# Can someone please explain where transistor graphs like this come from?

#### FuneralHomeJanitor

Joined Oct 12, 2019
31
I have been told about characteristic curves like this for a while but have never fully understood where they come from. I don’t see them on the datasheets, although they have a similar one where Ic and Ib are switched, and my books don’t really explain where they come from. Is this something I need to draw myself? If so where do I get the information required? Is it through testing the transistor on a meter or doing some math? Is the load line something I draw? Thanks in advance.

#### Attachments

• 568 KB Views: 31

#### Papabravo

Joined Feb 24, 2006
15,458
They come from the data taken by measurements in a laboratory. The process is amenable to automation and there are so called "curve tracer" units that come with high end oscilloscopes. We did this experiment when I was an undergraduate with the 2N3904(NPN) and 2N3906(PNP) (ca.1967). It was a tedious and painstaking enterprise.

Joined Jan 15, 2015
5,875
The image you posted looks like it came from a Transistor Curve Tracer for example the old Tektronix 575. Anyway it is just a graphic plot.

Ron

#### MrChips

Joined Oct 2, 2009
23,056
The concept of a load line is a graphical way of solving simultaneous equations.
In the case of circuit design, it allows you to find the operating point or Q-point (quiescent point) as well as DC and AC gain in a transistor circuit.

For example, given a diode D1 and series resistor R1, how does one determine the voltage across the diode and current flowing through the diode?

By superimposing the load line on top of the diode I-V characteristics, one can quickly solve for the diode current and voltage.

Joined Jan 15, 2015
5,875
That's a simple X-Y ploy using a scope, there are no current steps.

Ron

#### Audioguru again

Joined Oct 21, 2019
2,838
I saw a "load line" in a datasheet a long time ago. Here it is:

#### Attachments

• 54.6 KB Views: 10

#### FuneralHomeJanitor

Joined Oct 12, 2019
31
Is there a way to find this by hand? Do I need the resistance ahead of time? How do I figure out what resistance to use for the Q point taken from the load line? I am sorry if this seems dumb, I completely understand what the curves show but am confused how they are found when the base currents are included. Thanks again

#### Papabravo

Joined Feb 24, 2006
15,458
Is there a way to find this by hand? Do I need the resistance ahead of time? How do I figure out what resistance to use for the Q point taken from the load line? I am sorry if this seems dumb, I completely understand what the curves show but am confused how they are found when the base currents are included. Thanks again
Yes. You can put together an experimental setup that will allow you to take the measurements required to construct these graphs.

#### Audioguru again

Joined Oct 21, 2019
2,838
Transistors have a wide range of base current to collector current gain even when they have the same part number and are made by the same company. After you learn how to design transistor circuits then you will see how negative feedback reduces the effects that the range causes. You decide the power supply voltage and collector current then Ohm's Law calculates the resistor values.

#### FuneralHomeJanitor

Joined Oct 12, 2019
31
Yes. You can put together an experimental setup that will allow you to take the measurements required to construct these graphs.
So you’re saying I need to build the circuit and take measurements?

#### Papabravo

Joined Feb 24, 2006
15,458
So you’re saying I need to build the circuit and take measurements?
Yes, you could do that, but there is another option that was not available to me in the fall of 1967. You can use a free SPICE simulator to make the experimental circuit and run the simulation to produce a set of characteristic curves. I actually did that recently and I can show you the results if you're interested.

#### alan01346

Joined Aug 13, 2020
30
What it does isn't that complicated, but getting all those lines on the screen at once is, especially before computers. I worked for a guy that had one in his shop in the 70s.

#### Papabravo

Joined Feb 24, 2006
15,458
What it does isn't that complicated, but getting all those lines on the screen at once is, especially before computers. I worked for a guy that had one in his shop in the 70s.
You'll get no argument from me on that score. I remember the first time I saw a Lissajous curve on a scope with XY inputs and I thought it was one step removed from magic! I had access to other fancy toys in that era that I could spin a story or two about.

Joined Jul 18, 2013
22,631
I have been told about characteristic curves like this for a while but have never fully understood where they come from. I don’t see them on the datasheets, although they have a similar one where Ic and Ib are switched, and my books don’t really explain where they come from. Is this something I need to draw myself? If so where do I get the information required? Is it through testing the transistor on a meter or doing some math? Is the load line something I draw? Thanks in advance.
For me it was Valves (tubes).
Here is a link that may explain the origin.
https://www.rfcafe.com/references/p...d-line-story-nov-1960-popular-electronics.htm
Same principle.
Max.

#### alan01346

Joined Aug 13, 2020
30
I've seen them in sci-fi movies. I think Heathkit sold a curve tracer, without a scope. The one I used was hooked to some surplus military scope.

#### SamR

Joined Mar 19, 2019
3,180
They do sell/make kits that have a step mode selector switch for current to manually do individual traces on a scope. But you would still have to transpose each single trace onto a graph, current step by current step. That or buy a relatively expensive dedicated curve tracer scope system that does multiple traces at a time. A laborious process that the only real need for is to match transistor pairs? Unless I missed something... Google Octopus Curve Tracer.

#### atferrari

Joined Jan 6, 2004
4,235

Nobody has them available.

Most have seem them (in a book) once in their life when learning about transistors.

Few have tried to build them by patient and (more or less) well organized measurements.

Nobody gets them when they would like to have them handy.

Everybody learns to go away somehow, without them.

The one shown to you (to illustrate the reader only) seems to be always for the TPZ6591X39F-D732h transistor discontinued around 43 years ago.

#### SamR

Joined Mar 19, 2019
3,180
They don't put them in the PDF because it is different for each transistor due to manufacturing tolerances and variances. That is why curve tracers are used for matching transistor pairs as closely as possible. FWIW

#### MrChips

Joined Oct 2, 2009
23,056
You can buy a semiconductor curve tracer, assembled or in kit form.
Or you can build your own.

You can also build a computerized curve tracer with a couple of DACs and an ADC.

A curve tracer is very handy for checking transistors, diodes, zener diodes, tunnel diodes, etc.