Republic of the Philippines

BATANGAS STATE UNIVERSITY

The National Engineering University
Alangilan Campus
Golden Country Homes, Alangilan Batangas City, Batangas, Philippines 4200
Tel Nos.: (+63 43) 425-0139 local 2121 / 2221
E-mail Address: coe.alangilan@g.batstate-u.edu.ph | Website Address: http://www.batstate-u.edu.ph

College of Engineering

LABORATORY #1
TRANSISTOR AS A SWITCH

Pre-Lab:

1. Know the terminal pins of the transistor based on its packaging.

2. Secure a data sheet for your transistor.

Objectives:

1. To build a transistor switch and measure its input and output voltage/current levels for ON and
OFF conditions.
2. To be able to identify the saturation and cutoff regions of a transistor.

Materials:

● 10V DC power supply

● Variable DC power supply

● Diode

● 2 NPN Transistors

● 2 PNP Transistors

● 220 Ω resistor

● 10 kΩ resistor

● LEDs

Procedures:
1. Construct circuit A as shown using TinkerCAD software.

CIRCUIT A: Basic Transistor Switch

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 Republic of the Philippines
BATANGAS STATE UNIVERSITY
The National Engineering University
Alangilan Campus
Golden Country Homes, Alangilan Batangas City, Batangas, Philippines 4200
Tel Nos.: (+63 43) 425-0139 local 2121 / 2221
E-mail Address: coe.alangilan@g.batstate-u.edu.ph | Website Address: http://www.batstate-u.edu.ph

College of Engineering

2. Place an ammeter between the collector terminal of the transistor Q1 and the 220Ω. Gradually
increase Vin. Observe what happens. Record the maximum current level. This will be the value of
the output current (Icsat) when the transistor saturates (ON).
3. By using another voltmeter, measure the collector to emitter voltage (Vcesat) for this particular
collector current and then measure Vin record the reading in table 1.
4. Compare the Vin measured with the Vin calculated using:

Vin calc = Ib sat x Rb = (VccRb/βRc) + 0.7V

Ibsat and β are available on the transistors data sheet. Record the result in table 1.

5. Gradually reduce Vin until the transistor turns OFF. Measure Vin and record the result in table 1.
6. With Vin set at this value, the input is said to marginal. Measure and record the collector to emitter
voltage for this particular Vin in table 1.
7. Substitute the other transistor into Q1 and repeat the steps (replace Ibsat by Icbo on the data sheet).
Record the transistor behavior whether it is consistent or not in the space provided in table.
8. Replace the NPN transistor by a PNP transistor Q2. Repeat step 2. Observe what happens.
9. Try to saturate the PNP transistor. Fill up table 2.
10. On the last pages of your papers, document your activity through screen captures of your
simulations.

Leading Innovations, Transforming Lives, Building the Nation

 Republic of the Philippines
BATANGAS STATE UNIVERSITY
The National Engineering University
Alangilan Campus
Golden Country Homes, Alangilan Batangas City, Batangas, Philippines 4200
Tel Nos.: (+63 43) 425-0139 local 2121 / 2221
E-mail Address: coe.alangilan@g.batstate-u.edu.ph | Website Address: http://www.batstate-u.edu.ph

College of Engineering

EVALUATION SHEET 1:

Name: LIQUIDO, JOANA MARIE M.

Date: APRIL 16, 2025

I. TABLE 1: Data and Results

NPN Transistor ON Q1 NPN Transistor OFF (Cut-off) Q1

(Saturation)
Icsat, mA 45.1mA Ic, mA 0
Vce sat, V 0.0873V Vce, V 10V
Vin measured, V 5.25V Vin measured, V 0.7V
Vin calculated, V 5.25V Vin calculated, V 0.7V
Behavior Consistent Behavior Consistent

II. Computation: Solving for Vin calculated.

For NPN (2N3904)

GIVEN:
From the circuit,
Vcc = 10 V
Rb = 10 kΩ = 10000 Ω
Rc = 220 Ω

From transistor’s datasheet (2N3904),

Ic = 10 mAdc
IB = 1 mA
β = 100

SOLUTION:

NPN TRANSISTOR ON (SATURATED)

Calculating Vin:

