Is there any use for the active region of a Darlington transistor?

Thread Starter

JunkieNL

Joined Mar 19, 2020
22
Sometimes I buy stuff to later figure out what to do with it. This is also known as the “solution looking for a problem” approach. The solution of today is a TIP120 Darlington transistor. The question of today is: what can I do with it?

I decided to explore the active region of the TIP120. I have built the circuit below with the following features:

  • The 10 ohm resistor is rated for 20 W.
  • The 470 ohm resistor limits the base current to 6.4 mA.
  • The 50k resistor is a 10 turn pot, which gives me the ability to set the base current with 0.01 uA precision.
TIP120 circuit.png

I found that as I increased the base current Ib from 112 uA to 114 uA (in like a gazillion steps), the collector current Ic went up from 0 to 151 mA and Vce dropped from 3 to 1.44 V. At this point Ic is limited by the 10 ohm resistor and not because the transistor is saturated. Beta was well above 1000. This was pretty much what I expected as it agreed with the Youtube tutorials I watched this morning.

What surprised me is the fact that it took quite long to settle after I had changed the base current. Sometimes even more than 10 minutes. Also the setup shows a lot of hysteresis.

Now here comes my question: If the active region of this Darlington transistor is so “unstable”, what can it be used for? I can imagine that the TIP120 can be used as a switch, but wouldn’t a MOSFET be a better choice? What can I use a TIP120 for?

I guess that were three question instead of one. Is that too much? :)
 

ElectricSpidey

Joined Dec 2, 2017
1,160
Well they are useful as switches when the base drive available is limited.

I use them as power control and brightness controls when PWM is too noisy or the wattage is low. (emitter follower)
 
Last edited:

Audioguru again

Joined Oct 21, 2019
1,505
You show a single PNP transistor but a TIP120 is a darlington transistor with two NPN transistors and two resistors.
Like any transistor, it slowly conducts more as it heats up.
It is a darlington so its saturation voltage loss is up to 4V(!) as shown on its datasheet.
The datasheet shows a base current that is 1/250th the collector current when it is used as a saturated switch but even then it has the high saturation voltage loss.
 

ElectricSpidey

Joined Dec 2, 2017
1,160
JFTR if you want any BJT to be stable in that configuration, you need to provide negative feedback.

I didn't even notice the incorrect symbol, I just "assumed" it was a NPN because that is what a TIP120 is...:oops:
 

Thread Starter

JunkieNL

Joined Mar 19, 2020
22
Commonly used in the output stage of audio amplifiers. (used in Emitter follower mode)
I looked into "emitter follower mode" and if I understand it correctly it is a current amplifier. It sounds interesting and might be something I would like to try for myself. Could this be used to connect large speakers to a phone's audio jack? Would that be a possible beginner project or am I getting in over my head?

Someone else mentioned in a reply that the erratic behaviour a saw was due to temperature changes. I guess with a emitter follower you can keep the offset the same and after an initial warm up it should be stable.
 

Audioguru again

Joined Oct 21, 2019
1,505
The output voltage level of a phone is a low level but the voltage level of a speaker is a high level.
An emitter follower increases the current only if an amplifier increases the voltage.

Besides, a single darlington would be a class-A room heater amplifier instead of a two transistors push-pull class-AB amplifier.
Lookup class-A and class-AB. The class-A amplifier heats a lot even if the amplifier is producing no sounds.

An emitter follower also changes its output voltage a little when its temperature changes but it has no voltage gain so the output voltage change is not amplified.
 

Thread Starter

JunkieNL

Joined Mar 19, 2020
22
You show a single PNP transistor but a TIP120 is a darlington transistor with two NPN transistors and two resistors.
Like any transistor, it slowly conducts more as it heats up.
It is a darlington so its saturation voltage loss is up to 4V(!) as shown on its datasheet.
The datasheet shows a base current that is 1/250th the collector current when it is used as a saturated switch but even then it has the high saturation voltage loss.
You are correct. I accidently drew a PNP. I drew just one because I treat it as a single high gain NPN.

