Common emitter configuration of BJT

Common emitter configuration for BJT amplifer

The common emitter circuit is probably the most widely used transistor configuration. The emitter electrode is common to both input and output circuits. The common emitter amplifier has a typical input impedance of 1kilo ohms and a typical output impedance of 10 kilo-ohms. Also, the output will be the inverse of the input, which means the output experiences a 180° phase change. This results in a remarkable overall performance.

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Operation of common emitter amplier

The common emitter amplifier circuit comprises of a voltage divider bias and coupling capacitor C_Band C_C at the input and output and a bypass capacitor C_E which is connected from the emitter to the ground. The capacitor C_B couples the input signal to the input port of the amplifier. It also separates the AC signals from the DC biasing voltage.

Figure 1. Common Emitter Amplifier Circuit (a) circuit diagram, (b) circuit for DC bias Calculation. Image source: www.ee.iitb.ac.in

The capacitor C_C couples the output signal to the load resistance R_L. This is because an AC input signal is applied at the base and then taken from the collector terminal as the output signal. Both the input and output signals have the emitter as a common terminal.

The bypass capacitor C_C shorts the signal at the emitter to the ground, therefore, ensuring that no signal flows through the emitter at a particular frequency. This is done to supply base bias voltage. That is why the amplified output signal is 180̊ out of phase of the input signal.

The voltage divider biasing has two resistors (R₁ and R₂) which are connected midpoint to the base terminal of the transistor. This forms a potential divider which supplies the base with voltage bias. R₁ resistor is used to produce the forward bias voltage and R₂ is used to ensure that a bias voltage is developed.

The thermal stability of the amplifier is regulated through the resistor RE at the emitter terminal connection. R1, R2 and RE are extremely important for the stabilization of the circuit and the formation of the bias voltage. If these resistors fail to perform, a proper operating point would not be established. This could result in the negative half-cycle being cut off from the output signal.

Phase shift

Phase shift is the phase angle between the input and amplified output signal measured in degrees is known as phase shift. The conventional single-stage amplifiers provide a phase shift of either 180^o or 360^o.

DC Analysis of CE Amplifier

The common emitter amplifier operates when an AC input signal flows through the emitter-base junction. The input signals alternate between positive and negative peak values. The signal is in a forward bias direction and increases when it moves across the junction during the upper half cycle.

Common-emitter amplifier circuit diagram working — Figure 2. Operation of the Common Emitter Amplifier

The collector current also increases significantly due to an increase in the flow of electrons from the emitter to the collector through the base. The current also increases across the collector load resistor R_Cand a voltage drop is also observed across the collector. Therefore, the overall output signal is amplified. This shows that in the upper half of the cycle the common-emitter circuit is amplifying the input signal.

While the upper half cycle increases the forward bias voltage, the lower half cycle decreases the forward bias voltage across the emitter-base junction. Therefore, the decreased collector-base voltage causes the collector current to be reduced across the collector load resistance R_C. This is how we obtain the full cycle of the forward bias voltage.

This process of making the negative and positive peak values is achieved through the process of biasing. Biasing is very important for amplifier designs as it helps establish the operating point of the transistor amplifier at which it would receive signals. This way, the distortions in the output signal of the amplifier are greatly reduced as the proper parameters of the most efficient operating conditions are established. Bias refers to the DC current and voltage inside the amplifier circuit.

Under DC conditions all the three capacitors (C_B, C_E and C_C) in the common emitter amplifier are replaced with open circuits. The transistor gain β is assumed to be very large such that it goes to infinity. Due to this assumption the base current can be neglected. This way the R1-R2 network operates as a voltage divider. The transistor bias voltage is calculated by:

$V_B = \frac{R_2.V_C_C}{R_1 + R_2} \dots \dots \dots (1)$

V_CC is the supply voltage that also aids in determining the maximum collector current, IC when the transistor is fully switched ON, V_CE = 0.

To operate as an amplifier, the BJT needs to operate within the active region. The base-emitter junction should be in forward bias and the base-collector junction needs to be under reverse bias. The base-emitter voltage drop can be recorded by:

At first, we sample $f(x)$ in the $N$ ( $N$ is odd) equidistant points around $x^*$ :

$V_B_E =V_B - V_E \dots \dots \dots (2)$

For a silicon BJT, the difference between these voltages gives a voltage drop of about 0.7V. Hence, through this value, the V_E can be recorded as:

$V_E =V_B - V_BE = V_B - 0.7 = \frac{R_2.V_C_C}{R_1 + R_2} - 0.7 \dots (3)$

The emitter current I_E is:

$I_E = \frac{V_E}{V_R} \dots \dots \dots (4)$

$I_C = \frac{\beta}{\beta + 1} I_E \approx I_E \dots \dots \dots (5)$

The collector current is assumed to be equivalent to the I_C because it is assumed the gain of the transistor amplifier is very large. The current gain, of the common emitter amplifier is the ratio of the change in the collector current to the change in the base current. The voltage gain, in the product of the current gain and the ratio of the output collector resistance to the input resistance of the base circuits. Hence the following equations define the current gain and the voltage gain.

