# Transfer Functions in FE Electrical Exam

The topic of transfer functions in the FE Electrical exam offers a fundamental tool and mathematical framework to analyze and understand the behaviour of dynamic systems, allowing electrical engineers to unlock their full potential.

Whether designing filters, modeling control systems, or dealing with signal processing, if you prepare for the transfer functions in the FE electrical exam, you can unlock the mysteries of complex yet interesting electrical phenomena.

The transfer function models offer a clear lens to examine these electrical phenomena, offering a unique perspective on the interaction between the inputs and outputs of electrical systems.

This blog post will help you discover all the ins and outs of transfer functions in the FE electrical exam preparation. We will discuss how to understand and utilize the capabilities of transfer functions to shape and control the complex harmony between electrical signals.

So, get on an exciting journey through the depths of transfer functions as we explore their use cases and discuss the mathematical aspects of transfer function models.

## Basics of Transfer Functions

Transfer functions are mathematical representations that describe the relationship between the input and output of a linear time-invariant (LTI) system. These functions are crucial in analyzing and designing electronic circuits, control systems, and signal processing systems.

A transfer function is the ratio of the Laplace transform of the output of an LTI system to the Laplace transform of its input, assuming all initial conditions are zero.

Let’s consider a continuous-time LTI system with an input signal denoted as x(t) and an output signal denoted as y(t). The Laplace transforms of the input and output signals are denoted as X(s) and Y(s), respectively, where “s” is the complex frequency variable.

The transfer function, denoted as H(s), is defined as:

Where H(s) represents the system’s transfer function.

The transfer function characterizes the system’s behavior and provides insights into how the system responds to different inputs. We can determine the system’s frequency response, stability, and transient response by analyzing the transfer function.

Let’s consider an example to understand the relationship between the input, output, and transfer functions. Suppose we have an electrical circuit with an input voltage signal v_in (t) and an output voltage signal v_out (t). The transfer function of the circuit is denoted as H(s).

According to the definition of the transfer function, we can write:

where V_in (s) and V_out (s) are the Laplace transforms of v_in (t) and v_out (t) respectively.

If we know the transfer function H(s) and the Laplace transform of the input signal V_in (s), we can determine the Laplace transform of the output signal V_out (s) by using the relationship above.

Once we have the Laplace transform of the output signal, we can then apply the inverse Laplace transform to obtain the time-domain output signal v_out (t). This enables us to analyze and understand the system’s response to input signals.

Electrical engineers can design and optimize strategies for desired performance specifications by studying the transfer function and its properties. They can adjust the transfer function to achieve specific frequency response characteristics, such as amplification, filtering, or control behavior, to suit the application’s requirements.

To discover more about linear systems and how to apply Laplace transforms on linear systems for different purposes, read our detailed guide on Linear Systems in the FE Electrical exam

## Transfer Functions in Circuit Analysis of LTI Systems

LTI (linear time-invariant) system is a system in which the output is directly proportional to the input and does not depend on the specific time the input is applied. The system’s behavior remains constant over time and is characterized by its transfer function.

Consider a simple electrical circuit consisting of a resistor, an inductor, and a capacitor connected in series. This circuit can be modeled as an LTI system.

When an input voltage signal is applied to the circuit, the LTI system responds by producing an output voltage signal. The circuit’s transfer function can describe the relationship between the input and output voltage signals. In this case, the transfer function is determined by the electrical properties of the components and their arrangement within the circuit.

If the circuit is linear and time-invariant, the output voltage will be directly proportional to the input voltage and will not depend on when the input voltage is applied. If the same input signal is applied at different times, the resulting output signal will be identical, scaled by a constant factor determined by the transfer function.

The linearity property implies that if two input signals are applied simultaneously, the output signal will be the sum of the individual responses to each input signal. The time invariance property implies that if a delayed or advanced version of the input signal is applied, the corresponding output signal will also be delayed or advanced by the same amount, maintaining the same shape.

