I'm sorry my circuit is bizzare, it seemed like a good idea at the time. Where there is a switch, I actually have a counter, which does not have the current to drive the blue led. I am just trying to turn the LED on and off at something like maximum brilliance. I had anticipated an hfe of 200, and was quite disapointed with the low light level of the LED.
I agree with the feedback that everyone is giving the OP, but I was wondering something. Isn't he correct to wonder why the measured current gain is only 18?
That transistor is supposed to have a much higher gain, obviously. Also, it is clear that the gain will be much lower once in saturation. However, based on the voltage readings in his figure, the transistor is just barely entering the saturation region (i.e. Vbc=4.4 mV, Vce = 0.64 V).
I would still expect higher gain under the conditions he quoted.
Can anyone explain this?
Placing the LED in the emitter and the resistor in the collector is so incredibly bizarre?, it would be bad enough putting BOTH in the emitter. But doing it this way gives negative feedback as well, drastically reducing current gain. While the LED in the emitter isn't a resistor, it will still give negative feedback in association with the collector resistor.
The reason is because the transistor is in saturation (Vce=638.91mV) so the extra base current just passes out the emitter. Assuming a beta of 200 and a VLED of 2.6V: If you raised the 4.8V (on the collector resistor only) to 25V you'd see apx 160ma in the LED and then smoke and fire.I'm simply noting the quoted conditions for the transistor as follows:
Ic=14.21 mA
Ib=789.69 uA
Vce=638.91 mV
If I compare this to Fig. 1 in the data sheet for the BC547, it doesn't seem to agree. If I look at Vce and Ic on the graph, it seems to predict that Ib would be about 10 times less.
So, while I agree with the recommended circuit changes that people are suggesting, I'm just as baffled as the OP as to why the current gain is so low in his circuit.
The reason is because the transistor is in saturation (Vce=638.91mV) so the extra base current just passes out the emitter. Assuming a beta of 200 and a VLED of 2.6V: If you raised the 4.8V (on the collector resistor only) to 25V you'd see apx 160ma in the LED and then smoke and fire.
With real parts the current would be lower because transistors exhibit lower beta at higher collector currents.
Basically, I'm curious if anyone can explain why. The numbers presented don't add up. Although the transistor is in saturation, it has only just barely entered that region. Thus, I would expect higher current gain. The curves confirm this fact. The only thought that crosses my mind is that perhaps the OP took DC measurements with a meter (i.e. without an O-scope), and in fact had oscillations present in the circuit.
.... In the OPs circuit the collector current reached the maximum it could and the only way to get the transistor back on the curve would be to increase the collector voltage (as pointed out by kcriste).
Mike.
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