14-05-2013, 04:18 PM
Bipolar Junction Transistor Basics
Bipolar Junction.ppt (Size: 2.78 MB / Downloads: 281)
Physical Structure
Consists of 3 alternate layers of n- and p-type semiconductor called emitter (E), base (B) and collector ©.
Majority of current enters collector, crosses base region and exits through emitter. A small current also enters base terminal, crosses base-emitter junction and exits through emitter.
Carrier transport in the active base region directly beneath the heavily doped (n+) emitter dominates i-v characteristics of BJT.
Common-Collector BJT Characteristics
The Common-Collector biasing circuit is basically equivalent to the common-emitter biased circuit except instead of looking at IC as a function of VCE and IB we are looking at IE.
A Practical BJT Amplifier using Coupling and Bypass Capacitors
In a practical amplifier design, C1 and C3 are large coupling capacitors or dc blocking capacitors, their reactance (XC = |ZC| = 1/wC) at signal frequency is negligible. They are effective open circuits for the circuit when DC bias is considered.
C2 is a bypass capacitor. It provides a low impedance path for ac current from emitter to ground. It effectively removes RE (required for good Q-point stability) from the circuit when ac signals are considered.
AC coupling through capacitors is used to inject an ac input signal and extract the ac output signal without disturbing the DC Q-point
Capacitors provide negligible impedance at frequencies of interest and provide open circuits at dc.