### Analyzing LTI Circuits Using Transfer Functions

The following steps are involved in analyzing LTI circuits using Transfer functions:

Identify the circuit – Identify the electrical circuit you want to analyze. This circuit can consist of resistors, capacitors, inductors, and other passive or active components.

Formulate the differential equation – Write the differential equation describing the circuit’s behavior. This equation is typically derived using Kirchhoff’s laws and the component characteristics. The equation relates the input and output voltages or currents of the circuit.

Learn more about Kirchhoff’s Laws in our detailed guide on Circuit Analysis in the FE Electrical Exam.

Apply Laplace transform – Take the Laplace transform of the differential equation. This changes the time-domain differential equation into the frequency-domain algebraic equation. The Laplace transform converts differentiation and integration operations into algebraic operations.

Solve for the transfer function – Manipulate the Laplace-transformed equation to solve for the transfer function. The transfer function is the ratio of the output Laplace transform to the input Laplace transform, assuming zero initial conditions.

Simplify and interpret the transfer function – Simplify the transfer function by factoring out common terms and expressing them in a standard form. This form often reveals essential characteristics of the circuit, such as poles, zeros, and frequency response properties.

Analyze circuit behavior using transfer function properties – Once you have the transfer function, you can analyze the circuit’s behavior. Some critical properties include:

• Frequency response – The transfer function indicates how the circuit responds to different frequencies. By substituting complex frequencies into the transfer function, you can determine the amplitude and phase shift of the output signal at different frequencies.
• Stability – The poles of the transfer function determine the stability of the circuit. If all poles have negative parts, the circuit is stable. If any pole has a positive real part, the circuit may be unstable.
• System response – By applying specific input signals (e.g., step, impulse, or sinusoidal), you can determine the transient and steady-state responses of the circuit. This information helps us understand how the circuit behaves under different conditions.

But how can you derive transfer functions from complex circuit diagrams? Below is the step-by-step process with a sample problem of how to do this.

To derive the transfer function from a circuit diagram, you have to start with

Labeling the circuit components – Assign variables or symbols to represent the values of resistors, capacitors, inductors, and other elements in the circuit.

Applying Kirchhoff’s laws – Apply Kirchhoff’s voltage and current laws to develop equations that describe the relationships between voltages and currents in the circuit.

Transform the equations – Transform the differential equations into the frequency domain using the Laplace transform. Convert differentiation and integration into simple algebraic functions involving the Laplace variable ‘s.’

Solve for the transfer function – Manipulate the Laplace-transformed equations to isolate the output variable and express it as a ratio to the input variable. This ratio represents the transfer function of the circuit.

Simplify and interpret the transfer function – Simplify the transfer function by factoring out common terms and expressing it in a standard form. This form allows you to interpret the circuit’s characteristics and analyze its behavior.

Consider a series RL circuit consisting of a resistor and an inductor connected in series. An input voltage signal, V_in, is applied to the circuit, and the output voltage across the components, V_out, is given.

Your task is to derive the transfer function of this series RL circuit and analyze its behavior. The transfer function should represent the relationship between the input voltage, V_in, and the output voltage, V_out, of the circuit.

Once you have obtained the transfer function, simplify it and interpret its properties. Consider the circuit’s frequency response, stability, and transient response characteristics based on the transfer function.

Below is the practical implementation of the aforementioned step-by-step process to derive the transfer function of this circuit and analyze its behavior.

Label the circuit – Assign variables to the resistor (R) and the inductor (L).

Apply Kirchhoff’s laws – Apply Kirchhoff’s voltage law to write the equation for the circuit:

Where V_R is the voltage across the resistor and V_L is the voltage across the inductor.

Transform the equation – Apply the Laplace transform to the equation. Assuming zero initial conditions, we have:

Where I(s) is the Laplace transform of the current through the inductor.

