Common Emitter Amplifier (CE) Circuit Working and Applications

The Amplifier is an electronic circuit that is used to increase the strength of a weak input signal in terms of voltage, current, or power. The process of increasing the strength of a weak signal is known as Amplification. One most important constraint during the amplification is that only the magnitude of the signal should increase and there should be no changes in original signal shape. The transistor (BJT, FET) is a major component in an amplifier system. When a transistor is used as an amplifier, the first step is to choose an appropriate configuration, in which device is to be used. Then, the transistor should be biased to get the desired Q-point. The signal is applied to the amplifier input and output gain is achieved. In this article, we will discuss common emitter amplifier analysis.

Common Emitter Amplifier Configuration

In Common Emitter Amplifier Configuration, the Emitter of a BJT is common to both the input and output signal as shown below. The arrangement is the same for a PNP transistor, but bias will be opposite w.r.t NPN transistor.

NPN and PNP CE Configuration Amplifier
NPN and PNP CE Configuration Amplifier

Operation of Common Emitter Amplifier

When a signal is applied across the emitter-base junction, the forward bias across this junction increases during the upper half cycle. This leads to increase the flow of electrons from the emitter to a collector through the base, hence increases the collector current. The increasing collector current makes more voltage drops across the collector load resistor RC.

The negative half cycle decreases the forward bias voltage across the emitter-base junction. The decreasing collector-base voltage decreases the collector current in the whole collector resistor Rc. Thus, the amplified load resistor appears across the collector resistor. The common emitter amplifier circuit is shown below figure (a).

CE Amplifier
CE Amplifier

From the voltage waveforms for the CE circuit shown in Fig. (b) It is seen that there is an 180-degree phase shift between the input and output waveforms.

Common Emitter Amplifier Circuit Elements and their Functions

Biasing Circuit/ Voltage Divider

The resistances R1, R2 and RE used to form the voltage biasing and stabilisation circuit. The biasing circuit needs to establish a proper operating Q-point otherwise, a part of the negative half cycle of the signal may be cut-off in the output.

Input Capacitor (C1)

The capacitor C1 is used to couple the signal to the base terminal of the BJT. If it is not there, the signal source resistance, Rs will come across R2 and hence, it will change the bias. C1 allows only the AC signal to flow but isolates the signal source from R2

Emitter Bypass Capacitor (CE)

An Emitter bypass capacitor CE is used parallel with RE to provide a low reactance path to the amplified AC signal. If it is not used, then the amplified AC signal following through RE will cause a voltage drop across it, thereby dropping the output voltage.

Coupling Capacitor (C2)

The coupling capacitor C2 couples one stage of amplification to the next stage. This technique used to isolate the DC bias settings of the two coupled circuits.

CE amplifier circuit currents

Base current iB = IB +ib where,
IB = DC base current when no signal is applied.

ib = AC base when AC signal is applied and iB = total base current.

Collector current iC = IC+ic where,

iC = total collector current.

IC = zero signal collector current.

ic = AC collector current when AC signal is applied.

Emitter Current iE = IE + ie where,

IE = Zero signal emitter current.

Ie = AC emitter current when AC signal is applied.

iE = total emitter current.

CE Amplifier Frequency Response

The voltage gain of a CE amplifier varies with signal frequency. It is because reactances of the capacitors in the circuit changes with signal frequency and hence affects the output voltage. The curve drawn between voltage gain and the signal frequency of an amplifier is known as frequency response. Below figure shows the frequency response of a typical CE amplifier.

Frequency Response of Common Emitter Amplifier
Frequency Response of Common Emitter Amplifier

From the above graph, we observe that the voltage gain drops off at low (< FL) and high (> FH) frequencies, whereas it is constant over the mid-frequency range (FL to FH).

At low frequencies (< FL) The reactance of coupling capacitor C2 is relatively high and hence very small part of the signal will pass from amplifier stage to the load.

Moreover, CE cannot shunt the RE effectively because of its large reactance at low frequencies. These two factors cause a drops off of voltage gain at low frequencies.

At high frequencies (> FH) The reactance of coupling capacitor C2 is very small and it behaves as a short circuit. This increases the loading effect of the amplifier stage and serves to reduce the voltage gain.

Moreover, at high frequencies, the capacitive reactance of base-emitters junction is low which increases the base current. This frequency reduces the current amplification factor β. Due to these two reasons, the voltage gain drops off at high frequency.

At mid frequencies (FL to FH) The voltage gain of the amplifier is constant. The effect of the coupling capacitor C2 in this frequency range is such as to maintain a constant voltage gain. Thus, as the frequency increases in this range, the reactance of CC decreases, which tend to increase the gain.

However, at the same time, lower reactance means higher almost cancel each other, resulting in a uniform fair at mid-frequency.

Common Emitter Amplifier analysis

The first step in AC analysis of Common Emitter amplifier circuit is to draw AC equivalent circuit by reducing all DC sources to zero and shorting all the capacitors. Below figure shows the AC equivalent circuit.

AC Equivalent Circuit for CE Amplifier
AC Equivalent Circuit for CE Amplifier

The next step in the AC analysis is to draw h-parameter circuit by replacing the transistor in the AC equivalent circuit with its h-parameter model. Below figure shows the h-parameter equivalent circuit for CE circuit.

h-Parameter Equivalent Circuit for Common Emitter Amplifier
h-Parameter Equivalent Circuit for Common Emitter Amplifier

The typical CE circuit performance is summarised below:

  • Device input impedance, Zb = hie
  • Circuit input impedance, Zi = R1 || R2 || Zb
  • Device output impedance, Zc= 1/hoe
  • Circuit output impedance, Zo = RC || ZC ≈ RC
  • Circuit voltage gain, Av = -hfe/hie*(Rc|| RL)
  • Circuit current gain, AI = hfe. RC. Rb/ (Rc+RL) (Rc+hie)
  • Circuit power gain, Ap = Av * Ai

Applications Of CE Amplifier

  • The common emitter circuit is popular because it’s well-suited for voltage amplification, especially at low frequencies.
  • Common-emitter amplifiers are also used in radio frequency transceiver circuits.
  • Common emitter configuration commonly used in low-noise amplifiers.

This article discusses the working of the common emitter amplifier circuit and its applications. By reading the above information you have got an idea about this concept. Furthermore, any queries regarding this or if you want to implement Electronics and Electrical projects, please feel free to comment in the below section. Here is the question for you, what is the function of the common emitter amplifier?

Add Comment