I bought this transistor because a saw some projects were these were used to connect lights and motors to an Arduino. I read in the datasheet that the saturation is max 2.0 V voor 3 amps. Maybe it is less for lower currents and gets the dissipated power down to 1 watt. Anyway, a MOSFET will do a better job.
 

atferrari

Joined Jan 6, 2004
3,966
Sometimes I buy stuff to later figure out what to do with it. This is also known as the “solution looking for a problem” approach. The solution of today is a TIP120 Darlington transistor. The question of today is: what can I do with it?

I decided to explore the active region of the TIP120. I have built the circuit below with the following features:

  • The 10 ohm resistor is rated for 20 W.
  • The 470 ohm resistor limits the base current to 6.4 mA.
  • The 50k resistor is a 10 turn pot, which gives me the ability to set the base current with 0.01 uA precision.
View attachment 204224

I found that as I increased the base current Ib from 112 uA to 114 uA (in like a gazillion steps), the collector current Ic went up from 0 to 151 mA and Vce dropped from 3 to 1.44 V. At this point Ic is limited by the 10 ohm resistor and not because the transistor is saturated. Beta was well above 1000. This was pretty much what I expected as it agreed with the Youtube tutorials I watched this morning.

What surprised me is the fact that it took quite long to settle after I had changed the base current. Sometimes even more than 10 minutes. Also the setup shows a lot of hysteresis.

Now here comes my question: If the active region of this Darlington transistor is so “unstable”, what can it be used for? I can imagine that the TIP120 can be used as a switch, but wouldn’t a MOSFET be a better choice? What can I use a TIP120 for?

I guess that were three question instead of one. Is that too much? :)
Four questions in fact. Sorry.
 

MrChips

Joined Oct 2, 2009
21,159
Opamps typically have voltage gain in excess of 100,000.
A darlington might have current gain greater than 1000.
In either case, if you want to use it as a linear amplifier you have to tame the gain with negative feedback.

You can add negative feedback in a transistor circuit by adding a resistor at the emitter leg.
You can also add negative feedback by biasing the base with a resistor from the collector.

1586733990358.png
 

Thread Starter

JunkieNL

Joined Mar 19, 2020
22
The output voltage level of a phone is a low level but the voltage level of a speaker is a high level.
An emitter follower increases the current only if an amplifier increases the voltage.

Besides, a single darlington would be a class-A room heater amplifier instead of a two transistors push-pull class-AB amplifier.
Lookup class-A and class-AB. The class-A amplifier heats a lot even if the amplifier is producing no sounds.

An emitter follower also changes its output voltage a little when its temperature changes but it has no voltage gain so the output voltage change is not amplified.
Well, summer is coming and I do not need more heaters! Do you have any ideas for another project with an emitter follower? It should be fun, cheap, about an afternoons work and for learning purposes only. I already own a TIP120, so that should cut down on the cost factor. :)
 

DickCappels

Joined Aug 21, 2008
6,389
1586766287329.png

(Some text removed for clarity)
What surprised me is the fact that it took quite long to settle after I had changed the base current. Sometimes even more than 10 minutes. Also the setup shows a lot of hysteresis.

Now here comes my question: If the active region of this Darlington transistor is so “unstable”, what can it be used for? I can imagine that the TIP120 can be used as a switch, but wouldn’t a MOSFET be a better choice? What can I use a TIP120 for?

I guess that were three question instead of one. Is that too much? :)
Put it inside a feedback loop and it will be fine. I once used darlingtons in the output stage of a ±10A low noise highly stable (.01% deviation over temperature) linear power supply.
 

Alec_t

Joined Sep 17, 2013
11,311
I guess with a emitter follower you can keep the offset the same and after an initial warm up it should be stable.
It won't be stable unaided. Negative feedback, mentioned above, can automatically adjust the base bias to provide stability.
 

MisterBill2

Joined Jan 23, 2018
6,092
The linear (active) section of any transistor is useful as an amplifier. It will amaze some folks but a major use of transistors is as amplifiers, avoiding both the saturation and cutoff sections of the transfer function curve.
Darlington transistors do usually have slower response times, but slow is a relative term, of course. In the instance of your experiment there was probably a fair amount of heating, which will increase the leakage current, possibly quite a bit.
 
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