EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: zulunation on July 07, 2023, 07:43:12 pm
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Hello everyone.
I want to build a small signal amplifier using BC847C transistor.
Here is the datasheet
https://www.mouser.lt/datasheet/2/916/BC847X_SER-3081381.pdf (https://www.mouser.lt/datasheet/2/916/BC847X_SER-3081381.pdf)
The problem is that on Fig. 10 . BC847C: DC current gain as a function of collector current; typical values
is specified for Vce = 5V.
But my circuit has 4V power.
I need to set Q point. I should choose collector current that Hfe would be stable (horizontal line on the part of the
plot).
I can choose collector current at Q point to be let say 1mA.
Strange it is very small current. But when it will rise to 500mA the hFe for it will be non linear.
It is not plotted for on Fig. 10. What that mean?
This transistor can be linear amplifier when collector current is less than 30mA?
But the problem is how to recalculate Hfe to other Vce voltage? Let's say 1V.
Should i measure it by myself?
And another question.
In this datasheet there is no plot for collector current vs Collector emitter voltage and for different base currents.
Some datasheets have this type of plot but some don't.
Why this is happening?
Thanks,
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You need to design your circuit so it's not overly dependent on beta. Have a read:
https://www.electronics-tutorials.ws/amplifier/amp_2.html (https://www.electronics-tutorials.ws/amplifier/amp_2.html)
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That's a good amp, the simple ones are the only ones you really need. Pretty much just want to get that transistor activated without an error popping up, then uv'e pretty much got an amp.
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Just note that with 4V available the maximum level you'lle be able to smplify to is going to be rather less than 3Vpp unless you can tolerate major distortion and clipping of waveform peaks. I've been doing some amplification of what might be described as more medium sized signals, alittle under 3Vpp out is the maximum I can get with a 5V supply before my output starts to clip. It seems discrete transistors were mostly not aimed at analog applications when only single sided 5V (and less) power rails are available.
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And another question.
In this datasheet there is no plot for collector current vs Collector emitter voltage and for different base currents.
Some datasheets have this type of plot but some don't.
Why this is happening?
Because manufacturers are lazy and don't always bother to put all the interesting plots in the datasheet.
May also be that they don't want to guarantee anything in this regard, i.e. want the freedom to change the product slightly without updating the datasheet.
But as you can see from such plots in those cases when they are provided, it is typical for bipolar transistors operating at low collector current to have reasonably constant β down to Vce of maybe 1V or so. So β at 4V should be approximately the same as at 5V. Or at 15V.
Strange it is very small current. But when it will rise to 500mA the hFe for it will be non linear.
It is not plotted for on Fig. 10. What that mean?
This transistor can be linear amplifier when collector current is less than 30mA?
BC547 is a small transistor and not much use for 500mA. Its β at such currents is approximately zero (if it can even reach 500mA at all) so they don't plot it.
If you actually need β linearity it seems that this transistor is good for up to about 30mA as you noticed. Note that this only applies to the exact part from the exact manufacturer whose datasheet you have. BC547 is a generic transistor available from many vendors and they are not all entirely identical.
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LTspice is your friend :)
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What matters is: "Fig. 2. BC847A: DC current gain as a function of
collector current; typical values".
I've never needed to use a load line Q point for a small signal transistors.
It just doesn't find much use in designing low power and low voltage ciruits.
You would probably want to use load lines when designing with power transistors and valves though.
For a small signal low frequency transistor, assume Ie=Ic, Ib will be very small, Ic/Hfe Small signal low frq transitors tend to have a peak Hfe at in the low mAs as luck would have it.
Hfe (beta in oldspeak) is not a reliable figure, its more a worst case measure. Transistors from the same manufacturer, with the same part number, batch and suffix will show a wide variation.
A more real world aproach would be:
1. Pick Ic
2. Pick Ve. Ve is often set about a the same order of magnitude as Vbe, say one volt to obatain thermal stabilty via negative feedback. [As Ic rises with temperature, Ve (Ie.Re) will also increase. That will reduce the voltage Vbe decreasing Ic. Works every time!]
3. Re=Ve/Ic say 1k
4. Vb= Ve+0.65 note that for the purposes of small signals, Vcc is shorted to ground via the very low power supply impedance.
So the impedance looking in to the base is the parallel combination of the bias resistors.
Scale to suit your needs. The current in this voltage divider needs to be >> an estimated Ib. That's not often a problem.
5. Rc= (Vcc/2)/Ie 2k Vcc/2 centres the output swing. ie @ 2V on a 4V supply.
The voltage gain is simply Rc/Re.
You can increase the small signal gain by adding a bypass capacitor across Re. Doing this you can get a max gain in the order of 100x (Av is about 20*Vcc, thats another story)