![]() On the same silicon die, with 1cm spacing, the tau is 1.14 seconds at 1mm spacing (0.04 inches), the tau is 11.4 milliSeconds, thus some of the bass tones are tracked. Note with 0.96 seconds delay, even the bass tones will not be tracked by the diode/transistor combo. With 1cm spacing on copper, the thermal tau is 0.96 seconds. Share the same silicon die, and the diode is interdigitated into the transistor's emitter stripes.Įven if on the same heatsink, with 10cm spacing (copper heatsink), the thermal tau is 96 seconds. You cannot achieve tight thermal coupling, unless the diode and the transistor How does one achieve "tight thermal coupling", so the diodes track the transistor temperature? These guys were at the forefront of amplifier design in the 70's, but FET based designs changed it all. In terms of understanding this type of amplifier you can do no better than read the Linsley amplifier design, done before the days of FETs.Įqually great reading is Blomleys 'New Approach' design. The two diode bias obviously tracks better thermally than the four diode amplifier, but the larger amplifier is more capable and the output stage has larger heatsinks. This is done so that when you plug in a headset you typically only use the output as a Class A stage with very low distortion. Here the linear operation may cover 5 W or more. In the second circuit there are 4 diodes, and you'll see this commonly in high power (50-100 W) mains driven amplifiers. ![]() These amplifiers turn up in battery powered products a lot since they have low bias current Operation in Class A (linear) may only be 200-300 mW. Typically you might see this in a 5-10 W or so amplifier. What you describe with two diodes (which have Vf close to the Vbe of the output pair) are operating at very low Class A current. In the schematic below it shows both low and high A bias: When using two diodes to bias the output pair you are operating very close to Class B only. ![]() You can use resistors to bias an AB class amplifier (just look for Class AB on Google images to see a huge variation in designs), though typically you will see some other schema for setting the bias current through the output pair, or a constant current drive for the bases.Ī lot depends on whether you are setting very low class A capability or not. The answer is that it varies with the needs for the design.
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