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Legacy Personal Blogs Transistors: the Transistor as a Diode
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  • Author Author: jc2048
  • Date Created: 23 Feb 2017 10:45 AM Date Created
  • Views 4123 views
  • Likes 7 likes
  • Comments 14 comments
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Transistors: the Transistor as a Diode

jc2048
jc2048
23 Feb 2017

I thought I was going to do saturation next, but somehow I've diverted off into looking at the transistor as a diode. This came about from my

referring to Bob Pease's book, Troubleshooting Analog Circuits, to look up something else and then thinking that I'd like to measure for

myself the graph he shows of Vf against If for various diodes, including transistors connected as diodes.

 

A bipolar junction transistor contains two PN junctions, one between the base and the emitter (B-E) and one between the base and the collector (B-C),

and either can be used as a diode. When used in a circuit, the base-emitter diode normally has the collector shorted to the base (CB-E). It is

also possible to join the collector and emitter (B-CE), giving a final possible configuration (then the two diodes are in parallel).

 

 

image

 

The following graph shows the B-C B-E and CB-E forms for a  2N37042N3704 CB-E for a BD135(a medium power device intended for audio amplifiers

along with a few common diodes. I've also included a small-signal Schottky diode (BAT42) for comparison. This plot is in the usual form.

 

image

 

This is the same data with the current plotted on a log scale. This has the advantages that we can see the low current detail better and it allows us to see

and compare the forms of the curves.

 

 

image

 

All the curves are different. What I am measuring evidently isn't just a simple piece of semiconductor physics, though all the curves have a similar

form with a straight section at the lower currents and moving away from that higher up. I'm not going to sample lots of devices, but be aware that

there will be differences between different batches from the same manufacturer and differences between parts marked with the same part number

from different manufacturers.

 

The two CB-E transistor curves stand out as being almost straight lines over the five decades of current I've done the measurements for. In practice

the relationship holds much further and it points to one of the main uses of transistors in this configuration - linear to log and log to linear conversion,

often in conjunction with an op-amp.

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Top Comments

  • D_Hersey
    D_Hersey over 8 years ago +2
    They often use super-matched (inter-digitated) pairs for the log and anti-log amps, using one as the artifact and the other to null-out thermal offsets. One useful circuit that can be constructed (among…
  • D_Hersey
    D_Hersey over 8 years ago in reply to jc2048 +2
    Perhaps CB-E is faster because it uses base current to help it turn on.
  • clem57
    clem57 over 8 years ago +1
    Great idea to test a transistor as a diode.
  • COMPACT
    COMPACT over 8 years ago in reply to clem57

    So a theory is just a formally scribed conjecture.

    What you've said explains what that suppositories can do. It allows one to conjecture or derive theories from the throne.

    I just hope you don't get saturated by the current in the process and ensure everything is captured in the collector from the emitter and not left as a mess on the base.

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  • DAB
    DAB over 8 years ago in reply to clem57

    I stand corrected.

     

    DAB

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  • clem57
    clem57 over 8 years ago in reply to DAB

    conjecture ->A supposition based upon incomplete evidence; a hypothesis.

    theory -> A coherent statement or set of ideas that explains observed facts or phenomena, or which sets out the laws and principles of something known or observed; a hypothesis confirmed by observation, experiment etc

     

    Therefore my statement stands if you present no math! And they are not synonymous with each other.

    Clem

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  • DAB
    DAB over 8 years ago in reply to clem57

    All theories are conjecture.

     

    Until we get better measurement capability all we can do is develop new models, compare the models to the measurements we have and try to assess which model gets the best data correlation.

     

    At this point, my model fits the data and has mathematics that work at the small scale and fits physics, chemistry and biology effects.

     

    Truth is elusive, but I have learned to follow the math.

     

    DAB

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  • D_Hersey
    D_Hersey over 8 years ago in reply to jc2048

    It seems one could fab a pretty nice diode-ring mixer from a matched quad.  IIRC Gilbert was working along these lines. . .

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