FUCULTY OF ENGINEERING AND TECHNOLOGY

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

EEB433;ANALOGUE ELECTRONICS DESIGN

MOTSWAKE KAMOGELO 202107378

TITLE: COMMON BASE AMPLIFIER CIRCUIT

ABSTRACT
This experiment explores the characteristics and performance of a
common base amplifier, a transistor configuration widely used in high-
frequency applications due to its low input impedance and wide
bandwidth. By assembling and testing the amplifier circuit, various
parameters such as voltage gain, current gain, and impedance were
measured. Theoretical calculations were compared with experimental
results to assess the efficiency of the amplifier and its applicability in real-
world electronic systems.
LIST OF ABBREVIATIONS

CBA: Common Base Amplifier

DC: Direct Current

Q-point: Quiescent Point (operating point of the transistor)

Vb: Base Voltage

Vc: Collector Voltage

Ve: Emitter Voltage

Vcc: Supply Voltage

Vin: Input Voltage

Vout: Output Voltage

VCEQ: Collector-Emitter Quiescent Voltage

IBQ: Base Quiescent Current

ICQ: Collector Quiescent Current

β: Current Gain (hFE) of the transistor

re: Small signal emitter resistance

AC: Alternating Current

RC: Collector Resistor

RE: Emitter Resistor

R1, R2: Voltage Divider Resistors

Zin: Input Impedance

Zout: Output Impedance

VT: Thermal Voltage

LIST OF TABLES

Table-1: Measured Bias Circuit Parameters

Table-2: Voltage Gain Measurement

LIST OF FIGURES

Figure 1; circuit connection of the Common Base Amplifier

Figure 2; The Common Base Amplifier topology

Figure 3; The practical common Base Bias circuit

INTRODUCTION

The common base amplifier (CBA) is one of the three basic transistor
configurations, alongside the common emitter and common collector
amplifiers. It is particularly valued in high-frequency applications due to its
low input impedance and phase alignment between input and output
signals. Unlike other amplifier types, the common base amplifier provides
no current gain but can offer voltage gain and operates with a low input
impedance, making it useful in certain types of signal processing.

In this experiment, the common base amplifier configuration is analyzed

using a 2N2222 NPN transistor. The goal is to observe its amplification
characteristics and determine its input/output impedance, voltage gain,
and other operating parameters.

Figure 1; circuit connection of the common base amplifier

Figure 2; the practical common base bias circuit

OBJECTIVES
The objectives of this experiment are as follows:

To design and construct a common base amplifier circuit.

To measure the voltage gain, current gain, input impedance, and output
impedance of the common base amplifier.

To compare the theoretical values with experimental results to validate

the performance of the circuit.

To understand the impact of transistor biasing on amplifier characteristics.

To study the small-signal operation of the common base amplifier and its
frequency response.

LITERATURE REVIEW
Amplifiers are essential components in electronic circuits, used to increase
the power of a signal without changing its original content. The common
base amplifier (CBA) has distinct characteristics that differentiate it from
other configurations like the common emitter and common collector
amplifiers. It features low input impedance and provides significant
voltage gain but lacks current gain.

Historically, the common base configuration has been favored in RF

amplifiers and for impedance matching in transmission lines. According to
Sedra and Smith (2014), the CBA is suitable for applications where
minimal input impedance is required and for cascading stages in
wideband amplifiers. The CBA’s low impedance is advantageous in
reducing reflection and signal loss, especially in high-frequency
applications.

Biasing is critical in transistor amplifier design, as it establishes the

correct operating point (Q-point) to ensure linear amplification. The
biasing circuit for the CBA is similar to the common emitter amplifier,
typically using a voltage-divider network to provide stable operation.
Correct biasing prevents the transistor from operating in cutoff or
saturation, ensuring it remains in the active region for optimal
amplification.

