't do anything. here is the file for the falstad simulator, was quite a massive job. "/2fl8prex" just paste this behind a Falstad simulator "shared" link.

@@AnalogDude_ That's very strange, it should be public. Just try googling "low Tim amplifier"

In the case of the first amplifier, diodes D3-D6 increase the voltage drop and allow reducing the resistance of R25. In addition, they respond to the temperature of the interior of the amplifier, not just the power output heat sink itself. If the interior temperature increases, the current of transistor Q7 also increases, that is, the voltage CE deposited on it decreases. Therefore, eventually the bias of the power transistors also decreases. In the second case, the diodes spread the voltages that control the bases of transistors Q8 and Q9 by about 1.4 volts, so that both transistors remain on all the time and have bias.