Op amp based DBT bias control put to test
A while back I posted in the Bob Cordell Power Amp Book thread an op-amp based bias control schematic for a Diamond Buffer Triple (DBT) output stage utilizing the On-semi ThermalTrak output devices. Encouraged by the positive response I got around and put this design to bench test.
I rigged up a simple OTL amp based on two separate aluminum plates that serve as heatsinks, one big one for the TT transistors, the other one smaller for the pre-driver and driver transistors.
The op-amp is On-semi NCS2001. It's designed to operate off single cell Ni-MH battery swinging rail to rail, and is guaranteed to work on a 0.9V supply voltage. Can you find it in the picture?
In my original circuit I was assuming the bias voltage needed for a DBT output would be in the range of 1.1-1.4V, so I had the op-amp directly take supply on the bias voltage. The test result indicated that may not be a good idea. I found that the pre-driver transistors have a higher Vbe drop than do the driver transistors by 0.166V combined which jacks up the bias voltage by that much. At measured heatsink temperature of 70C I read a 0.975V bias voltage between the base of the pre-driver transistors, only 75 mV into the "green". So in the final circuit the op-amp gets power at the output of the pre-driver, comfortably at about 2.3V.
The bias voltage tracks temperature quite well. The power is turned on at 22C and I started taking readings at 24C. The voltage was taken between the emitter of the output transistors.
I'm now working on a power amp PCB design based on Fig. 3.14 of "The Book" featuring a DBT with max. 5 pairs of TT output transistors, TMC, solid state speaker relay, and I'll put this bias control in it.
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