Solve for the transfer function – Rearrange the equation to isolate the output voltage v_out (s) in terms of the input voltage v_in (s):

Hence, the transfer function is:

Simplify and interprets the transfer function: The transfer function is in a simple form, indicating a first-order system with a single pole at s=-R/L. This indicates that the circuit has a frequency response that attenuates higher frequencies and exhibits a phase shift.

To dig deeper into the mathematics of transfer functions and Laplace transforms, learn more about how Laplace transformation helps derive transfer functions from a circuit.

## Transfer Functions in Control Systems

A control system is a system that manages, directs, or regulates the behavior of other systems or processes. It consists of various components working together to maintain desired outputs by manipulating inputs and considering feedback.

Below are the critical components of a control system:

### Plant/Process

The plant or process refers to the system or process being controlled. It can be a physical system, such as a mechanical system, electrical circuit, or chemical process.

#### Use-Case of Transfer Function in Plant/Process

• By analyzing the plant’s transfer function, electrical engineers can understand the relationship between the input and output variables and capture the system’s characteristics.
• The transfer function incorporates the plant’s physical properties, parameters, and dynamics, enabling accurate modeling and analysis.

### Actuator

The actuator is responsible for converting control signals from the controller into physical actions or inputs applied to the plant. It may involve devices such as motors, valves, or transducers.

#### Use-Case of Transfer Function in Actuator

• Transfer functions representing the actuator dynamics can be included in the system model.
• It allows electrical engineers to account for the actuator’s response time, saturation limits, and nonlinearities, ensuring accurate analysis and controller design.

### Sensor

Sensors measure or sense the outputs or relevant variables of the plant. They provide feedback information to the controller, enabling it to monitor the system’s behavior and make necessary adjustments.

#### Use-Case of Transfer Function in Sensor

• By incorporating the sensor’s transfer function into the overall system model, engineers can account for the sensor’s influence on the system’s behavior and assess its impact on stability and controller design.

### Controller

The controller is the central component that determines the control actions based on the desired output and the feedback received from the sensors. It computes control signals or commands that are sent to the actuator.

#### Use-Case of Transfer Function in Controller

• Transfer functions capture the dynamics of the controller, including its algorithms, gains, and filters.
• The controller’s transfer function is designed to achieve desired system performance, considering stability, response time, robustness, and other control objectives.

### Feedback Path

In a closed-loop system (as illustrated above), the feedback path connects the plant’s output to the controller’s input, providing information about the system’s actual performance. This feedback loop enables the control system to adjust the control signals continuously based on the discrepancy between the desired and actual outputs.

Simply, Transfer Functions help in system modeling, stability analysis, and controller design, enabling engineers to understand and improve the performance of control systems in various applications.

The table below outlines the implications of transfer functions to achieve optimization in these three crucial areas of control systems.

## Transfer Functions in Signal Processing

Transfer functions are widely used to analyze and design filters and other signal-processing systems. A transfer function represents the input-output relationship of a system in the frequency domain.

It allows us to understand how a system affects different frequencies in the input signal. The use case below showcases how transfer functions are applied in filter analysis and design.

For instance, you must design a low-pass Butterworth filter with a cutoff frequency of 2 kHz for a sampling rate of 10 kHz.

*It is recommended to choose an appropriate transfer function form that satisfies the desired specifications. Common forms include Butterworth, Chebyshev, and elliptic (Cauer) filters.

Let’s see how to evaluate its frequency response and verify that it meets the desired specifications.

Specify the desired specifications

Cutoff frequency: 2 kHz

Sampling rate: 10 kHz

Filter type: Low-pass

Frequency Transformation

Calculate the normalized cutoff frequency:

Transfer Function Calculation

Choose the Butterworth filter transfer function:

Where n is the filter order.

Determine the filter order based on the desired specifications. In this case, let’s choose n=4 for a sharper roll-off.

The transfer function for the Butterworth filter of order 4 is:

Frequency Response Analysis

*The transfer function of a filter provides its frequency response, which describes how the filter affects different frequencies in the input signal.