V ¿ calc=
( β × Rc )
V cc × R b
+0.7 V

V ¿ calc= ( 10100V ×10000

×220 Ω )
Ω
+ 0.7 V

V ¿ calc=4.545454545V + 0.7 V

V ¿ calc=5.245454545 V
V ¿ calc=5.25 V

NPN TRANSISTOR OFF (CUT-OFF)

Calculating Vin:
V ¿ calc=V BE
V BE=V B −V E

Leading Innovations, Transforming Lives, Building the Nation

 Republic of the Philippines
BATANGAS STATE UNIVERSITY
The National Engineering University
Alangilan Campus
Golden Country Homes, Alangilan Batangas City, Batangas, Philippines 4200
Tel Nos.: (+63 43) 425-0139 local 2121 / 2221
E-mail Address: coe.alangilan@g.batstate-u.edu.ph | Website Address: http://www.batstate-u.edu.ph

College of Engineering

V BE=0.7 V −0V
V BE=0.7 V
V ¿ calc=0.7 V

CONCEPTUAL QUESTIONS:

1. What did you notice about the value of the output voltage and output current when the transistor is
saturated?

There is a wide array of choices within electrical components that can be included in an electrical
circuit. The most common ones, at least in the field of Electrical Engineering, are resistors, inductors, and
capacitors. These electrical elements perform specific duties when they are placed in a circuit. However,
since electronic devices were being dealt with in this laboratory report, other several components will be
used and emphasized; one of them is transistors.
A transistor, in electrical context, is a small semiconductor material that not just amplifies and
generates electrical signals but also functions as a switch or a gate to manage or regulate the flow of
current and voltage (Cameron Hashemi-Pour, 2024). Given the definition and also the focus of this report,
transistors can serve as a switch, which can be turned on or turned off. By this, the flow of either current
or voltage can be controlled. Moreover, it also works as an amplifier, which means that it can make a
small signal stronger.
A transistor has three terminals namely base, collector, and emitter. Moving forward, there might
be instances where the transistor is saturated. This saturation occurs when a system reaches its threshold
or the maximum value. This saturation happens when the voltage across the base-emitter exceeds the
threshold and the base current is adequate to drive the transistor fully on. Now, when a transistor is in its
saturated mode, it acts like an open circuit between the collector and the emitter terminals, resulting in
minimal voltage drop in VCE. A transistor in saturation mode also functions like a fully turned on switch,
allowing the current to flow freely. (Hommer Zhao, 2025).
Now, dealing with the values obtained through the conducted simulation. At saturation mode, the
output current ICsat reaches its maximum value of 45.1 mA while the output voltage across the collector-
emitter VCEsat declines to a very low level of 0.0873 V. These values satisfy the concept and theory
presented above because the value of the output current is relatively high while the output voltage is
relatively low. This happens because in the saturation process, the base current is applied, which makes
the transistor act like a fully on switch. This results in a large amount of current flowing through.
Meanwhile, the voltage drop happens to stay low because just like what was stated earlier, it acts like a
short circuit between the collector and the emitter. In this case, the transistor functions like a closed
switch, only allowing the current to flow through while the voltage remains very low. To sum it all up,
the saturation of the transistors result in high output current while the output voltage is low.

References:
Cameron Hashemi-Pour. September 2024. What is a transistor?. Retrieved from
https://www.techtarget.com/whatis/definition/transistor#:~:text=A%20transistor%20is%20a
%20miniature,switch%20or%20gate%20for%20them.
Zhao, H. (2025, January 23). Transistor Saturated: What it is and how to Identify one. Circuit Board
Fabrication and PCB Assembly Turnkey Services - WellPCB.
https://www.wellpcb.com/blog/components/transistor-saturated/

Leading Innovations, Transforming Lives, Building the Nation

 Republic of the Philippines
BATANGAS STATE UNIVERSITY
The National Engineering University
Alangilan Campus
Golden Country Homes, Alangilan Batangas City, Batangas, Philippines 4200
Tel Nos.: (+63 43) 425-0139 local 2121 / 2221
E-mail Address: coe.alangilan@g.batstate-u.edu.ph | Website Address: http://www.batstate-u.edu.ph

