Electronic Circuit Design
Title : Design of a Common Emitter Amplifier with given Specifications
AV=125 and f =1kHz
Submitted by :
Taha Ahmed 23-ENC-20
Raees Ali 23-ENC-13
M. Sarim 23-ENC-49
Submitted to :
Sir Farhan
EXPERIMENT NO. 4
Design of a Common Emitter Amplifier with given Specifications
AV=125 and f =1kHz
PRE LAB TASK
Objectives
1. To design Common Emitter Amplifier using transistor 2N3904/2N2222 with the
given specifications.
2. To implement the designed CE Amplifier.
Introduction
Theory
A common emitter amplifier is widely used as a voltage amplifier and is one of the most
frequently employed configurations of bipolar junction transistors (BJTs). The circuit
depicted in Figure 1 illustrates a typical common emitter amplifier. In this setup, the base
terminal acts as the input, while the collector terminal serves as the output. The emitter
terminal, shared between the input and output, is typically connected either to a ground
reference or a power supply rail.
Common emitter amplifiers are known for their high voltage gain and introduce a 180° phase
shift between the input and output signals. However, the gain of the amplifier is affected by
temperature variations and bias current fluctuations.
Some common limitations of this circuit include a restricted input dynamic range, where
exceeding the small-signal limit results in high distortion. When the transistor surpasses this
limit, it no longer behaves in accordance with its small-signal model. A common technique
used to address these challenges is the incorporation of negative feedback, typically achieved
through emitter degeneration.
Emitter degeneration involves adding a resistor (or another impedance element) between
the emitter and the common signal reference (such as ground or a supply rail). This added
impedance (RE) effectively lowers the circuit’s transconductance (GM = gm) by a factor of
gmRE + 1, thereby influencing the voltage gain.
Circuit Diagram of a Common Emitter Amplifier
By implementing emitter degeneration, the voltage gain primarily depends on the ratio of
RC to RE rather than the intrinsic characteristics of the transistor, which can vary
unpredictably. As a result, stability and distortion performance improve, although this
comes at the cost of a reduced overall gain.
Design Guidelines
Ensure the base-emitter voltage (VBE) remains within 0.6V - 0.7V for silicon
transistors.
The current gain (β) of the transistor is generally high (typically between 100-300).
Assume that IC ≈ IE.
Also, consider ICRC = ICRE.
The supply voltage VCC should be chosen such that VCC ≤ VCEO.
The collector-emitter voltage VCE should fall within the range VCC/3 ≤ VCE ≤
VCC/2.
The collector current (IC) should typically be less than 10 mA.
The collector resistor (RC) is calculated based on circuit requirements.
After determining RC, the next steps involve:
1. Calculating IB and IE.
2. Choosing a nominal base current (I0) such that I0 = 25IB = IR2.
3. Determining the value of R2.
4. Computing R1.
5. Calculating the gate resistor (Rg) using the relation:
RE′=RE−RgRE' = RE - RgRE′=RE−Rg
For capacitor selection:
Input coupling capacitor (C1): Choose XC << RE' at the operating frequency, then
determine C1 accordingly.
Bypass capacitor (CE): Select XCE << (R1 || R2) or XCE << RB, then compute CE.
Output coupling capacitor (C2): Ensure XC2 << (RL || RC) and determine C2 from
this condition.
Guidelines help in designing a stable and efficient common emitter amplifier while
mitigating its inherent limitations.
LAB SESSION
Lab Task
Design the CE Amplifier of the given specification and implement the circuit in lab and
record the values in table.
AV = 125
VCC = 12 V
f = 1000 Hz
VCE = 3.6V
β = 150
IC= 20 mA
R1=3.6kΩ
Equipment and Materials:
Power supply
Function generator
Digital Storage Oscilloscope
Resistors of different value (1.5K,220,22K,220)
Capacitors (10uf,22uf)
Digital Multimeter
Transistors 2N2222
Experimental Procedure:
1. Follow the procedure given in design rules step by step to get the common emitter
amplifier of required specifications.
2. Verify the results obtained with calculations to the one obtained by using the
specific parameter values with the help of the oscilloscope.
3. Record the results in tabular form. Circuit
Diagram of a Common Emitter Amplifier
Observations
Table 1
Common Emitter Amplifier
DC Voltages AC Voltages
IC Frequency
(mA) VBE (V) VCE (V) VE (V) Input Vp-p Output Vp-p AV (Hz)
125 1k
20 1.7 3.6 4.4 20mV 2.50
