I spent an hour trying to figure out why my math was different than a simulation on a particular circuit. It turned out my DC current gain I chose, 100 as per the minimum value on the datasheet, was incorrect; It was approximately 200. It got me thinking, is there something I should be looking at that can help determine the actual DC current gain value prior to solving a transistor circuit beside the min and max values on a datasheet? Is it intuition I'm lacking?

  • \$\begingroup\$ Ideally, you should design your circuits so that the \$\beta\$ of the transistor doesn't change the overall behavior very much. Other than that, a "typical" value on the datasheet, or the value from a vendor-supplied SPICE model is the only thing that can tell you what value might be more likely. Real samples are likely to vary quite a bit from part to part and over temperature, so trusting the SPICE model isn't safe either. \$\endgroup\$
    – The Photon
    Mar 4 '20 at 21:56
  • \$\begingroup\$ Discrete transistor current gain varies greatly. However, inside a chip, \$ \beta \$ variations transistor-to-transistor might be reasonably small, making them well-matched. \$\endgroup\$
    – glen_geek
    Mar 4 '20 at 22:02
  • \$\begingroup\$ Your experience is lacking the use of negative feedback to make use of high gain and resistor ratios to stabilize Q point and AC gain. your H bias or other will suffer from non-linearity and offset for max swing. \$\endgroup\$ Mar 4 '20 at 23:25
  • \$\begingroup\$ Jaywalk, why don't you share with us the circuit and your calculations and the simulation results? I think it would go a long way in helping us answer your deeper questions about self. Give us a chance to see what you see. And I think we may be able to help find the right keys to unlock that intuition you wonder about. But we need a peak inside to do that. Help us, eh? \$\endgroup\$
    – jonk
    Mar 5 '20 at 6:36
  • \$\begingroup\$ It's one of our standard circuits, I honestly can't let anyone take a look at it. I can draw up a similar circuit after work but FYI it's a solenoid driver, the command bit for it takes a lot more power than expected to run. The circuit itself works for any arbitrary beta value within spec but it depending on the beta value it consumes a lot more power. \$\endgroup\$
    – Jaywalk
    Mar 5 '20 at 14:00

If you want to know the correct value of current gain for a specific transistor then you must measure it yourself. But you should know that the gain also varies with collector current and temperature.

If you are analyzing a circuit by hand you can estimate typical behavior by using the specified typical gain. If the datasheet doesn't give you that, then analyze your circuit for both the minimum and maximum gain and make sure the circuit meets your needs for both cases.

Circuit simulators typically use a more sophisticated model rather than relying on any fixed value of the transistor parameters.

  • \$\begingroup\$ I see, I'll pay attention to the minimum/maximum before assuming the math is incorrect next time. Thank you. \$\endgroup\$
    – Jaywalk
    Mar 5 '20 at 14:01

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