hi, friends, here is the diagram of a semicomplementry OTL power amplifier. what puzzles me is : how to decide (or compute) the voltages of point A and B?where A is base of bg5, and B is base of bg6.

 

Feedback through R6 controls the voltage on the collector of BG8. The Voltage on the base of BG1 is set by the ratio of R1 and R2. The emitter of BG1 will be Vbe below that, so R1 and R2 are chosen to set the base of BG1 to be half supply plus Vbe.

Then you need to set the current through the output stage (BG7/BG8) to be about 25mA*. You do that by calculating the voltage across R14 and adding the various Vbe voltages, and that gives you the voltage at A and B.

By the way, the term OTL (Output TransformerLess) is rarely used with semiconductor amplifiers, because they have all been designed without transformers apart from the very early days of semiconductors. OTL is generally only used when referring to valve amplifiers without output transformers.

*It doesn't specify the output power of the amplifier. 25mA is a guide, you have plenty of latitude. 100mA would still be OK, it would just run warmer.

 

Feedback through R6 controls the voltage on the collector of BG8. The Voltage on the base of BG1 is set by the ratio of R1 and R2. The emitter of BG1 will be Vbe below that, so R1 and R2 are chosen to set the base of BG1 to be half supply plus Vbe.

Then you need to set the current through the output stage (BG7/BG8) to be about 25mA*. You do that by calculating the voltage across R14 and adding the various Vbe voltages, and that gives you the voltage at A and B.

By the way, the term OTL (Output TransformerLess) is rarely used with semiconductor amplifiers, because they have all been designed without transformers apart from the very early days of semiconductors. OTL is generally only used when referring to valve amplifiers without output transformers.

*It doesn't specify the output power of the amplifier. 25mA is a guide, you have plenty of latitude. 100mA would still be OK, it would just run warmer.

ty.
I wonder if you could tell me how bg1,bg2,bg3,bg4 decide voltage of A and B.
thx.

 

One thread is enough.
https://forum.allaboutcircuits.com/threads/power-resister-and-capacitor.186876/