Here is my circuit adopting the famous LME49811 amplifier front-end and ThermalTrakTM
transistors. This one is a rather conventional topology but implementing some tricks to improve the performance.
DC SERVO CONTROL
The goal of this project is to accomplish a full DC operation. No capacitors were used in the signal path. In order to keep the output DC offset to 0V, a servo stage is used. This one is based on a simple integrator. It’s built around a FET input op amp – IC2. For the role I’ve chosen to use TI’s OPA2604. The quality of the op amp is critical as it is a part of the signal path. OPA2604 is a low-noise, low-distortion, high-quality, audio op amp. A simple TL072 would also do the job. This however could compromise the performance.
The voltage sense is taken from the output at the point where R13 and the output inductor is soldered to the board. The signal is then filtered using a low-pass filter – R14, C10. This effectively removes any AC signal and leaves the DC portion only. The filter is tuned at 0.2Hz. D5 and D6 serve as protection for the op amp. The output from the servo is then injected via R11 to the negative feedback path formed by R12 and R3.
COMPENSATION AND STABILLITY
To allow any further compensation and stability adjustments I’ve included one additional option – R26, C15. However upon inspection, no further compensation was necessary, so these could be omitted.
The dominant pole compensation is implemented via C6, C7, R4. This one is a bit different from the traditional compensation used with LME49811. A two-pole compensation was chosen. This one is a bit tricky and when used the wrong way could cause oscillations and instability. When used wisely however, it has an advantage. In other words a two-pole compensation allows for more negative feedback towards high frequency region. This lowers the distortion and improves performance.
THE OUPUT STAGE
The output stage is a traditional emitter-follower topology with shared driver emitter resistor. Thanks to the ThermalTrakTM transistors the thermal compensation is now a bit different. The two tracking diodes are now part of the Vbe multiplier. This allows for the fastest possible temperature tracking.
The drivers however can not benefit from the usage of ThermalTrakTM. To ensure improved thermal stability, the sensor transistor Q1 is thermally coupled to one of the drivers Q2.
The output devices as already clear are the ONsemi’s ThermalTrakTM NJL1302D/NJL3281D. Datasheet is available here:
Coming in part 2 – PCB design and explanation.
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