Question regarding darlington pair

[motbuddy]

Hi,

I have a few doubt about the darlington pair characteristic. After reading all the sources, i know that darlington pair has a higher current gain and a low input impedance. Then i start to prove it through experiments.

Figure1 is the common collector by using one transistor. and Figure2 is common collector using a darlington pair. Both has the same resistors value.

Figure1
1.JPG

Figure2
2.JPG

after calculation i found that, the Figure2 design just increase the input impedance by 4k ohm. And Ic for Figure1 is ~4.7mA while Ic for Figure2 is ~4.37mA. So what i curious about is isnt it Figure2 should have a more higher current compare to Figure1? If not what is the purpose i use the darlington pair since i get almost the same Ic by using the Figure1 design.

This is the question i cant figure out.

Hope someone can help me on this.

thank you.

[Mooly]

Your example is setting a fixed voltage on the base of the transistors via R1 and R2.

So the darlington will actually give a lower voltage across Re and thus an apparently lower Ic. It's lower because there are two base emitter junctions in the darlington configuration.

What the darlington does do is give a huge current gain. Remember bjt transistors are CURRENT driven, not voltage driven.

So if you remove R1 and R2 and replace just R1 with say 1meg ohm the first circuit will not develop much voltage across Re. The darlington will... the current gain is much higher.

[Mooly]

Or try this...

scale R1 and R2 upward by a factor of 10 and then by 100 and then by 1000 and see what happens

[motbuddy]

hi Mooly, thanks for the reply.

I can see that if i scale R1 and R2 upward (R1x10) and (R2x10) then the input impedance will become higher and Vout has a very very little increase which is almost the same as the Vin. Power delivered to the load still the same.

Am i correct?

thank you.

[Mooly]

That's essentially correct yes.

The darlington gives an effective current gain of the combined hfe's of the two transistors.

[Mooly]

Imagine Re to be a bulb... say a car headlight that draws 5 amps or so and you want to switch it on or off with a small control current perhaps from a microprocessor that can only supply a very low current. This is where the darlington could be used as it would operate with only a few milliamps of base current, the single transistor would need hundreds of milliamps of base current.

[motbuddy]

Originally Posted by Mooly

That's essentially correct yes.

The darlington gives an effective current gain of the combined hfe's of the two transistors.

But Mooly,

Although i increase the R1 and R2 value but it still wont affect my Vb value and so the Ie. And Ie = Ic . Am i correct? So i still can get the same Ic by using the Figure1 design right? then why i want to choose the darlington design?

thank you

[Mooly]

You have to start thinking in terms of current as well as voltage.

Ve = Vb less 0.7 volts (for a single silicon transistor).
Ie = Ib + Ic.

Vb is affected if there is not enough current available to supply the bias requirement of the transistor. The transistor pulls current from R1 and R2 and lowers the voltage (Vb).

If the transistor is high gain (a darlington) the current it draws from R1/2 is less.

Not easy to explain in a few words

[motbuddy]

Hi Mooly,
I just dont understand that, by applying the first design i still can get a low Ib value with a high Ic value. Then why darlington?

thank you

[motbuddy]

or maybe i should say if i want to prove that a darlington pair has a higher current than a basic common collector amplifier. Then i use the same R1 , R2 , Re and RL value for both figure1 and figure2. I still get almost the same Ie and Ic. Just that figure2 has higher impedance. This is the main point i cant understand, why darlington pair has higher current and my experiment couldnt prove it. And this makes me hard to understand and confuse.haha...

thank you