Substitute s=jω in the transfer function to obtain the frequency response in terms of ω.

Evaluate the magnitude and phase response for different frequencies to analyze the filter’s behavior.

*Gain and Phase Response – The transfer function’s magnitude response represents each frequency component’s gain or attenuation. At the same time, the phase response represents the phase shift introduced by the filter.

Plotting and Verification

Plot the magnitude and phase response of the transfer function using the frequency response data.

*Bode plots are created by plotting the magnitude and phase response of the transfer function on logarithmic scales, visually illustrating the frequency response characteristics of a filter.

Implement the designed filter using digital signal processing techniques, such as finite impulse response (FIR) or infinite impulse response (IIR) filter structures.

Verify that the filter meets the desired specifications, such as the stopband’s cutoff frequency and attenuation level.

*Desired Specifications – Specifying the desired filter characteristics, such as the cutoff frequency, passband ripple, stopband attenuation, and filter type (e.g., low-pass, high-pass, band-pass), is mandatory. Moreover, you must apply frequency transformation techniques, such as the bilinear transformation or frequency scaling, to map the desired specifications from the analog to the digital domain.

## Study Tips for Transfer Functions in the FE Electrical Exam

The following helpful study tips and hacks will help you tackle complex problems related to Transfer Functions in the FE Electrical Exam and excel in your electrical engineering career.

### Understand the Foundation

Develop a deep understanding of Laplace transforms, which form the basis of Transfer Functions. Study the properties of Laplace transforms, inverse transforms, and common transform pairs.

Familiarize yourself with concepts like poles, zeros, and frequency response. Understand how to analyze and interpret the behavior of systems based on their pole-zero configurations.

It is recommended to solve a variety of Transfer Function problems that reflect real-world electrical engineering scenarios. Work on sample problems that involve designing filters, analyzing control systems, or evaluating signal processing systems.

Use resources like the FE Electrical exam practice questions from Study for FE to gain exposure to different Transfer Function models and their related problems.

### Visualize with MATLAB or Simulations

Utilizing MATLAB or simulation software to visualize the frequency response of systems represented by Transfer Functions is a plus to gaining more profound practical knowledge. Plot Bode plots to observe the magnitude and phase response at different frequencies.

Experiment with varying the Transfer Function parameters and observe the corresponding changes in the frequency response. Gain insights into the impact of poles, zeros, and filter characteristics.

### Collaborate and Discuss

Engage in study groups or online forums to discuss Transfer Functions with peers. Participate in discussions to exchange ideas, clarify doubts, and deepen your understanding of challenging concepts. Teach and explain Transfer Functions to others.

By articulating the concepts in your own words, you reinforce your understanding and identify gaps in your knowledge.

### Utilize Authentic FE Exam Preparation Resources

Take advantage of the specialized and trusted FE electrical exam preparation course offered by Study for FE. Access comprehensive study materials, including detailed lectures and practice problems, specifically designed to help you master Transfer Functions.

### Conclusion

Now you clearly understand all the nitty-gritty of transfer functions in the FE electrical exam preparation. The significance of this crucial topic extends far beyond the exam and academics. Mastering the art of transfer functions not only enhances your understanding of signals and systems but also unlocks new career opportunities.

Proficiency in transfer functions in the FE electrical exam equips you with a powerful analytical tool that is highly sought after in industries ranging from telecommunications and power systems to control systems and signal processing.

To embark on your journey to prepare for the FE Electrical exam, consult – Study for FE – your most trusted and go-to platform for all FE exam-related queries and concerns. We provide all the knowledge and resources you need to crack the exam easily.

Explore what Study for FE has to offer. Equip yourself with the tools, knowledge, and confidence needed to conquer the FE Electrical exam and unlock the vast potential that awaits you in electrical engineering. Your future starts now! Licensed Professional Engineer in Texas (PE), Florida (PE) and Ontario (P. Eng) with consulting experience in design, commissioning and plant engineering for clients in Energy, Mining and Infrastructure.