As i am studying about DC Biasing of transistor, i am just able to fully understand only Base Bias after that i am stuck at Emitter Bias which is also called Dual Supply Emitter bias where we apply voltage Vee to emitter, Now my question is, In Emitter bias we know Ic become independent of Beta when Base voltage become zero, it means as it gets independent of Beta so its operating point will be stable.. and the problem is solved.. than why we move to other bias conditions like Collector feedback bias or voltage divider bias etc etc why we dont end the game at Emitter bias after studying Base Bias. Thankyou.
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\$\begingroup\$ Write up the details of your question using schematics and equations. It works lots better in getting your point across than bits of English tossed together. \$\endgroup\$– jonkMay 12, 2021 at 4:34
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2 Answers
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With an BJT transistor in a linear circuit, you need to make the collector current (Ic) not depend on the HFE (used to be called Beta) of the transistor.
The usual method is to make some resistor on the emitter have a lot more voltage drop than the Vbe (Base to Emitter) of the transistor.
This can be done by having a positive and negative supply voltage and keeping the Base near zero volts. Life, however, often doesn't give you two supply voltages. In that case, setting the base at some voltage part way between the positive and negative supplies is much easier than having to make a new power supply circuit. This is the two resistor voltage divider in many designs.
Sometimes you may find your self designing in a case where adding one more component will be objected to by management. A resistor from the collector to the base of a BJT takes fewer parts than the voltage divider.
Sometimes the supply voltage you have is much too low for a normal design. In this cases, you must get very creative about biasing of the transistors.
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1\$\begingroup\$ Put some schematics in. There's no point answering a wall of text with a wall of text, especially where the subject of the question is the difference between different circuit topologies. Use the board's schematic editor, it's the diode/resistor button in the edit toolbar when you edit your answer. \$\endgroup\$– Neil_UKMay 12, 2021 at 5:28
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It can be shown - as far as DC bias point stability against tolerances and temperatur changes is concerned - that the best bias method is to use
a resistive voltage divider at the base node for providing a bias voltage Vb as "stiff" as possible. For this purpose the resistor niveau should be as low as possible - depending on some limits set by allowed DC power consumption as well as input resistance of the whole circuit. In this case, the "unknown" value of the base current (due to large beta-tolerances) plays a minor role only.
an additional resistor Re in the emitter path for providing negative DC feedback. In this case, any unwanted increase of the collector current will increase the voltage Ve at the emitter node (npn case) and, thus, reduce Vbe=Vb-Ve. Hence, this effect will counteract the Ic increase up to a certain degree.
