MJ11016 darlington

hen I

That's what the datasheet says.

Sometimes you have to actually think, a computer simulation will not do it. They got an 8K across the input transistor and a 40 ohm across the output. The values are probably selected to make for the most linear gain.

If these transistors are still in production they are probably only for aud io outputs for low end equipment, like DJ ad KJ stuff, with their gain drop ping like a rock. It is like -23dB at at 1Mhz.

And with regular low frequency gain, they ain't all that bad but they ain't all that good either.

If you are on a standard DVM and on the ohms scale you should read 8,400 oh ms each way. If you read with your -|>|- scale it should read about a volt and a half, unless your meter does not put out enough, in which case it wil l read open.

In servicing, you are just looking for shorts, that is to either base or em itter to collector. In designing you need to look for a bit more. But it sh ould not read 40 ohms. If it does I would say the head transistor has a B-E short.

The fact that they put a low ohm resistor there at the output transistor le ads me to believe they meant this think for high temperatures at which the input transistor's leakage might become significant. Allison disagreed with me on that but that doesn't make him right. Lots of Darlingtons have those resistors, and get it through your head that they put NOTHING in that is n ot needed. Business is business. He has educated me on a couple of things, but not on Darlingtons.

But really, on a modern DVM they should not read 40 ohms, they should read

8.040 Kohms. If the front transistor got fried like that you got a problem in the driver circuitry, and if the whole thing is not shorted it is a myst ery to me.

Now if you got some special high voltage meter and all that let me know. Th at blows all I said out, except it DOES apply when using a regular DVM.

But you did not say that.

** Yep.

** That is a comment about YOUR simulation.

Try biasing it on properly. The output device takes over when the current exceeds 20mA.

... Phil

The usual problem with Darlingtons without that resistor is that the first stage gets starved of current, especially at light loads. That makes the stage slow and hurts its linearity.

Since V_BE of the output stage varies much less than its base current, the 40 ohm resistor is a simple way to ensure that (with enough quiescent bias) the first stage always has enough current to run well.

As the other Phil pointed out, you need about 15 mA to get the ~0.6V needed to bias the output stage into conduction.

Cheers

Phil Hobbs

** The 40 ohm B-E resistor has a major effect on the turn off time of the output device plus increases the Vceo. Such darlingtons were once common in switching inverters, PWM drives & regulators running at a few kHz.

IME, integrated power darlingtons always have such resistors incorporated.

.... Phil

If used as an amplifier (not hard switched), without the resistor, the first transistor would also introduce a pole (phase lag) at fairly low frequencies, which would tend to make the circuit more likely to oscillate. The pole would be at a frequency determined by the capacitance on the intermediate node multiplied by the output impedance of the first stage (roughly 30 Ohms divided by the collector current of the first transistor in milliamps). By adding the resistor and thereby running more current in the first stage, the pole will be at a higher frequency, so that hopefully it does not contribute troublesome amounts of phase shift up to the unity-gain frequency of the feedback loop in which the Darlington is used.

My recollection of those big ole Moto darlingtons is that they were shockingly leaky.

Thank goodness mosfets were invented.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

** Power Darlingtons have just one thing in common with MOSFETS.

You get a free diode in every pack !

.... Phil

** When well matched, complementary Darlingtons became available for use in audio amplifiers a few makers took them up while most did not. Problems wi th thermal runaway and poor SOA were two likely reasons.

Some manufacturers like to have their own code numbers printed on devices r ather than the makers number ( Crown and SAE come to mind )and this resulte d in a novel problem for repairers. With no recognisable part numbers to go by and a pair of TO3 output devices which had failed short - they replaced the Darlingtons with regular BJTs.

The result was an amp that ran very hot, did not respond to bias adjustment and strangely also had low power output. While some techs might twig to wh at was going on, a lot did not and quite a few such amps wound up on my ben ch with the above symptoms.

.... Phil