DC Biasing of BJT'S

Biasing of BJT'S

DC Biasing of BJT'S:

Objectives:

 To Understand : 

• Concept of Operating point and stability 

• Analyzing Various biasing circuits and their comparison with respect to stability 

BJT – A Review 

• Invented in 1948 by Bardeen, Brattain and Shockley 

• Contains three adjoining, alternately doped semiconductor regions: Emitter (E), Base (B), and Collector (C) 

• The middle region, base, is very thin

 • Emitter is heavily doped compared to collector. So, emitter and collector are not interchangeable. 

Three operating regions:

• Linear – region operation: 

– Base – emitter junction forward biased – Base – collector junction reverse biased 

• Cutoff – region operation: 

– Base – emitter junction reverse biased – Base – collector junction reverse biased 

• Saturation – region operation: 

– Base – emitter junction forward biased – Base – collector junction forward biased

Introduction - Biasing:

The analysis or design of a transistor amplifier requires knowledge of both the dc and ac response of the system. In fact, the amplifier increases the strength of a weak signal by transferring the energy from the applied DC source to the weak input ac signal 

• The analysis or design of any electronic amplifier therefore has two components: 

• The dc portion and 

• The ac portion 

During the design stage, the choice of parameters for the required dc levels will affect the ac response.

What is biasing circuit? 

• Once the desired dc current and voltage levels have been identified, a network must be constructed that will establish the desired values of IB, IC and VCE, Such a network is known as biasing circuit. A biasing network has to preferably make use of one power supply to bias both the junctions of the transistor.

Purpose of the DC biasing circuit:

• To turn the device “ON” 

• To place it in operation in the region of its characteristic where the device operates most linearly, i.e. to set up the initial dc values of IB, IC, and VCE

Biasing circuits: 

• Fixed – bias circuit 

• Emitter bias 

• Voltage divider bias 

• DC bias with voltage feedback 

• Miscellaneous bias