Common Emitter Amplifier Circuit Working & Its Characteristics
- Jun 06, 2022
There are different types of transistor amplifiers operated by using an AC signal input. This is interchanged between the positive value and negative value, hence this is the one way of presenting the common emitter amplifier circuit to function between two peak values. This process is known as the biasing amplifier and it is an important amplifier design to establish the exact operating point of a transistor amplifier which is ready to receive the signals hence it can reduce any distortion to the output signal. In this article, we will discuss common emitter amplifier analysis.
What is an Amplifier?
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 the 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 the 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.
What is a Common Emitter Amplifier?
The common emitter amplifier is a three basic single-stage bipolar junction transistor and is used as a voltage amplifier. The input of this amplifier is taken from the base terminal, the output is collected from the collector terminal and the emitter terminal is common for both the terminals. The basic symbol of the common emitter amplifier is shown below.
Common Emitter Amplifier Configuration
In electronic circuit design, there are three kinds of transistor configurations are used like common emitter, common base, and common collector, In that, the most frequently used one is common emitter due to its main attributes.
This kind of amplifier includes the signal which is given to the base terminal then the output is received from the collector terminal of the circuit. But, as the name suggests, the main attribute of the emitter circuit is familiar for both the input as well as output.
The configuration of a common emitter transistor is widely used in most electronic circuit designs. This configuration is evenly appropriate to both the transistors like PNP and NPN transistors but NPN transistors are most frequently used due to the widespread use of these transistors.
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.
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 an increase in 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 above.
From the voltage waveforms for the CE circuit shown in Fig. (b), It is seen that there is a 180-degree phase shift between the input and output waveforms.
Working of Common Emitter Amplifier
The below circuit diagram shows the working of the common emitter amplifier circuit and it consists of voltage divider biasing, used to supply the base bias voltage as per the necessity. The voltage divider biasing has a potential divider with two resistors are connected in a way that the midpoint is used for supplying base bias voltage.
There are different types of electronic components in the common emitter amplifier which are R1 resistor is used for the forward bias, the R2 resistor is used for the development of bias, the RL resistor is used at the output it is called the load resistance. The RE resistor is used for thermal stability. The C1 capacitor is used to separate the AC signals from the DC biasing voltage and the capacitor is known as the coupling capacitor.
The figure shows that the bias vs gain common emitter amplifier transistor characteristics if the R2 resistor increases then there is an increase in the forward bias and R1 & bias are inversely proportional to each other. The alternating current is applied to the base of the transistor of the common emitter amplifier circuit then there is a flow of small base current. Hence there is a large amount of current flow through the collector with the help of the RC resistance. The voltage near the resistance RC will change because the value is very high and the values are from 4 to 10kohm. Hence there is a huge amount of current present in the collector circuit which amplified from the weak signal, therefore common emitter transistors work as an amplifier circuit.
Voltage Gain of Common Emitter Amplifier
The current gain of the common emitter amplifier is defined as the ratio of change in collector current to the change in base current. The voltage gain is defined as the product of the current gain and the ratio of the output resistance of the collector to the input resistance of the base circuits. The following equations show the mathematical expression of the voltage gain and the current gain.
β = ΔIc/ ΔIb
Av = β Rc/Rb
Circuit Elements and their Functions
The common emitter amplifier circuit elements and their functions are discussed below.
Biasing Circuit/ Voltage Divider
The resistances R1, R2, and RE used to form the voltage biasing and stabilization 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 the 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.
Common Emitter Amplifier analysis
The first step in AC analysis of Common Emitter amplifier circuit is to draw the AC equivalent circuit by reducing all DC sources to zero and shorting all the capacitors. The below figure shows the AC equivalent circuit.
The next step in the AC analysis is to draw an h-parameter circuit by replacing the transistor in the AC equivalent circuit with its h-parameter model. The below figure shows the h-parameter equivalent circuit for the CE circuit.
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
CE Amplifier Frequency Response
The voltage gain of a CE amplifier varies with signal frequency. It is because the reactance 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. The below figure shows the frequency response of a typical CE 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 the 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 a 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 tends 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.
We can observe the frequency response of any amplifier circuit is the difference in its performance through changes within the input signal’s frequency because it shows the frequency bands where the output remains fairly stable. The circuit bandwidth can be defined as the frequency range either small or big among ƒH & ƒL.
