# Help Understanding BJT “On Characteristics” in Datasheet

I am currently in the process of designing my first transistor circuit and I am using another as a basis to work off of. In this process I am trying to learn how to read a datasheet, as in university so far, we have rarely used them. I am using a 2N5089 NPN transistor for my design and below I have posted the part of the datasheet in question. First, what exactly is Base-Emitter On voltage? I think this means the voltage required to forward bias the BE junction, but I figure this would be a minimum value, not a maximum. Secondly, why does the hFE for the first scenario have a maximum value, but the other two do not (just hyphens)? Finally, is V_CE of 5.0 V a good bias point? The design I am basing off of, when simulated with a generic NPN, gives a V_CE of roughly 5V, so this makes me think so.

The final picture is the DC analysis of the amplifier I am working on in case it helps. I used 800 A/A as it is the average of 400 and 1200.

## 2 Answers

I think this means the voltage required to forward bias the BE junction, but I figure this would be a minimum value, not a maximum.

That's correct. They're specifying a maximum to tell you, "you won't have to provide more than 0.8 V to turn the device on".

Secondly, why does the hFE for the first scenario have a maximum value, but the other two do not (just hyphens)?

Because they don't commit to any maximum limit on that parameter.

Every limit in the datasheet adds test time when manufacturing the part, which adds to the cost. So the fewer parameters that have limits, the lower their cost to produce the part.

Finally, is V_CE of 5.0 V a good bias point? The design I am basing off of, when simulated with a generic NPN, gives a V_CE of roughly 5V, so this makes me think so.

Given you have a 9 V supply, biasing the collector at ~4.5-5 V (relative to ground) gives you near the maximum dynamic range. So Vce of ~5 V is probably about right.

I used 800 A/A as it is the average of 400 and 1200.

Before you're done, you should test that your design works with either 400 or 1200. Ideally you'll want to design so that the $$\h_{FE}\$$ can go to infinity and your circuit will still work.

• Thank you very much! That was very insightful and concise. One final question, how do you know that biasing the collector at 5V gives almost the maximum dynamic range? Does this refer to maximum positive/negative signal swing? – ModularMan May 14 '20 at 18:44
• @ModularMan, yes, in this case I mean the maximum peak-to-peak swing. – The Photon May 14 '20 at 20:01
• A bit of an update, I discovered that V_BE(ON) max is not some sort of recommendation. It is telling you that that value, or larger, cannot be used, as too much current would be flowing and it would begin to damage the transistor. Diodes will always "compensate" for the voltage placed across them by allowing more current to pass and keeping about 0.7 V across them, but as more current flows, slightly more voltage will build up around the diode. If anything, it seems to be more of an indirect current warning. – ModularMan May 19 '20 at 16:37

the base voltage of biolars has the predictable behavior, when biased into amplification as you have done, of

10:1 increase in collector current will, over many decades of current, show a constant delta_Vbe.

For ideal bipolars (abrupt-junction), the delta_Vbe is 0.058 volts per 10:1 current change.

Let us assume the Vbe at 1mA is 0.600 volts.

Then the Vbe at 0.1 mA will be 0.600 -0.058 volts

And the Vbe at 0.01 mA will be 0.600 - 2 * 0.058 volts.

And the Vbe at 0.001mA will be 0.600 - 3 * 0.058 volts.