5spice transistor biasing

Discussion in 'General Electronics Chat' started by omaroski, Apr 29, 2012.

1. omaroski Thread Starter Active Member

Jun 19, 2008
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0
I'm trying to analyze through spice the dc biasing of a transistor.

I choose 15V for Vcc and load collector resistor 400ohm while base resistor 140K ohm. Along with bc848 curve characteristic the operating point should be at 4,5V Vce and 0,25mA Ic cause base current is 100uA.

Why my spice analysis is putting out different things?!

2. #12 Expert

Nov 30, 2010
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9,685
Apparently spice thinks the current gain of that transistor is 100.00 and Vcc is 6.0 volts.
As for why a computer program can make a mistake? I don't have a clue.

But there are lessons to be had.
The current gain of a transistor is not very predictable. The "right" way to bias this is to use a voltage divider to set the base at a certain voltage, like 2 volts, then use an emitter resistor to set the current through the transistor.
(2-.7)/emitter resistor = collector current..aproximately.

3. Audioguru Expert

Dec 20, 2007
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1,316
I was taught to NEVER, EVER bias a transistor with a single resistor to the supply voltage like that. The current gain of every transistor is different so some might be saturated and others might be cutoff.
The current gain is affected if the temperature changes.

4. omaroski Thread Starter Active Member

Jun 19, 2008
34
0
Thank you both, i'm aware that this is not the best way for biasing a transistor, it's just for teaching myself through the use of a software like this.

5. #12 Expert

Nov 30, 2010
18,076
9,685
This demonstrates that trying to simulate your way to an understanding has its limits. Connecting real transistors will give you real results. Even when you do it wrong, you will learn from it. I recommend that method.