Simple diamond buffer based on a WJ design. Enclosed in the feedback loop of an OPA827.
DB open loop distortion is 0.0006xx% THD20 at 2Vrms into a 250 Ohm load.
Enclosed in the OPA827 feedback loop distortion is 0.000020%THD20 at 5Vrms into a 250 load. Note : I find this too optimistic, maybe the OPA827 model is too good?
Output transistor bias is set to 50mA, a little high, but gives very low distortion, even open loop.
Output transistors are the great KSC3503E/KSA1381E complementary pair from Fairchild. Usually used as VAS transistors, these highly linear TO-126 transistors are great for a headamp outputstage. Only downside is the 100mA(200mA pulsed) maximum collector current, but otherwise they work marvelously.
Using the KSC3503E/KSA1381E output pair, the design is good for 150-600 Ohm loads with extremely low distortion. Want to drive 32 Ohm loads? Use BD139/BD140 or similar.
Stable into 1pF-10nF capacitive loads.
I went with a 4 layer PCB with Top Layer = Signal, Layer 2 = Power Ground Plane, Layer 3 = Power Plane, Bottom Layer = Signal.
It might be overkill, but the complete circuit is good to around 15MHz(-3dB) so I would rather be safe than sorry and besides, I have never done a 4 Layer PCB before so I might as well get some experience with it.
DB open loop distortion is 0.0006xx% THD20 at 2Vrms into a 250 Ohm load.
Enclosed in the OPA827 feedback loop distortion is 0.000020%THD20 at 5Vrms into a 250 load. Note : I find this too optimistic, maybe the OPA827 model is too good?
Output transistor bias is set to 50mA, a little high, but gives very low distortion, even open loop.
Output transistors are the great KSC3503E/KSA1381E complementary pair from Fairchild. Usually used as VAS transistors, these highly linear TO-126 transistors are great for a headamp outputstage. Only downside is the 100mA(200mA pulsed) maximum collector current, but otherwise they work marvelously.
Using the KSC3503E/KSA1381E output pair, the design is good for 150-600 Ohm loads with extremely low distortion. Want to drive 32 Ohm loads? Use BD139/BD140 or similar.
Stable into 1pF-10nF capacitive loads.
I went with a 4 layer PCB with Top Layer = Signal, Layer 2 = Power Ground Plane, Layer 3 = Power Plane, Bottom Layer = Signal.
It might be overkill, but the complete circuit is good to around 15MHz(-3dB) so I would rather be safe than sorry and besides, I have never done a 4 Layer PCB before so I might as well get some experience with it.