BC547 hfe

I'm afraid I don't understand your question, the OP didn't mention a Vce, or are you quoting the simulation values. If so I would suggest that the simulator doesn't simulate these extreme conditions correctly. The Ib and Ic values agree with Fig. 1.

Mike.

The curves show a "typical" transistor, not a minimum one and not one that is saturated like a switch.
The maximum saturation voltage in the written spec's shows a pretty poor saturation voltage loss of max 0.6V when the collector current is only 100mA with a base current as high as 5mA.

Pommie said:

I'm afraid I don't understand your question, the OP didn't mention a Vce, or are you quoting the simulation values. If so I would suggest that the simulator doesn't simulate these extreme conditions correctly. The Ib and Ic values agree with Fig. 1.

Mike.

Click to expand...

Ah OK. I see that I misunderstood. Those values in the figure are simulated values, not measured values. So the answer is that the simulation model is not representative of the transistor he is using. That seems reasonable. Thank you!

audioguru said:

The curves show a "typical" transistor, not a minimum one and not one that is saturated like a switch.
The maximum saturation voltage in the written spec's shows a pretty poor saturation voltage loss of max 0.6V when the collector current is only 100mA with a base current as high as 5mA.

Click to expand...

I agree that the curves are for a 'typical' transistor, and most curves are not for saturation like a switch. However, the Fig. 1 I'm referring to does include full saturation effects. Both linear region and saturation region are shown.

Further, the discrepancy I'm noting is too large to be manufacturing variations. Note that the spec you quote shows a current gain of 20 with 0.60 V for Vce, 5 mA Ib and 100 mA Ic. This is not inconsistent with Fig. 1 of the spec sheet. However, the point I was concerned about shows a current gain of 18 with 0.64 V for Vce, 0.79 mA Ib and 14 mA for Ic. This is clearly light years off the curves if you consider the exponential functions in the transistor equations.

I am now comfortable with Pommie's explanation that the simulation's transistor model is not consistent with the real transistor. If the OP were to measure Vce in his original circuit, he should see something closer to 0.3 V, and not 0.64 V!

Pommie,

Thanks for the response. I finally settled on a 10K base resistor and a 100 ohm series resistor at the collector. This allows me to light a red, green, blue, or white LED with a input current of 420 uA.

steve,

Sorry for causing you the confusion. My failure to understand that the transistor was saturated caused me to "tune" beta so that the input and output currents matched my measurements. Vce, Vbe, and Vbc were simulated.

DavidBear (the OP [I think?])

davidbear said:

Actually, I don't think need the 220 Ω resistor (BTW the LED is blue), since the transistor is limiting the current to 30 mA when on. Would it be better design to put the series resistor with the LED and remove the base resistor?.

Click to expand...

You MUST have a series resistor for the LED - and it was already in your original circuit in the collector, notice I told you to move the LED to the collector, I didn't tell you to remove the resistor - don't leave it out!.

Likewise you MUST have the resistor feeding the base.

steveB said:

Ah OK. I see that I misunderstood. Those values in the figure are simulated values, not measured values. So the answer is that the simulation model is not representative of the transistor he is using. That seems reasonable. Thank you!

Click to expand...

Hfe is also increases with temperature so if you're not careful it can go into thermal runaway and blow up.

Never rely on simulations for this kind of thing, they won't help you, they'll only encourage you to turn our really bad designs which blow up in your face when you actually build them.