Hi there,

I am wondering if anyone can help me with a problem I am having trying to get an amplified current out of the Darlington pair configuration I have built.

I have constructed two circuits, one using a normal transistor(BC108) as a current amplifier and another using the same transistor (BC108) in the Darington pair configuration.

From the research I have done about the Darlington pair I have gathered that the base current Ib can be amplified greater than that of a single transistor circuit due to the Darlington's current gain being the gain of the two transistors multiplied together.

I find it strange that though experimenting I am unable to prove this theory!

I seem to get an amplified current (Ic) in the Darlington pair that is about the same as that of the amplified current in a single transistor circuit.

Why is this the case if the two circuits have completely different current gains??? How do I use the Darlington pair to amplify my base current greater than that of a single transistor circuit described in theory?

Thank you, I look forward to your help

 

You have probably put them in a configuration that self limits. We need schematics to see this. I use MSPaint, save as .png, back to this page, go advanced, manage attachments, browse, find it on the desktop, open, upload.

 

Please post a schematic of your circuit.

 

Hello,

It's correct that the Darlington pair results in an output current equal to multiplying the gain of each transistor in the pair. Can you post your circuit?

 

Thanks for the speedy reply guys.

I have attached two simple circuit diagrams of the exact configuration of what I have built.
Fig.1 is of the single transistor as a current amplifier and Fig.2 is of the Darlington pair as a current amplifier.

Notice how Ic is greater in the single transistor circuit.

 

You have noting to control the input current of the transistor so it will take all the current you apply to it (the base-emitter junction looks like a forward biased diode).

To properly do the test you adjust the input base current using a resistor and a variable DC source to give a certain collector current (say 10mA) without saturating the transistor. If you then measure the input current for both configurations you will see that the Darlington takes much less base current for the given collector current.

 

Thanks a lot for that Crutschow.

You mentioned using a resistor to control the amount of current flowing into the base, if the amount of current flowing into the base is substantial (about 4mA, within the parameters according the data sheet) will i still need a base resistor?

Thanks

 

Actually, the easiest way to see the current gain is to use the common collector (emitter follower) configuration. The emitter voltage will be less than the base voltage by one, or two (Darlington), diode drops, and the emitter current Ie will simply be Ve/Re. The base current will then be Ie/(β+1), where β is the current gain of the transistor or the Darlington. See the attachment.

 

Cheers Ron, I'll give that a try

 

Thanks a lot for that Crutschow.

You mentioned using a resistor to control the amount of current flowing into the base, if the amount of current flowing into the base is substantial (about 4mA, within the parameters according the data sheet) will i still need a base resistor?

You do if you want to measure the current gain of a common-emitter circuit.

 

see what you mean,
Thanks

 

You do if you want to measure the current gain of a common-emitter circuit.

Which is why I recommended the common-collector configuration. If Vce and Ic are essentially the same in both configurations, then the current gain will be the same. CC is self-biasing. With CE, you have to adjust base current, unless you use feedback.

 

Which is why I recommended the common-collector configuration. If Vce and Ic are essentially the same in both configurations, then the current gain will be the same. CC is self-biasing. With CE, you have to adjust base current, unless you use feedback.

I completely agree with that. My comments were just to help him understand why his measurements with a CE configuration weren't correct.

 

I completely agree with that. My comments were just to help him understand why his measurements with a CE configuration weren't correct.

I figgered.
I was also directing my comments to the OP.