So from this, we can decide the voltage gain for any sinusoidal input in a given range of frequency. The frequency response of a logarithmic presentation is the Bode diagram. Most of the audio amplifiers have a flat frequency response that ranges from 20 Hz – 20 kHz. For an audio amplifier, the frequency range is known as Bandwidth.
Frequency points like ƒL & ƒH are related to the lower corner & the upper corner of the amplifier which are the gain falls of the circuits at high as well as low frequencies. These frequency points are also known as decibel points. So the BW can be defined as
BW = fH – fL
The dB (decibel) is 1/10th of a B (bel), is a familiar non-linear unit to measure gain & is defined like 20log10(A). Here ‘A’ is the decimal gain which is plotted over the y-axis.
The maximum output can be obtained through the zero decibels which communicate toward a magnitude function of unity otherwise it occurs once Vout = Vin when there is no reduction at this frequency level, so
VOUT/VIN = 1, so 20log(1) = 0dB
We can notice from the above graph, the output at the two cut-off frequency points will decrease from 0dB to -3dB & continues to drop at a fixed rate. This reduction within gain is known commonly as the roll-off section of the frequency response curve. In all basic filter and amplifier circuits, this roll-off rate can be defined as 20dB/decade, which is equal to a 6dB/octave rate. So, the order of the circuit is multiplied with these values.
These -3dB cut-off frequency points will describe the frequency where the o/p gain can be decreased to 70 % of its utmost value. After that, we can properly say that the frequency point is also the frequency at which the gain of the system has reduced to 0.7 of its utmost value.
Common Emitter Transistor Amplifier
The circuit diagram of the common emitter transistor amplifier has a common configuration and it is a standard format of transistor circuit whereas voltage gain is desired. The common emitter amplifier is also converted as an inverting amplifier. The different types of configurations in transistor amplifiers are common base and the common collector transistor and the figure are shown in the following circuits.
Characteristics of Common Emitter Amplifier
- The voltage gain of a common emitter amplifier is medium
- The power gain is high in the common emitter amplifier
- There is a phase relationship of 180 degrees in input and output
- In the common emitter amplifier, the input and output resistors are medium.
The characteristics graph between the bias and the gain is shown below.
Transistor Bias Voltage
The Vcc (supply voltage) will determine the utmost Ic (collector current) once the transistor is activated. The Ib (base current) for the transistor can be found from the Ic (collector current) & the DC current gain β (Beta) of the transistor.
VB = VCC R2/R1+R2
Sometimes, ‘β’ is referred to as ‘hFE’ which is the forward current gain of the transistor within the CE configuration. Beta (β) is a fixed ratio of the two currents like Ic and Ib, so it doesn’t contain units. So a small change within the base current will make a huge change within the collector current.
The same type of transistors as well as their part number will contain huge changes within their ‘β’ values. For instance, the NPN transistor like BC107 includes a Beta value (DC current gain in between 110 – 450 based on the datasheet. So one transistor may include a 110 Beta value whereas another may include of 450 Beta value, however, both the transistors are NPN BC107 transistors because Beta is a feature of the structure of the transistor but not of its function.
When the base or emitter junction of the transistor is connected forward bias, then the emitter voltage ‘Ve’ will be a single junction where voltage drop is dissimilar to the voltage of the Base terminal. The emitter current (Ie) is nothing but the voltage across the emitter resistor. This can be calculated simply through Ohm’s Law. The ‘Ic’ (collector current) can be approximated, as it is approximately a similar value to the emitter current.
Input and Output Impedance of Common Emitter Amplifier
In any electronic circuit design, impedance levels are one of the main attributes that need to consider. The value of input impedance is normally in the region of 1kΩ, while this can differ significantly based on the conditions as well as values of the circuit. The less input impedance will result from the truth that the input is given across the two terminals of the transistor-like base & emitter because there is a forward-biased junction.
Also, the o/p impedance is comparatively high because it varies significantly again on the values of selected electronic component values & allowed current levels. The o/p impedance is a minimum of 10kΩ otherwise possibly high. But if the current drain permits high levels of current to be drawn, then the o/p impedance will be decreased significantly. The impedance or resistance level comes from the truth that the output is used from the collector terminal because there is a reverse-biased junction.
