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Which transistor configuration?

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armandas:
In the following image, what are the advantages and disadvantages of the two transistor configurations?
(The 10K resistors represent source impedance)



On one hand, I found this website that discourages the use of Darlington configuration:



On the other hand, The Art of Electronics lists this circuit in the "bad circuits" section.

hlavac:
In the left schematic you are sending unnecessarily high current thru base-emitor diode of the right transistor. About 100mA. Its wasted. Need to limit that with resistor in collector of left transistor.

Right schematic is proper darlington. It is frowned upon for its (double Vbe) higher voltage drop when saturated therefore higher losses and lesser voltage swing. Not good for switching loads.

JackOfVA:
Is this supposed to be a switching circuit where the 1mH inductor is a relay coil or is it some sort of linear amplifier where the signal is picked off Q2's collector?

armandas:

--- Quote from: hlavac on April 28, 2013, 05:47:56 pm ---In the left schematic you are sending unnecessarily high current thru base-emitor diode of the right transistor. About 100mA. Its wasted. Need to limit that with resistor in collector of left transistor.

Right schematic is proper darlington. It is frowned upon for its (double Vbe) higher voltage drop when saturated therefore higher losses and lesser voltage swing. Not good for switching loads.

--- End quote ---

Thanks. I now understand the first concept, but I'll have to read more about darlington configuration.


--- Quote from: JackOfVA on April 28, 2013, 06:25:38 pm ---Is this supposed to be a switching circuit where the 1mH inductor is a relay coil or is it some sort of linear amplifier where the signal is picked off Q2's collector?

--- End quote ---

The inductor represents the coil of a relay.

Dave:
Why not just use a N-FET? Most of them will open just fine at 5V.

Also, don't forget the flyback diode for the relay coil.

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