# Bjt Biasing techniques

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#### kristotech

Joined Jan 24, 2009
1
I've read through the allaboutcircuits worksheet that asks and answers questions on the the transistor biasing techniques but i still do not understand how the resistors, capacitors and their various methods of connections actually achieves the transistor biasing functions. I need someone to explain the biasing networks to me in simple and clear terms.

#### Wendy

Joined Mar 24, 2008
22,939
Actually, capacitors have very little to do with DC biasing, which is where you start. They have everything to do with AC frequency response however.

There are more than one model for describing how a transistor works. The one I use starts from the base assumetion, Ic = β X Ib . From there you can derive almost every biasing formula used.

Given the 3 basic configurations, Common Emitter, Common Collector, and Common Base, and their subclassifications you have to figure each set of equations on a case by case bases, using the root one I described. If you want to understand a specific example you have to pick one. My personal favorite is this one.

It is a general purpose amplifier.

#### GirishC

Joined Jan 23, 2009
58
I've read through the allaboutcircuits worksheet that asks and answers questions on the the transistor biasing techniques but i still do not understand how the resistors, capacitors and their various methods of connections actually achieves the transistor biasing functions. I need someone to explain the biasing networks to me in simple and clear terms.
Hi

In all transistor biasing method, what you need to make sure that you have Q point (biasing point) at the center of the load line, for faithful amplification. Based on this need there are different methods of biasing. Since you have collector junction reverse biased by connected supply (Vcc) to collector w.r.t. emitter. The main purpose of the collector resistor, it acts as load for given load line.

As far as base circuitry is concern, all these resistors make sure that emitter junction is forward biased with enough base current to take Q point at the center of load line. So you can derive the base voltage from connector, or collector supply or have separate additional supply at base. Adding emitter resistor limits the emitter current i.e. it adds voltage component to base-emitter circuit. So you need to increase base drive by that much value to make emitter diode forward biased.

The capacitors are used to block any DC component and allow AC component to transistor for faithful amplification.

Hoe, I gave you idea..

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