Single Stage Common Emitter Amplifier
The single-stage common emitter amplifier is shown below and different circuit elements with their functions are described below.
The circuits like biasing as well as stabilization can be formed with resistances like R1, R2 & RE
Input Capacitance (Cin)
The input capacitance can be denoted with ‘Cin’ which is used to combine the signal toward the base terminal of the transistor.
If this capacitance is not used, then the resistance of the signal source will approach across the resistor ‘R2’ to alter the bias. This capacitor will allow simply AC signal to supply.
Emitter Bypass Capacitor (CE)
The connection of the emitter bypass capacitor can be done in parallel to RE to give a low reactance lane toward the amplified AC signal. If it is not utilized, then the amplified AC signal will flow throughout RE to cause a voltage drop across it, so the o/p voltage can be shifted.
Coupling Capacitor (C)
This coupling capacitor is mainly used to combine the amplified signal toward the o/p device so that it will allow simply AC signal to supply.
Once a weak input AC signal is given toward the base terminal of the transistor, then a small amount of base current will supply, because of this transistor act, high AC. current will flow throughout collector load (RC), so high voltage can come into view across the collector load as well as the output. Thus, a feeble signal is applied toward the base terminal which appears in the amplified form within the collector circuit. The amplifier’s voltage gain like Av is the relation between the amplified input and output voltages.
Frequency Response & Bandwidth
The amplifier’s voltage gain like Av for several input frequencies can be concluded. Its characteristics can be drawn on both the axis like a frequency on X-axis whereas voltage gain is on Y-axis. The graph of frequency response can be attained which is shown in the characteristics. So we can observe that the gain of this amplifier can be decreased at very high and low frequencies, however, it stays stable over an extensive range of mid-frequency area.
The fL or low cut off frequency can be defined as when the frequency is below 1. The range of frequency can be decided at which the amplifier gain is double the gain of mid-frequency.
The fL(upper cut off frequency) can be defined as when the frequency is in the high range at which the amplifier’s gain is 1/√2 times the gain of mid-frequency.
Bandwidth can be defined as the interval of frequency among low-cut off & upper cut-off frequencies.
BW = fU – fL
Common Emitter Amplifier Experiment Theory
The main intention of this CE NPN transistor amplifier is to investigate its operation.
The CE amplifier is one of the main configurations of a transistor amplifier. In this test, the learner will design as well as examine a fundamental NPN CE transistor amplifier. Suppose, the learner has some knowledge on the theory of transistor amplifier like the use of AC equivalent circuits. So the learner is estimated to design his/her own process to perform the experiment in the lab, once the pre-lab analysis is completely done, then he can analyze & summarize the experiment results in the report.
The required components are NPN transistors – 2N3904 & 2N2222), VBE = 0.7V, Beta = 100, r’e = 25mv/IE in the analysis of Pre-lab.
As per the circuit diagram, calculate the DC parameters like Ve, IE, VC, VB & VCE with approximate technique. Sketch the ac equivalent circuit & calculate the Av (voltage gain ), Zi (input impedance) & Zo (output impedance). Also sketch the composite waveforms predictable at different points like A, B, C, D & E within the circuit. At point ‘A’, assumer Vin like 100 mv peak, Sine wave with 5 kHz.
For a voltage amplifier, draw the circuit with input impedance, a voltage source which is dependant as well as o/p impedance
Measure the input impedance value like Zi through inserting a test resistor within a series through the input signals toward the amplifier & measure how much the signal of the ac generator will appear really at the amplifier’s input.
To determine output impedance, take out the load resistor momentarily & calculate the unloaded ac o/p voltage. After that, put back the load resistor, again measure the ac o/p voltage. To determine the output impedance, these measurements can be used.
Experiment in Lab
Design the circuit accordingly and check all the above calculations. Utilize DC coupling as well as dual-trace on the oscilloscope. After that takeout common-emitter momentarily & again measure the o/p voltage. Evaluate the outcomes using your Pre-lab computations.
The advantages of a common emitter amplifier include the following.
- The common emitter amplifier has a low input impedance and it is an inverting amplifier
- The output impedance of this amplifier is high
- This amplifier has the highest power gain when combined with medium voltage and current gain
- The current gain of the common emitter amplifier is high