|
Okay I've laid out the 4 basic Darlington topologies, starting with the PNP version, then the NPN version, then the PNP driving Sziklai version and the NPN driving Sziklai version. Let's compare their parameters. The first two examples have a base voltage drop at around 1.4V below which, they'll be disabled and won't pass any current. The Sziklai pair as you can see is very sensitive and at 1V input voltage, it's already giving out 7A of current. To get the same amount of current, you'd have to drive the Darlington pairs with 9V. The current draw is once again very low and the same for all topologies but the driving voltage is different. That's because the Sziklai pair is wired in such way, that the driving transistor is in common emitter topology providing some voltage gain. Notice that all base connections of the output transistors have 8V on them. So basically a sziklai pair is a workaround of the huge voltage drop needed to maintain the same current capability. This although a huge advantage, also has some disadvantages too. The slew rate is more limited of a Sziklai pair. The difference is imperceptible at audio frequencies but at let's say 100Mhz, you have a definite slew rate deterioration, while the normal Darlington pair, remains almost perfect.
|
