Push-pull power amplifier on complementary darlingtons

Question

I decided to make a power DC-amplifier. I used complementary darlington pair TIP142 and TIP147. Also TL082 used for negative feedback. The schematic is as below:

I gave 100 kHz sine signal and under oscilloscope I noticed the crossover distortion. It seems opamp is too slow to compensate ±1.4V "dead zone" (distorion on oscilloscope above).

Then I decided to bias darlingtons bases using four 1n4148 diodes as follow:

Unfortunately after 5 seconds TIP147 blew up although transistors were mounted on a heat sink and the fun was over. :D

I was reflecting what actually gone wrong. I suppose that:

1. I didn't put diodes on the same heat sink as darlingtons. Consequently BE voltage dropped under the diodes bias.
2. In Horowitz's "The Art of Electronics" I have read if I we are using emiter resistors, then four diodes biasing are insufficient.

I would to know are my conclusions correct and also how to effectively get rid of crossover distortion.

Answer 1

Four diodes provide too strong bias. Together with positive temperature coefficient of output transistors and lack of thermal coupling, the result is sadly expected.

An option is to use three diodes, but the circuit is still thermally unstable until diodes are on the heatsink, and it has smaller but still very high crossover distortion.

^{simulate this circuit – Schematic created using CircuitLab}

Better solution is to use transistor. It must be mounted on the same heatsink, preferably on top of one darlington. Quiescent current should be adjusted to about 20mA (start with maximum resistence, measure mV on emitter resistor).

Answer 2

On 1 you are correct. The diodes are really temperature sensors.

On 2 the answer depends on your definition of "sufficient". Generally you want to have the same number of pn junctions in the bias section. As the idle current is small typically in the ma range the emitter resistor isn't really a factor.

In your case I think issue 1 is a far bigger driver . insufficient biasing would have saved you, everything else being equal.

Btw, oscillation many times kills amps too. So keep an eye on that.

Edit as your rail is single ended you will need a capacitor on the output to block dc.

Answer 3

Place a 1 microfarad ceramic cap between the darlington bases and look for waveform improvement on the scope .This improvement should be more visible at say 20KHz. This is because loop gain decreases with frequency paticularly for opamps.Increase the value of the emitter resistors .A rule of thumb is up to 10% of the load resistance .It is a trade off between output swing and thermal stability .Your diode circuit is adequate but the thermal coupling should be adressed .What I have done is make my own diodes out of TO126 transistors like BD139 or what ever is handy .Bolting this to the same heatsink has made for reliable Audio amplifiers operating well into class A .I have tied the base to the collector to make a diode .Also I have jacked it up with BE and BC resistors making a VBE multiplier. I have seen this VBE multiplier called a Rubber Diode .The 1 mic cap is still just as useful . The key here is that the thermal resistance of the TO126 package to the heatsink is really low compared to your 4148 or other peoples BC547 .There is little point in designing fancy bias circuits before your thermal impedance is dealt to .

Answer 4

The crossover occurs with both transistors switched off, causing ugly overswing. Instead of biasing diodes, you can put a resistor (like 470ohms or whatever the opamp can comfortably drive in addition to the base current while being up to 1.4V away from the output rail) directly between opamp output and speaker output. That makes for a more controlled transition while the transistors are both off.

Answer 5

try to avoid BJT transistors because BJT has large collector emitter resistor (CE(on)). in high current applications they loss more power and badly heat up. Try MOSFETs because its (RDS(on)) is very low. Transistors are current controlled device and MOSFET is voltage controlled device. Try this circuit