Which transistor configuration?

[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]

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.

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

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?

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.

[hlavac]

You would be fine with just a single BC547B. They have beta of 200-450. For max collector current of 100mA you would need at most 0,5mA of base current, 8k2 base resistor would do for 5V logic control. And yes don't forget the protection diode across the relay or magic smoke will fly.

[armandas]

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

I use mosfets in my own designs. I came up with this question when helping someone out and they already had BJTs.

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

I haven't. I only drew out the components necessary to make sense of my question.

[armandas]

You would be fine with just a single BC547B. They have beta of 200-450. For max collector current of 100mA you would need at most 0,5mA of base current, 8k2 base resistor would do for 5V logic control. And yes don't forget the protection diode across the relay or magic smoke will fly.

I picked up transistor out of a batch and got around 70mA with 10K on the base. I wanted to be pretty sure the circuit would work, so I opted for a two-stage solution.

[hlavac]

You can lower the base resistor a bit to get more current, modern MCUs can drive 25mA or more, 1mA will be fine

[vlf3]

Apart from what has been said, and it's valid information with the pair exhibiting current gains of 10,000 or more, their is the impedance issue too.

By using a darling configuration or (super-alpha pair), you also create a very high input impedance beyond several Megohms, dependant on the transistor type used; that's okay if you require a high input impedance, but other factors come into play... e.g. slow switching action thus, low frequency operation; also a through phase shift occurs, if used in negative feedback designs.

The preferred transistor configuration is the right-hand circuit, where the collectors are joined together, to maintain equal current sourcing, through the collector load.

[armandas]

Thanks for the info, guys!