The transistor is used to amplify the weak signal a CE amplifier connection is shown in the below figure.
The output from the CE amplifier is either taken across the resistance RC or at terminals 1 and 2. In both cases, the magnitude of the output signal is the same. Since
Output = Voltage drop across RC = ICRC
This method is used in amplifiers having single stage amplification only.
When the output is taken from terminals 1 and 2 the
Output = voltage across the terminals = VCE = VCC – ICRC
Since VCC is the DC Voltage, it cannot pass across the coupling capacitor Cc. Only varying output appears across the output terminals. Therefore
Output = -ICRC
Thus output is same in magnitude, but negative sign indicates the phase reversal. This method is used in multistage amplifier circuits.
As an amplifier, CE connected transistor can be represented as a 2 port network.
Input Resistance
When a small change of base-emitter voltage (∆ VBE) is applied. It results in a change in base current as ∆ IB at constant collector-emitter voltage, then their ratio is called input resistance Ri.
Mathematically,
Ri = ∆VBE/∆IB = VBE/IB
So when 1 V is applied and if the base current is 2 mA, then the input resistance of the transistor amplifier will be
Ri = 1 / 2 X 10-3 = 500 ohms
Output Resistance
When the change in collector-emitter voltage (∆VCE) occurs to produce a change in collector current (∆IC) at a constant base current, then their ratio is called output resistance (RO).
Mathematically,
RO = ∆VCE/∆IC
This value is very high due to the reverse biasing of the collector-base PN junction.
Effective Collector Load
It is load offered to the A.C collector current. In the case of single amplifiers, it is the parallel combination of RC and RO.
Mathematically,
RAC = (RC X RO) / (RC + RO)
Since, RO is the output resistance and it is large, so
RC + RO = RO
Therefore
RAC = RC
Hence in a single-stage amplifier, the effective load is equal to the collector load RC.
In the case of a multistage amplifier, the input of the next stage is connected to the output of the first stage. So the effective output resistance will now be:
(RO X Ri) / (RO + Ri) ≈ Ri
Since RO is very much more significant than Ri. Now the effective load resistance will be:
RAC = (RC X Ri) / (RC + Ri)
Thus the effective load resistance is decreased corresponding to the value of Ri.
Current Gain
Is the ratio of change in collector current ∆IC to the change in the base current?
Mathematically,
Current gain = β = ∆IC/∆IB
Its value ranges between 20 to 500.
Voltage Gain
Is the ratio of change in output voltage ∆VCE to the change in the input voltage ∆VBE.
Mathematically,
AV = ∆VCE/∆VBE = (∆IC X RAC) / (IB X Ri)
= (∆IC / ∆IB) X (RAC/Ri) = β X (RAC/Ri)
For single stage Ra = RC and for multistage:
RAC = (RC X Ri)/(RC + Ri)
Power Gain
It is the ratio output signal power to the input signal power.
Mathematically.
AP = β AV = Current gain X Voltage gain