College of Engineering

2. What did you notice about the value of the output voltage and output current when the transistor is
cutoff?

Modern electronic circuits rely on different components to manage and control electrical signals.
Among these, transistors stand out for their ability to function as both switches and amplifiers. Their role
is critical in regulating how and when current flows through a circuit. This experiment specifically
examined the cut-off operation of an NPN transistor to observe how it behaves when current is
intentionally blocked.
A transistor, particularly the NPN type, can be compared to a special tiny valve that controls
current. It has three terminals: the base, collector, and emitter (Ashely, 2021). When a small voltage is
applied to the base, typically around 0.7 V, and enough base current is supplied, the transistor switches
ON, allowing current to flow freely from collector to emitter. However, if this voltage or current is
insufficient, the transistor remains OFF, preventing any current from flowing, just like an open switch
When in the cut-off region, the base-emitter junction of the transistor is not forward-biased. As a result,
no base current flows, and the transistor does not conduct between the collector and emitter. This causes
the collector current I c to drop to zero and the entire supply voltage to appear across the collector-emitter
terminals. In this mode, the transistor behaves like a disconnected wire or a switch that is left open
(Weinman, 2023).
Referring to the simulation results, the transistor remained OFF when the input voltage was at 0.7
V. During this state, the measured collector current was 0 mA, and the collector-emitter voltage reached
10 V, the same as the supply voltage. These outcomes are consistent with the theory of transistor cut-off.
Since no base current was applied, even though the threshold voltage was met, the transistor did not
switch ON. The output current stayed at zero, and no voltage drop was observed across the load resistor
To conclude, the transistor's behavior in the cut-off mode demonstrated its role as an open switch. No
current flowed through the collector-emitter path, and the output voltage remained at its highest value.
These results support theoretical expectations and emphasize how transistors can effectively control
circuit operations by turning current flow on or off.

References:

Ashely, E. (2021). What is NPN Transistor? Definition, types & applications. Www.rs-

Online.com. https://www.rs-online.com/designspark/what-is-npn-transistor-definition-types-

applications

Weinman, J. (2023). Transistors. Grinnell.edu.

https://weinman.cs.grinnell.edu/courses/CSC211/2023S/electricity/transistors.html

Leading Innovations, Transforming Lives, Building the Nation

 Republic of the Philippines
BATANGAS STATE UNIVERSITY
The National Engineering University
Alangilan Campus
Golden Country Homes, Alangilan Batangas City, Batangas, Philippines 4200
Tel Nos.: (+63 43) 425-0139 local 2121 / 2221
E-mail Address: coe.alangilan@g.batstate-u.edu.ph | Website Address: http://www.batstate-u.edu.ph

College of Engineering

EVALUATION SHEET 2:
Name: LIQUIDO, JOANA MARIE M.
Date: APRIL 16, 2025

I. TABLE 2: Data and Results

PNP Transistor ON (Saturation) Q1 PNP Transistor OFF (Cut-off) Q1

Icsat, mA 45.5mA Ic, mA 0mA
Vce sat, V 10V Vce, V 0V
Vin measured, V 4.75V Vin measured, V 9.30V
Vin calculated, V 4.755V Vin calculated, V 9.30V
Behavior Consistent Behavior Consistent

II. Computation: Solving for Vin calculated.

For PNP (2N3906)

GIVEN:
From the circuit,
Vcc = 10 V
Rb = 10 kΩ = 10000 Ω
Rc = 220 Ω

From transistor’s datasheet (2N3906),

Ic = 10 mAdc
Vce (sat) 0.25 V
β = 100

SOLUTION:

PNP TRANSISTOR ON (SATURATION)

Calculating Vin:

V ¿ calc=V cc −
( β × Rc )
V cc × Rb
+ 0.7 V

V ¿ calc=10 V − ( 10V100×10000
× 220 Ω )
Ω
+0.7 V

V ¿ calc=10 V −5.245454545 V
V ¿ calc=4.754545455V
V ¿ calc=4.7545V

PNP TRANSISTOR OFF (CUT-OFF)

Calculating Vin:
V ¿ calc=V cc −(I b sat × Rb )−0.7 V
V ¿ calc=10 V (0 mA ×10000 Ω)−0.7 V
V ¿ calc=9.3 V

CONCEPTUAL QUESTIONS:

Leading Innovations, Transforming Lives, Building the Nation

 Republic of the Philippines
BATANGAS STATE UNIVERSITY
The National Engineering University
Alangilan Campus
Golden Country Homes, Alangilan Batangas City, Batangas, Philippines 4200
Tel Nos.: (+63 43) 425-0139 local 2121 / 2221
E-mail Address: coe.alangilan@g.batstate-u.edu.ph | Website Address: http://www.batstate-u.edu.ph

