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Old 14th July 2014, 01:05 AM   #1
WSJ is offline WSJ  United States
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Default LM4702 Driving a Quasi Complementary Darlington Output

The design of this amp was center around the 2N6059 Darlington. The Fairchild FT359 is a cross with Vceo of 350V. I have a lot of FT359, TO-3 devices which was the motivation for the Quasi Complementary output stage. I measured hFE of over 1000 at 5 amps.
Additional Darlingtons and higher supply voltages could be incorporated to increase the output power.
Any ideas for improvements are welcome.

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Old 14th July 2014, 06:21 AM   #2
sangram is offline sangram  India
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If the pot wiper disconnects the output will be at full bias. The normal practice is to connect the pot from base to ground instead.

IME, and I don't have a good enough reason, the cap across the multiplier needs to be very large. I arrived at 100uF though I've seen 220 and larger values there. Also seen many circuits with typically small values and some with no cap at all. For the 4702, i ended up using the 100uF number.
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Old 14th July 2014, 06:29 PM   #3
AndrewT is offline AndrewT  Scotland
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most of us strive for a fairly linear current gain in our transistors.

The wider the range of current for similar gain the higher we presume the quality of performance.

Darlingtons break that rule.
They are far from linear.
They can peak out at >10000 hFE and even beyond 30000hFE.
and at other currents end up <1000hFE.

whereas a combination of two transistors connected as EFs can achieve excellent linearity over a range of currents.

The gain of the driver stage will have to try to make up for that Darlington gain variation.
regards Andrew T.
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Old 15th July 2014, 04:56 PM   #4
WSJ is offline WSJ  United States
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That sounds like a good design guideline. My goal is evaluate a Quasi Complementary Darlington output configuration, although it may not be the optimum output configuration.

The LM4702 application note AN-1490 shows a schematic with Darlingtons and (THD+N) 0.0006%.

"National's LM4702 is a stereo audio amplifier driver designed to drive
high-power discrete transistors, or Darlington pairs, in systems delivering
25W per channel to more than 300W per channel. When implemented in a
complete amplifier as shown in reference design AN-1490
(http://www.national.com/an/AN/AN-1490.pdf ), the total harmonic distortion
plus noise (THD+N) is 0.0006 percent into an 8-Ohm load. The amplifier's
output power is scaled by changing the supply voltage and corresponding
output power transistors. For higher current and power levels, the output
transistors can be configured in parallel without changing the basic LM4702
driver circuit."

National Semiconductor Debuts Industry's First Integrated, 200V Stereo Driver -- re> SANTA CLARA, Calif., Sept. 19 /PRNewswire-FirstCall/ --
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