# Where does the name Diode-Connected BJT come from?

Discussion in 'Homework Help' started by Finaleblue, Sep 21, 2015.

1. ### Finaleblue Thread Starter New Member

Sep 22, 2014
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0
In npn BJT, when the collector is shorted with the base we call it a diode-connected bjt.
How I understand it is, when we put a voltage source in shorted collector-emitter node and ground the emitter, the bjt acts like a small resistor.
What happens when we connect the voltage source on the emitter end and ground the collector in diode-connected bjt?
Assuming from the name "diode" I assume there should be no current until the voltage reaches some breakdown point.

So, long story short, can someone explain me why we call such bjt diode-connected?
Thanks

2. ### WBahn Moderator

Mar 31, 2012
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What does the I-V characteristic of a diode-connected BJT look like?

3. ### Finaleblue Thread Starter New Member

Sep 22, 2014
7
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Shouldn't it be just linear since it acts like a resistor?

4. ### WBahn Moderator

Mar 31, 2012
18,088
4,917
On what basis to you claim that it acts like a resistor?

If it acts like a resistor, then if you double the current through it, you double the voltage across it. How does this reconcile with your understanding of the voltage across the base-emitter junction?

5. ### crutschow Expert

Mar 14, 2008
13,505
3,379
Why would you think something called a diode-connected BJT act like a resistor?

It actually acts like a diode which conducts current in the forward direction (the direction of the base-emitter arrow) with the logarithmic current versus voltage relationship of a diode, and blocks in the reverse direction until the reverse breakdown voltage of the base-emitter junction is exceeded.
The advantage of this connection is that it acts very close to an ideal diode at low currents, more so than a typical standard diode.

6. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,047
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This configuration actual provided a constant current source; it's prevalent inside most op-amps. It's called diode connected because there are only two terminals from the "outside" universe.

Eric

Sep 22, 2014
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8. ### WBahn Moderator

Mar 31, 2012
18,088
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????

As I read it, the configuration he is talking about is a voltage source connected directly across a diode-connected BJT. How does that provide a constant current source?

9. ### WBahn Moderator

Mar 31, 2012
18,088
4,917
Your confusion comes from not distinguishing between large-signal and small-signal models.

Hint: What is the small-signal model of a regular old diode?

10. ### Finaleblue Thread Starter New Member

Sep 22, 2014
7
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Thanks WBahn!

11. ### crutschow Expert

Mar 14, 2008
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3,379
You are confusing that with a current mirror, half of which consists of a diode-connected BJT.