EXPERIMENT 1: Non-Inverting Amplifier
and High-Frequency Roll-Off
1. Theory s Biomedical Context
The non-inverting amplifier is a fundamental building block in biomedical instrumentation, offering a high input
impedance that prevents the loading of delicate bio-signal sensors (like electrodes). The theoretical closed-loop
voltage gain ACL is determined entirely by the external feedback network:
��
��� = 1 +
𝑅�
High-Frequency Roll-Off: While ideal op-amps have infinite bandwidth, practical op-amps have an internal
capacitance that causes the open-loop gain to drop at higher frequencies. The product of the amplifier's
closed-loop gain and its upper cutoff frequency (fc, where gain drops by 3dB) is constant. This is known as
the Gain-Bandwidth Product (GBWP). Understanding this is critical in biomedical engineering; for
example, amplifying a high-frequency Electromyogram (EMG) signal requires an op-amp with a sufficient
GBWP to avoid distorting the signal.
Pin Diagram of LM741:
F
i
g
u
r
e
1
:
P
i
n
d
i
a
g
ram of 741
Figure 2: Circuit Diagram of Non-Inverting Amplifier
2. Equipment Required
1.
2.
3.
4.
5.
6.
Oscilloscope C Function Generator
Dual Output DC Power Supply (±15�)
Digital Multimeter (DMM)
Op-Amps: LM741 (x1) and LM358 (x1)
Resistors: 1kΩ (x1), and an assortment of other standard values.
Breadboard and connecting wires.
3. Pre-Lab Computer Simulation
To be completed individually prior to hardware assembly.
1. Launch Proteus.
2. Construct a non-inverting amplifier using an LM741 model powered ±15� by DC supplies.
3. Set Ri = 1kΩ and Rf = 10kΩ.
4. Apply a 100mVpp sine wave at 1kHz to the non-inverting terminal.
5. Task: Run an AC Sweep (Bode Plot) from 10Hz to 1MHz.
6. Record: Locate the upper cutoff frequency (fc) on your Bode plot where the gain drops by 3dB from
its mid-band value. Print or sketch this plot for your final report.
4. Hardware Implementation s Guided Design
Task 1: Amplifier Design
1. Design Challenge: Using the provided 1kΩ resistor as Ri, design a non-inverting amplifier to achieve
a theoretical voltage gain ACL of approximately 11.
2. Calculate the required value for the feedback resistor Rf and select the closest standard
available resistor from your component kit.
○ Target Rf Calculation:
Ω
○ Actual Rf Selected:
Ω (Measure with DMM)
Task 2: Mid-Band Gain Verification
1. Assemble your designed circuit on the breadboard using the LM741 op-amp. Connect V+ to
+15V (Pin 7) and V- to -15V (Pin 4).
2. Set the function generator to a 1kHz sine wave, 100mVpp (mid-band frequency).
3. Connect channel 1 of the oscilloscope to the input and channel 2 to the output.
4. Measure the output peak-to-peak voltage. Record this in Table 1 and calculate the actual
experimental gain.
Task 3: High-Frequency Roll-Off Observation
1. Maintain the 100 mVpp input amplitude. Slowly increase the frequency on the function generator
while observing the output waveform on the oscilloscope.
2. Find the experimental cutoff frequency (fc): Continue increasing the frequency until the output peak-topeak voltage drops to 0.707 × ����(���−����) (This represents a -3dB drop in power).
3. Record this experimental cutoff frequency for the LM741 in Table 2.
4. Op-Amp Swap: Power down the circuit. Carefully replace the LM741 with the LM358 op-amp (Verify
pinouts, though they are typically compatible for single op-amp packages).
5. Repeat the frequency sweep to find the new experimental fc for the LM358 and record it in Table 2.
5. Data & Observations
Op-Amp
Table 1: Mid-Band Gain (f = 1kHz)
Calculated
Measured Vin Measured Vout Experimental
Gain
Theoretical Gain
% Error (Exp
vs Theo)
LM741
Op-Amp
Table 2: High-Frequency Roll-Off Comparison
Mid-Band Vout (1kHz) -3dB Target Voltage
Experimental Cutoff
(Vout×0.707)
Frequency (fc)
LM741
LM358
Simulation on Proteus
Fig 3. Simulation of non-inverting Amplifier Circuit on Proteus
Fig 4. Bod Plot of non-inverting Amplifier Circuit
6. Post-Lab Analysis & Understanding
Answer the following questions comprehensively in your lab report.
1. Error Analysis: Discuss any discrepancies between your theoretical gain, simulated gain, and actual
experimental gain in Table 1. What real-world factors (e.g., resistor tolerances, instrument loading)
contribute to this deviation?
Answer: We observed a small difference between the calculated gain and the measured gain. The
measured gain was slightly higher than the theoretical value. This deviation occurred because in our
calculations we assumed ideal components, while in the actual experiment real-world factors affected
the result. Resistor tolerances, instrument loading from the oscilloscope and signal generator, and small
variations in component values contributed to this difference.
2. Gain-Bandwidth Product: Based on your data in Table 2, calculate the GBWP for both the LM741
and the LM358 ���� = ���� × ��. How do they compare?
Answer: For the LM358, the midband gain and cutoff frequency were approximately similar to the
LM741. After converting the gain from dB to linear form and multiplying it by the cutoff frequency,
the Gain-Bandwidth Product was found to be approximately 1.12 MHz. This value is close to the
typical GBWP of the LM358, and it is similar to the LM741 under our experimental conditions.
3. Biomedical Application: An Electromyogram (EMG) signal typically contains frequency
components up to 500Hz. A researcher wants to amplify a raw 1mV EMG signal to 1V using a single
non-inverting amplifier stage. Based on your experimental GBWP findings, would the LM741 be
suitable for this specific task? Justify your answer mathematically.
Answer: To amplify a 1 mV EMG signal to 1 V, a gain of 1000 is required. The maximum EMG
frequency is 500 Hz, so the required Gain-Bandwidth Product is 0.5 MHz. Since the experimental
GBWP of the LM741 was approximately 1.12 MHz, which is greater than 0.5 MHz, the LM741 is
mathematically suitable for this task in terms of bandwidth.
7. Teamwork & Lab Ethics Sign-Off
To be filled by the group and verified by the Lab Instructor.
● [ ] We actively participated in the circuit design and troubleshooting process as a team.
● [ ] We verified power supply limits (±15�) before applying power to protect the ICs.
● [ ] We have returned all components, turned off equipment, and cleaned our workstation.
Group Members Signatures:
1.
2.
3.
4.