$\beta = \frac{\Delta I_C}{\Delta I_B} \dots \dots \dots (6)$

$A_V = \beta \frac{R_C}{R_B} \dots \dots \dots (7)$

Figure 3. DC Bias computation for the common-emitter amplifier with finite base current

The base current I_B can be calculated using the Thevenin equivalent circuit. It is found by the collector current I_C and the DC current gain β. The Kirchhoff voltage loop in the Base to Emitter loop can be represented as:

$V_T_H= I_B R_T_H + V_B_E + (\beta+1)I_BR_E \dots \dots \dots (8)$

$I_C = \beta I_B + \beta \frac{V_T_H - V_B_E}{R_T_H + (\beta+1 R_E} \dots \dots \dots (9)$

Where R_Th represent the parallel resistance between R₁ and R₂ and V_Th= V_B .

These calculations sum up the DC analysis of the collector-emitter amplifier.

Operating point and quiescent values
shown on the load line for a bipolar transistor operating in common-emitter mode — Operating point and quiescent values are shown on the load line for a bipolar transistor operating in the common-emitter mode

AC analysis of CE Amplifier

To calculate the alternating current behaviour of transistor circuits, substitute circuit diagrams are often used that show the transistor with regards to its terminal behaviour. There are a large number of equivalent circuit diagrams, however, the h-parameter equivalent circuit diagram is preferred to calculate circuits with bipolar transistors.

‘h’ parameters are four parameters that describe the behaviour of a two-port network system. This linear network is formed when the resistance, inductance, capacitance remain fixed and only the voltage across them changes. Therefore, these parameters are divided into h₁₁, h₁₂, h₂₁ and h₂₂ where the first subscript number denotes the input variable and the second denotes the output variable. It is in a subscript form which means the first number is always divided by the second number. ‘h’ parameters are also known as hybrid parameters as they are a mixture of two variables.

Figure 4. The equivalent circuit diagram with h parameters for the Emitter circuit

The equations resulting from this equivalent circuit are as follows: –

$U_1= h_1_1.I_1+h_1_2. V_2 \dots \dots \dots (9)$

$U_2= h_2_1.I_1+h_2_2. V_2 \dots \dots \dots (10)$

The h parameters can be easily determined and are measured by the parameter values which are different as they depend on which pair of terminals are viewed as input or output. Where:

h₁₁ is the input resistance.
h₁₂ is the input resistance.
h₂₁ is the input resistance.
h₂₂ is the input resistance.

In the case of high frequencies, capacities between the base-emitter, base-collector and collector-emitter connections must also be taken into account in the equivalent circuit diagram. This also includes the base-collector resistor, which however is neglected in most cases because it is very large. The capacitances determine the cut-off frequencies of the basic transistor circuits. This type of circuit is suitable for lab testing as the capacities produce negligible frequencies.

Characteristics of Common Emitter Amplifier

A few notable characteristics of the common emitter amplifier are:

The common emitter amplifier has a high power gain.
The input and output have a phase relationship of 180 degrees.
It has a medium voltage gain.

Various characteristics of Common emitter configuration of BJT

Parameter	Values
Voltage gain	Medium – typically 40
Current Gain	High – typically 200
Power gain	Very High – typically 8000
Input resistance	Medium – 2.5kΩ
Output resistance	High – 20kΩ
Phase shift	180⁰
Typical application	General Purpose AF and RF amplifiers

Pros and Cons of Using the Common Emitter Amplifier Configuration

Advantages

A few advantages of using the common emitter amplifier are:

The output impedance of the amplifier is high.
The input impedance is low and it works as an inverting amplifier
It has medium voltage and high current gain which results in a high power gain.

Disadvantages

A few disadvantages of using the common emitter amplifier are:

The common emitter amplifier does not respond to high frequencies.
The voltage gain of the amplifier is not stable.
It has a very high output resistance.
Despite the presence of resistors, there are still chances of thermal instability in the circuit.

Nowadays common-emitter amplifiers are being used in applications that require low-frequency voltage and noise amplifiers. Radiofrequency transceiver circuits also make use of common-emitter amplifiers.

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