Figure 3;The Common Base Amplifier topology

EQUIPMENT

The following equipment and components were used to carry out the
experiment:

Experimental Test Board

DC Power Supply

Function Generator

Two-Channel Oscilloscope

Digital Multimeter

NPN Silicon Transistor (2N2222)

Resistors: 10 kΩ, 3.3 kΩ, 1.5 kΩ, 1 kΩ.

Capacitors: 2.2 μF and 10 μF

PROCEDURE

1. The DC bias circuit shown in Fig. 3 was connected.

2. The DC voltages Vb , Ve , and Vc were measured using a digital

voltmeter. The transistor current gain beta was measured using a multi-
meter, and the results were recorded in Table-1.

3. The amplifier circuit was then connected as shown in Fig. 4.

4. A sinusoidal source signal with a 0.1 V peak-to-peak amplitude and a

frequency of 10 kHz was applied to the input, Vppin = 0.1V . The input
signal was displayed at channel 1 of the oscilloscope, and the output
signal was displayed at channel 2.

5. The amplitudes of both the input and output signals were measured,
and the practical voltage gains Av and Avs were calculated, as shown in
Table-2.
RESULTS

Table-1: Measured Bias Circuit Parameters

PARAMETER VALUE
β
Vb 3.02
Vc 2.39
Ve 8.75
ICQ 2.40
VBCQ 5.67
VBEQ 0.64
IB 0.9mA
re

Table-2: Voltage Gain Measurement

QUANTITY VALUE
Vppin 0.1V
Vppout 0.8V
Av Vout-pp/Vin-pp
Avs

THEORETICAL CALCULATIONS AND ANALYSIS

1. Bias Point Calculations:

The bias point of the transistor was calculated using the given formulas
and the measured values.

Base voltage (Vb):

Vb = (R2 * Vcc) / (R1 + R2)

= (3.3k * 12V) / (10k + 3.3k)

= 3.02V

Emitter voltage (Ve):

Ve = Vb – Vbe
 = 3.02V – 0.64V

= 2.39V

Collector voltage (Vc):

Vc = Vcc – (Ic * Rc)

= 12V – (2.4mA * 1.5k)

= 8.75V

Quiescent collector current (Icq):

Icq = Ve / Re

= 2.39V / 1k

= 2.4mA

The calculated values match the experimental measurements, confirming

that the biasing circuit was correctly designed.

2. Theoretical Voltage Gain (Av):

The theoretical voltage gain for the common base amplifier is given by:

Av = Rc / re

Where:

Re = (26mV / Icq)

= 26mV / 2.4mA

= 10.83 ohms

Therefore:

Av = 1.5k / 10.83

Av = 138.48

3. Measured Voltage Gain (Av):

From the measured values:

- Vppin = 0.1V

- Vppout = 0.8V

Measured gain:

Av = Vppout / Vppin
Av = 0.8V / 0.1V

Av = 8

The large difference between the measured voltage gain (8) and
theoretical gain (138.48) can be attributed to several factors:

- The low input impedance (\( Z_{in} \)) of the common base amplifier
which reduces the input signal magnitude.
- Parasitic capacitances and non-ideal behavior of the transistor,
especially at higher frequencies, which can attenuate the signal.
- Non-ideal components such as the internal resistance of the signal
generator.

4. Input Impedance (Zin):

The input impedance seen from the signal source is given by:

Zin = RE // re

Zin = (1k * 10.83 ohms) / (1k + 10.83 ohms)

Zin = 10.71 ohms

Output Impedance (Zout):

The output impedance of the amplifier is approximately:

Zout = Rc = 1.5k ohms

5. Internal Source Resistance \( R_s \)

The internal resistance of the signal generator \( R_s \) can be estimated

by comparing the theoretical voltage gain \( _v \) and the measured gain \(
Av(s) \). From the modified gain equation.

A_{v(s)} = V_{out(p-p)} / V_{in(p-p)} = 0.8V / 0.1V = 8

Using the gain equation with source resistance:

A_{v(s)} = (R_C \parallel re) / (re + Z_{in} + Rs)

By solving for \( R_s \), the internal resistance of the signal generator
can be inferred to be significant, causing a reduction in gain.