College of Engineering

3. What will happen when the NPN transistor is replaced by a PNP transistor?

This experiment started by testing an NPN transistor in a basic switching circuit. The NPN
transistor’s behavior was analyzed in both ON and OFF states, and then replaced with a PNP transistor to
compare the outcomes. This focuses on analyzing how circuit functions change when replacing NPN
transistors with PNP transistors.
Based on the simulation and resulting data, it was observed that the PNP transistor did not work
well when simply placed in the same position as the NPN transistor. It can only work well when the
accurate voltage is applied to the base which allows current to flow from the collector to the emitter.
During saturation, the NPN transistor had a low voltage and high current, which meant it was turned on,
making the circuit work as intended. On the other hand, when the PNP transistor is replaced without
adjusting the circuit, the transistor remains in OFF condition. This happened because a PNP transistor
needed a specific condition to activate. This should be the base needs to be negative or lower than the
emitter. Since the circuit was initially developed for an NPN transistor, the PNP transistor did not turn on
because it received the least amount of correct signal. As a result, there is less electricity to pass through
the collector, and the output voltage remains at a high level. This clearly demonstrated the transistor was
in the cut-off region, which means it will not function as it should.
This observation and explanation support the study that the known concept of NPN and PNP
transistors cannot be replaceable without changing the other circuit component. According to Sedra and
Smith Microelectronics (2016), there are two types of transistors that operate in opposite ways. These are
the NPN transistors that require a positive base-emitter voltage and the PNP transistors need a negative
base-emitter voltage. By evaluating the circuit’s illustration, it was concluded that the two transistors
cannot perform at the same time. If transistor 1 is on, then transistor 2 is off, and vice versa. Therefore, if
one wants to utilize a PNP transistor in the place of an NPN transistor, the current and voltage polarity
will also be reversed. Additionally, the base-emitter voltage for an NPN transistors necessarily needs the
base to be positive that is relative to the emitter. While in a PNP transistor, the base must be negative
relative to the emitter to turn it on. The voltage polarity of the collector-base is also reversed. This means
simply replacing an NPN transistor with a PNP transistor without reversing other components of circuit
will cause it to remain off and the circuit to break down (Shaik A., 2015).
To sum up, without changing the circuit, replacing an NPN transistor with a PNP transistor
causes the transistor to remain OFF and prohibits the circuit from working effectively. The PNP
transistors require more different biasing conditions than the NPN transistor. The results matched the
outcome from the transistor theory which shows the need for correct setup when working with various
transistors.

References:

Sedra and Smith Microelectronics 7th edition Example 6.12. Electrical Engineering Stack Exchange.
https://electronics.stackexchange.com/questions/243154/sedra-and-smith-microelectronics-7th-edition-
example-6-12
Shaik, A. Transistor terminal voltages.
https://www.physics-and-radio-electronics.com/electronic-devices-and-circuits/transistors/
bipolarjunctiontransistor/transistorterminalvoltages.html

Leading Innovations, Transforming Lives, Building the Nation

 Republic of the Philippines
BATANGAS STATE UNIVERSITY
The National Engineering University
Alangilan Campus
Golden Country Homes, Alangilan Batangas City, Batangas, Philippines 4200
Tel Nos.: (+63 43) 425-0139 local 2121 / 2221
E-mail Address: coe.alangilan@g.batstate-u.edu.ph | Website Address: http://www.batstate-u.edu.ph

College of Engineering

SIMULATION:

NPN CIRCUIT SATURATION:

NPN CIRCUIT CUT OFF:

Leading Innovations, Transforming Lives, Building the Nation

 Republic of the Philippines
BATANGAS STATE UNIVERSITY
The National Engineering University
Alangilan Campus
Golden Country Homes, Alangilan Batangas City, Batangas, Philippines 4200
Tel Nos.: (+63 43) 425-0139 local 2121 / 2221
E-mail Address: coe.alangilan@g.batstate-u.edu.ph | Website Address: http://www.batstate-u.edu.ph

College of Engineering

PNP CIRCUIT SATURATION:

PNP CIRCUIT CUT OFF:

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