MALAYSIA-JAPAN INTERNATIONAL INSTITUTE OF
TECHNOLOGY
ELECTRONIC CIRCUIT (SMJE 2083)
SEMESTER I 2023/2024
NAME: MUHAMMAD FARUQI BIN AHMAD RAMZI
MATRIC NO: A22MJ0104
YEAR/PROGRAM 2/SMJEH
SECTION 1
LECTURER’S NAME DR. RASLI ABD GHANI / DR. SITI RAHMAH
BINTI AID
DATE 22 JANUARY 2024
ASSIGNMENT 2
Question 1
a) Illustrate and explain in details the basic structure and operation of the dual gate nchannel MOSFET.
With two gates, Gate 1 (G1) and Gate 2 (G2), the dual-gate n-channel Metal Oxide
Semiconductor Field-Effect Transistor (DG-MOSFET) has a special construction that
improves its controllability. The base is the silicon substrate, which is linked to the
source terminal. G1 mainly regulates the flow of charge carriers, or electrons, and is
positioned between the source and the drain. In the meantime, G2, which is nearer the
source, offers more control and modifies the channel's overall conductivity. Direct
electrical contact between the gates and the semiconductor material is prevented by the
insulating layer, which is usually composed of silicon dioxide (SiO2). Voltages must be
applied to G1 and G2 separately for operation. While G2 enables precise modulation, G1
chooses the mode of action (enhancement or depletion). Precise transistor behaviour can
be achieved with complex modifications thanks to this dual control. DG-MOSFETs are
utilized in radio frequency (RF) circuits, where their improved control capabilities are
useful, especially in mixers and amplifiers. All things considered, the dual-gate n-channel
MOSFET provides better performance and versatility for applications needing complex
modulation and control.
b) Point out the reason why the D-MOSFET is used in this particular application.
The BF998 Dual-Gate n-channel MOSFET is selected for the pH sensor circuit in
electronic instrumentation for wastewater treatment facilities due to its advantageous
characteristics. Its high input impedance is crucial in pH sensor circuits, preventing
signal degradation and ensuring accurate readings by minimizing loading effects. The
MOSFET's low noise figure enhances sensitivity in pH measurements, contributing to
precise signal processing. The dual-gate configuration provides flexibility in adjusting
the transistor's behavior, optimizing the sensor circuit's sensitivity and response to
variations in pH levels. With broadband operation, the BF998 effectively handles
signals across a range of frequencies, suitable for applications where pH levels result
in varying signal frequencies.
c) Based on the graph shown in Figure 2, classify the relation between sensor voltage and
pH scale/class.
Based on Figure 2, the sensor voltage is directly proportional to the pH scale. This means
when the sensor voltage is increased, the pH value is also increased.
d) What is the correlation of the answers from Question 1-b) and c). Discuss about it.
In Question 1-b), there are various reasons for selecting the BF998 MOSFET for
electronic instrumentation in wastewater treatment. These reasons included high input
impedance, low noise figure, dual-gate flexibility and broadband operation. These
features collectively contribute to the accuracy, sensitivity, and reliability of electronic
instrumentation, especially in challenging environments such as wastewater treatment
facilities. Now, in Question 1-c), it is noted that the sensor voltage is directly
proportional to the pH scale. This implies that changes in the pH level directly
influence the sensor voltage output. The high input impedance, low noise figure, and
high gain of the BF998 MOSFET become particularly relevant in this context. The
high input impedance prevents signal degradation, ensuring that the sensor voltage
accurately represents pH variations. The low noise figure enhances the sensitivity of
the pH sensor circuit, allowing for precise detection of small changes in sensor
voltage corresponding to changes in pH levels. The high gain amplifies the weak
sensor signals, making it easier to detect and measure variations in pH.
e) Determine the approximate sensor voltage for a pH of 8 and pH of 3.
Based on Figure 2, the approximate sensor voltage for pH 8 is 100 mV, while for pH 3,
the approximate sensor voltage is -400 mV.
Question 2
a) In the sensor design of Figure 3, what is the value of the voltage on the second gate?
2
𝑉𝐺𝑆2 =𝑉𝐷𝐷 ( 𝑅2𝑅+𝑅
)
1
= (12v) ( 100𝑘100𝑘
)
+ 100𝑘
= 6V
We can find the value of voltage for the second gate using voltage divider.
b) If the value stated in Question 2-a) need to be set to the lower value, what should be
done. Discuss the solution for this.
As shown in question 2a) we can see that 𝑉𝐺𝑆2 is directly proportional to 𝑉𝐷𝐷 , since
they are directly proportional to each other, we can reduce the value of 𝑉𝐷𝐷 making
the value of 𝑉𝐺𝑆2 lower. Aside from that, we can increase the value of 𝑅2 𝑎𝑛𝑑 𝑅1.
This will result in a larger denominator in the voltage division formula, leading to a
smaller voltage at 𝑉𝐺𝑆2 .
c) What is the specified typical transconductance (transadmittance) for the BF998?
