Analysis of a CE amplifier
The CE amplifier circuit is often constructed using voltage divider method as shown in figure shown below. This voltage divider provides a negative feedback that can be used to stabilize both the DC operating point and the AC gain. The source has AC voltage Vs and resistance Rs. C1 and C2 are the coupling capacitors. C1 couples the signal into the base and C2 couples the amplified output signal to the load resistor Rl.
Ce is a bypass capacitor.
When there is no signal entering the circuit, the base current Ib, collector current Ic, base voltage Vb and collector voltage Vc are fixed as according to the Q-point. When AC signal is applied to the base, it produces AC variations in the base voltage and the base current. The ac variations in the base current produce large variations in the collector current. Due to the ac current flowing through the collector resistor Rc, large AC vartiations are produced in the collector voltage.
The analysis of a amplifier of a CE amplifier is divided into two parts
1. DC analysis
2. AC analysis
DC equivalent circuit
In the case of dc equivalent circuit, all capacitor acts as short. Hence, the entire AC sources are reduced to zero. Also, all the capacitors are opened and then the circuit is analysed as dc equivalent circuit.
Reducing ac source to zero is as good as replacing it by a short circuit. Opening all the capacitor means disconnecting them. The circuit, which remains after these changes, is the called dc equivalent circuit. The dc equivalent circuit is a circuit for calculating only dc currents and voltages.
Initially, ac signal is removed. Then the generator, load resistor Rl and the emitter capacitor Ce are disconnected. The circuit is now transformed into dc equivalent circuit.
In this type of circuit, the capacitor acts as short to ac. hence, all the dc sources are reduced to zero and all capacitors are shorted.
When the dc source Vcc is reduced to zero, the top of Rc and Rl get grounded. When the capacitor C1 is shorted, it connects the generator to the transistor base.
When C2 is shorted, it connects Rl to the collector of the transistor and when CE is shorted, it shorts the emitter Re to the ground.