6. **Effect of Coupling Capacitor \( C_{out} \):**

If the coupling capacitor \( C_{out} \) in Figure 4 were shorted, the DC

voltage at the collector would be approximately zero because the
capacitor would no longer block the DC component, and the output signal
would be severely affected.

7. Effect of Opening CB
 If CB in Figure 4 were opened, the base would no longer be at AC ground,
altering the voltage gain and significantly increasing the input impedance,
which would reduce the overall amplifier performance.

8. Main Disadvantage Compared to Common Emitter Amplifier:

The main disadvantage of the common base amplifier compared to the

common emitter amplifier is its extremely low input impedance. This
makes it less suitable for circuits with high source impedance, as it tends
to load the input signal significantly, reducing the voltage gain in practical
scenarios. Additionally, the common base amplifier provides no current
gain, which limits its use in some applications where both voltage and
current gain are required.

DISCUSSION
The experiment focused on analysing the performance of a common base
amplifier, which is critical in applications requiring high-frequency
response and low input impedance. A comparison between the theoretical
calculations and the measured results provides insight into the behaviour
of the amplifier and the influence of various circuit elements. The
calculated bias point voltages were \( V_b = 3.02V \), \( V_e = 2.39V \),
and \( V_c = 8.75V \), closely matching the measured values of \
( 3.02V \), \( 2.39V \), and \( 8.75V \) respectively. This agreement
indicates that the biasing network was effectively designed, providing
stability to the transistor’s operating point. The close match between the
theoretical and experimental results confirms that the chosen resistor
values (R1 and R2) successfully established the desired operating
conditions for the transistor, ensuring reliable amplification. One of the
most significant observations from this experiment was the stark contrast
between the theoretical voltage gain \( A_v \) of 138.48 and the measured
gain of only 8. The substantial deviation can primarily be attributed to the
low input impedance characteristic of the common base configuration.
The theoretical calculations did not fully account for the loading effects of
the source impedance and the internal resistance of the signal generator.
When the input impedance of the common base amplifier is significantly
lower than that of the signal source, it leads to a voltage divider effect,
effectively reducing the input signal voltage and, consequently, the output
voltage. Moreover, the calculated emitter resistance \( r_e \) of 10.83Ω,
derived from the quiescent current \( I_{CQ} \), and the practical input
impedance of 10.71Ω indicate that the amplifier is well-matched for
specific applications. However, in a typical laboratory environment,
variations in component tolerances, parasitic capacitances, and non-ideal
behaviours of the transistor could introduce additional attenuation, further
compounding the differences between the theoretical and measured
gains. The role of the coupling capacitors, particularly \( C_{out} \), also
deserves attention. If \( C_{out} \) were shorted, the DC component of the
collector voltage would essentially be zero, which would disrupt the
amplifier’s ability to function effectively. This highlights the importance of
coupling capacitors in isolating the DC biasing conditions from the AC
signals, ensuring that only the intended signal is amplified. The capacitors
maintain stability in the DC operating point while allowing for the
amplification of AC signals, which is critical in high-frequency applications.
The estimated internal resistance \( R_s \) of the signal generator played a
vital role in the gain measurements. The discrepancy between the
expected and observed gains suggests that the signal generator’s
resistance impacted the overall circuit performance, resulting in additional
signal attenuation. This highlights the importance of considering the entire
circuit configuration, including source resistances, when designing
amplifiers for practical applications.

CONCLUSION

The experiment demonstrated that the common base amplifier’s

measured bias voltages closely matched theoretical values, confirming
proper circuit operation. However, the theoretical voltage gain of 138.48
was much higher than the measured gain of 8, likely due to the low input
impedance (10.71 ohms), which affected the input signal. Despite this, the
amplifier’s output impedance of \( 1.5 k\Omega \) suggests it could still be
effective in specific high-frequency applications, though its low input
impedance limits its use with higher source impedance circuits.
REFERENCES