(a) Characteristic Curves: The circuit diagram for determining the static characteristic curves of an n-p-n transistor in common-emitter configuration is shown infigure.
Common Emitter Characteristics:
(i)Input characteristics: These characteristic curves are obtained by plotting base current (IB) versus base-emitter voltage VBE for fixed collector-emitter voltage VCE. Fig. represents these characteristics.
(ii) Output characteristics: These characteristics are obtained by plotting collector current IC versus collector-emitter voltage VCE at a fixed value of base current IB . The base current is changed to some other fixed value and the observations of IC versus VCE are repeated. Fig. represents the output characteristics of a common-emitter circuit. Input Resistance. It is the ratio of change in base-emitter voltage (ΔVBE) to the corresponding change in base current (ΔIB) at constant collector-emitter voltage VCE i. e,.,
The input resistance of a common emitter circuit is of the order of a few hundred ohms.
Current amplification factors of a transistor (α and β):
The current gain a is defined as the ratio of change in collector current to the change in emitter current for constant value of collector voltage in common base configuration i.e.,
Practical value of a ranges from 0.9 to 0.99 for junction transistor. The current gain α is defined as the ratio of change in collector current to the change in base current for constant value of collector voltage in common emitter configuration i. e.,
The value of β ranges from 20 to 200. The current gains α and β are related as
(b) A transistor as an Oscillator:
Circuit Operation. When the collector supply voltage is switched on by closing switch *, collector current starts increasing and the capacitor C is charged. When the capacitor attains maximum charge, it discharges through coil L, setting up oscillations of natural frequency.
These oscillations induce a small voltage in coil L' by mutual induction. This induced voltage is the feed back voltage; its frequency is same as that of resonant LC circuit but its magnitude depends on the number of turns in L' and coupling between L and L'. The feedback voltage is applied between the base and emitter and appears in the amplified form in the collector circuit. A part of this amplifier energy is used to meet losses taking place in oscillatory circuit to maintain oscillations in tank circuit and the balance is radiated out in the form of electromagnetic waves. Positive Feed back. The feed back applied in tuned collector oscillator circuit is positive. This may be seen as follows: A phase shift of 180° is created between the voltages of L and L' due to transformer action. A further phase shift of 180° arises between base-emitter and collector circuit due to transistor action in CE configuration. Thus the net phase becomes 360° (or zero); which is the required condition for a positive feed back. Due to positive feed back the energy fed back to the tank circuit is in phase with the generated oscillations, thus maintaining oscillations.