Referring to datasheet provided for BF998, the specified typical transconductance for
the BF998 is 𝑔𝑚 = 24mS.
d) If the drain-to-source voltage is 10V, determine the maximum allowable drain current.
By using the formula,
𝑉𝐷𝑆 = 𝑉𝐷 – 𝑉𝑆
𝑉𝐷𝑆 = 𝑉𝐷𝐷 − 𝐼𝐷 𝑅𝐷 − 0
10V = 12V - 𝐼𝐷 (620Ω + 100Ω)
= 2.78 x 10−3 A
e) If one gate is biased to 1V, determine the 𝐼 𝐷 when the other gate is 0V.
When 𝑉𝐺𝑆1 = 1V, 𝑉𝐺𝑆2 = 0V, 𝐼𝐷 = 3𝑚𝐴.
When 𝑉𝐺𝑆1 = 0V, 𝑉𝐺𝑆2 = 1V, 𝐼𝐷 = 5𝑚𝐴.
Question 3
a) Construct the circuit diagram in Figure 5 and execute the simulation. Please include
your Multisim circuit in the submission report.
a.400mV
b.350mV
c.250mV
d.150mV
e.0mV
f.-150m
g.-250mV
h.-350mV
i.-400mV
c) From the simulated results, discuss the relation between 𝑉𝑠𝑒𝑛𝑠𝑜𝑟 and 𝑉𝑜𝑢𝑡 .
In order to show relation between the two value we can use table 1.1 𝑉𝑜𝑢𝑡 vs 𝑉𝑠𝑒𝑛𝑠𝑜𝑟 .
𝑉𝑜𝑢𝑡 (𝑚𝑉) 9.835 9.913 10.153 10.463 10.991 11.504 11.782 11.961 11.988
350
250
150
0
-150
-250
-350
-400
𝑉𝑠𝑒𝑛𝑠𝑜𝑟 (𝑚𝑉) 400
Table 1.1
From table 1.1, we can see that as 𝑉𝑠𝑒𝑛𝑠𝑜𝑟 is decrease, 𝑉𝑜𝑢𝑡 increase.
e) Relate the 𝑉 𝑜 𝑢 𝑡 and pH for each measurement by plotting the graph (𝑉 𝑜 𝑢 𝑡 vs pH).
𝑉𝑜𝑢𝑡 (𝑚𝑉) 9.835 9.913 10.153 10.463 10.991 11.504 11.782 11.961 11.988
10.5
9.5
8.5
7
5.5
4.5
3.5
3
11
pH
𝑉𝑜𝑢𝑡 vs pH
14
12
𝑉𝑜𝑢𝑡
10
8
6
4
2
0
0
2
4
6
pH
As pH level decrease, the 𝑉 𝑜 𝑢 𝑡 is increased.
8
10
12
Future Application
Based on the pH sensor circuit given in this assignment, please propose one potential
system (other than waste water treatment) where you can apply the pH sensor.
Please include:
• The background of the proposed system
• The problem that needs to be solved
• How the system can work with the pH sensor
• References
One potential system where we can apply the pH sensor is Aquaponics Monitoring
System.
Background:
Aquaponics is a sustainable farming method that combines aquaculture (raising fish)
with hydroponics (cultivating plants in water). It creates a symbiotic relationship where
fish waste provides nutrients for plants, and the plants help filter and clean the water for
the fish. Maintaining proper pH levels is crucial for the health of both fish and plants in
an aquaponics system.
Problem:
In aquaponics, maintaining the right pH balance is essential for optimal nutrient uptake
by plants and the well-being of fish. Fluctuations in pH can lead to nutrient deficiencies,
affecting plant growth and potentially harming fish. Therefore, there is a need for a
precise pH monitoring system to ensure a stable and suitable environment for
aquaponics.
How the System Can Work with the pH Sensor:
Integrating a pH sensor circuit into the aquaponics monitoring system allows continuous
monitoring of the water's pH level. The pH sensor provides real-time data, enabling
automated control systems to make adjustments if pH levels deviate from the desired
range. For instance, a feedback loop could trigger actions such as adjusting the rate of
aeration, controlling nutrient dosing, or notifying the system operator about any
potential issues.
References:
Al Baihaqi, M., & Ahmad, I. (2022). Development of IoT Based Aquaponic Monitoring
System for Agriculture Application. Journal of Engineering Technology, 10(1), 1–6.
https://bmi.unikl.edu.my/wp-content/uploads/2022/11/1_6_Development-of-IoT-BasedAquaponic-Monitoring-System-for-Agriculture-Application_2022100001.pdf
Alselek, M., Alcaraz-Calero, J. M., Segura-Garcia, J., & Wang, Q. (2022). Water IoT
Monitoring System for Aquaponics Health and Fishery Applications. Sensors, 22(19),
7679. https://doi.org/10.3390/s22197679