H-Bias BJT amplifier question

Discussion in 'Homework Help' started by c1rcu1ts, Oct 31, 2013.

Oct 19, 2013
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I need help in understanding why i would want to select a quiescent current
(collector current) between a range of 0.1mA-10mA and why should i choose a particular value in that range. (this is for a 2N3904 transistor if necessary)

Oct 19, 2013
62
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Can the base emitter voltage affect the maximum symmetrical swing?

3. WBahn Moderator

Mar 31, 2012
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Sounds like a rule of thumb recommendation. The specific value you choose depends on the performance specs you are designing to.

Oct 19, 2013
62
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Thanks but i need to know how to justify the selection of the quiescent current. Basically trying to explain why the rule of thumbs is the rule of thumbs.

5. WBahn Moderator

Mar 31, 2012
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You set the quiescent current to achieve a desired operating point. The quiescent current dictates Re, gm, and other parameters. It's also going to greatly influence you input signal range and your output swing.

Any given transistor is going to have a range of currents that it is generally intended to operate at. This will, in part, depend on the collector-emitter voltages it will be seeing as this greatly affects the power dissipation.

But all of this depends on the specifics of the application and the circuit topology to be used, which you have given no hint of.

c1rcu1ts likes this.

Oct 19, 2013
62
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Voltage gain of 50, supply voltage of 15V and lower cut off frequency of 100 Hz is the specs to design for. Some specs of the 2N3904 are also known. There is good linear range of the graph of current gain vs Ic from 0.1mA-10mA and Vbe is 0.65V (which is minimum voltage for on) when a current of 1mA is applied to base (Ic=10mA).

What would influence your decision in selecting quiescent current?

Feb 17, 2009
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8. LvW Active Member

Jun 13, 2013
674
100
Perhaps the following hints can help to find a suitable value for Ic in your case:
* Larger Ic leads to larger power consumption (of course)
* Larger Ic values give higher transconductances (gm) leading to a larger gain, however:
* At the same time there is a larger DC voltage drop across the collector resistor Rc causing a reduction of Vce (DC),
* Larger Ic values are connected with a reduction of the input resistance, which normally is desired to be large
______________

Summary: Selection of a suitable Ic is the result of a trade-off between (conflicting) requirements for the gain stage.

Edit: There is something like a "general rule of circuit design philosophy":
"Improving one parameter of an electronic circuit causes deterioration of another parameter."

Last edited: Nov 1, 2013

Oct 19, 2013
62
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Could it be that the lowest current for the transistor to turn on would be most appropriate since it results in least power consumption ? Ic is 1mA

Also, what would u consider to be a large gain (because as u said we wont need such a high ic)

(Thanks alot, that helped)

10. LvW Active Member

Jun 13, 2013
674
100
As said already before (post#5) a general answer is not possible. It depends really on some specific requiremnents and also on the circuit topology chosen (bias circuitry, emitter degeneration/feedback, signal amplitudes, distortion requirements,...).
Of course, looking on the power consumption only, a small current Ic is best, however ....(as mentioned before).

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