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Old 13th August 2007, 06:27 AM   #1
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Default LME49180 Based Amp Experiment

I want to build an amplifier based on LME49810 and NJL3281/NJL1302. The idea was posted in the other thread "LME49810 - a new cousin for LM4702". Although it have an advertized driving current of 60 mA, I want to do some simulations to see whether a driver stage can be omitted. I download the SPICE model of NJL3281/1302 from OnSemi. A symbol for LTspice was built. The schematic for simulation is shown here. A driver is added already. Supply rail is +/- 48 V. DC analysis gives NJL3281/1302 Iq = 110 mA.

Here I want to see the base current of NJL3281/1302 when driving a dummy load. The input sine source is 1 kHz and Vp = 30 V. The dummy load is 8R. Peak output current is 3.5 A. The corresponding base current of U1 is peak at 36 mA.

Will LME39810 maintains good performance when sourcing/sinking 36 mA? When the load is reduced to 4R, a larger driving current of 86 mA is obtained via simulation. It seems the beta drop effect reducing the current gain. Hence, a driver is definitly needed, as mentioned in http://www.diyaudio.com/forums/showt...5#post1235665.
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Old 14th August 2007, 10:02 AM   #2
glennb is offline glennb  Australia
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A resistive dummy load does not model the reactive currents which the output stage needs to deliver into a real-world speaker system. A good rule of thumb is to at least double the resistive current.

This means the 50mA output drive capability of the LME49810 will fall a long way short of your expectations of not being able to use intermediate driver transistors.

Unless you are going for a low output power (<30Wrms?) or want to double or triple-up the output devices to get some effective gain, the extra drivers will be needed.
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Old 14th August 2007, 12:27 PM   #3
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Hello Glenn

Thanks for your comments.

Is it possible to model a speaker with simple L,R,C network? Any standard model available?

I did some tests to see the load effect on the chip performance. I used the test circuit (Fig 3) shown in its data sheet and connected different resistors (10 k, 1 k, 600 R, 100 R) as load. The compensation cap Cc was 22 pF instead of the original 10 pF. It should affect the output drive capability.

THD vs output level (Vrms) of a 1 kHz test signal for different loads was measured. For load = 600 R, the THD is still very low indeed. The output peak current is about (20 * 1.414) /600 = 47 mA before clip. With load = 100 R the chip clips at about the same current, (3.5 * 1.414)/100 = 49 mA. However, THD increases substantially even at low output level, Vo=1 V, 100 R vs Vo=6V, 600 R. Why?
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Old 14th August 2007, 04:57 PM   #4
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50mA is pretty good but depending on your output power level and transistor type used you may want a driver stage. Check out the AN-1645, LM4702 driving FETs directly with no driver stage and the LM4702 only has like 5mA of output drive. There is a section in the note on a driver but it is minimal. If you are doing a lower power (<100W) amp then it may be enough drive current. It all depends on the full design and target specs. But if I was making a PCB I would include the option to have a driver stage and test to see what I prefer.

-SL
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