Interviewing Prep

Can you explain what a Bode plot is and what information it conveys about a system?

A Bode plot is a graphical representation of a system's frequency response, composed of a magnitude plot and a phase plot. The magnitude plot shows how the amplitude of the output signal varies with frequency, while the phase plot shows how the phase of the output signal shifts.

What is the difference between a buck converter vs LDO? What are the tradeoffs?

A buck converter and LDO both step down voltage, but they operate differently. A buck converter is a type of switch-mode power supply, It offers high efficiency over a wide range of load currents but can be more complex and produce noise due to its switching action. An LDO, on the other hand, regulates voltage by dissipating excess power as heat through a pass transistor operating in its linear region. This makes LDOs simpler and quieter, but they are less efficient with large voltage differences between input and output or at high load currents.

How does a buck converter work?

A buck converter reduces voltage using a combination of inductors, diodes, capacitors, and a switch (usually a transistor). When the switch is on, current flows through the inductor, storing energy in its magnetic field. When the switch turns off, the inductor discharges its stored energy into the load, which is smoothed by the capacitor.

What is Noise in Op-Amps

A. Noise in op-amps refers to the unwanted internal and external signals that can corrupt the desired output. 1. Internal Noise - Internal noise sources include thermal noise, flicker noise (also known as 1/f noise), and shot noise. 2. External Noise - Arises from external circuit components and environmental sources such as power supply variations, electromagnetic interference from other devices, and noise in resistors

What is aliasing?

Aliasing is a phenomenon that occurs when a signal is sampled at a rate that is insufficient to capture its frequency content accurately

When can an LDO be more efficient than a buck converter?

An LDO can be more efficient than a buck converter when the voltage difference between the input and output is small, which minimizes the power lost as heat.

How does an LDO work?

An LDO regulates output voltage by using a pass element controlled by a differential amplifier. The amplifier compares the output voltage with a reference voltage and adjusts the pass element to maintain the desired output level, even as the input voltage or load conditions change.

What is an opamp? How does it work?

An operational amplifier, or op-amp, is an integrated circuit that can amplify voltage. An op-amp amplifies the difference between its two input voltages. It has a very high gain, which means even a small difference between the input terminals can result in a large output voltage.

Why are buck converters more efficient than LDOs?

Buck converters are typically more efficient LDOs because they transfer energy with minimal heat dissipation, using switching elements that are either fully on or off, reducing voltage drops and power loss.

What components are most responsible for efficiency losses?

Components such as diodes, transistors, inductors, and capacitors can contribute to efficiency losses due to factors like conduction losses, switching losses, core losses in inductors, and equivalent series resistance (ESR) in capacitors.

What is efficiency in a power supply? How can it be measured? Why is it important?

Efficiency in a power supply is a measure of how well it converts input power to output power. It's calculated as the ratio of output power to input power

Tell me about your project on "Adjustable Power Supply"

I took the lead in designing and simulating an adjustable DC power supply, where I selected and integrated key components such as resistors, capacitors, and an LM317 LDO regulator onto a PCB. The challenge was to make the power supply adjustable. To address this, I introduced electrical switches to the design, which allowed for variability in the output voltage. Throughout the project, I was deeply involved in the simulation process using LTspice. This tool helped us to simulate different output voltages based on switch positions. I carefully compared the simulation data with our theoretical calculations to ensure the design's viability. Interestingly, we noticed some deviations between our theoretical predictions and the simulated outcomes. On investigating, we realized that practical issues like the non-ideal behavior of resistors and unexpected power contributions were influencing our results. These discrepancies were critical learning experiences for me, highlighting the importance of accounting for real-world conditions in our designs.

Tell me about your self?

I was born in Ethiopia but raised in California, where I developed an early interest in electronics by tinkering with household appliances, like radios, broken laptops, and phones. My first project was building a helicopter that used a brushed DC motor and a battery. That passion led me to pursue a degree in Electrical Engineering. During my studies I joined the SJSU Robotics that helped me gain experiences in areas such as PCB design, serial communication protocols(I²C, CAN bus, and UART), and working with multidisciplinary teams.

What is the difference between Ideal and non-Ideal op amps?

Ideal op-amps are theoretical constructs with perfect characteristics for ease of analysis. Non-ideal op-amps, the real-world devices, have limitations like finite gain, limited bandwidth, and input/output impedance, affecting their performance.

An LDO can often be modeled as a single passive component, what is this passive component?

In simplified electrical models, an LDO is often represented as a series resistor, which accounts for the linear regulation's power dissipation characteristics. The value of this resistor typically corresponds to the LDO's dropout voltage at a given load current.

Tell me about your project on "Encoding and Decoding Touch-Tone (DTMF) Signals"

In the AM Radio Receiver project, I was tasked with building a circuit that could demodulate AM signals. The heart of this circuit was the LC resonant circuit, chosen for its ability to pick out specific frequencies — like tuning into just the right station on a radio dial. I connected an antenna to this circuit to catch the radio waves right out of the air. My design work centered around tweaking this resonant circuit to zero in on the 500 kHz frequency range. I also added a diode to rectify the signal, which is a way of converting the captured radio waves into a form that we can listen to. Then, to actually hear the demodulated signal, I incorporated an audio amplifier into the mix. It was a hands-on project that not only reinforced theoretical knowledge but also provided practical experience in electronic design and signal processing.

Tell me about your project on "AM Radio Receiver"

In the AM Radio Receiver project, my objective was to construct a circuit capable of demodulating AM radio signals. To achieve this, I utilized an LC resonant circuit, which is effective in selecting signals at a specific frequency due to its resonant properties. The antenna, which I connected to the LC circuit, is responsible for intercepting radio frequency signals. My role involved designing the resonant circuit to match the desired reception frequency around 500Khz, integrating a diode for rectification, and incorporating an audio amplifier to make the demodulated signal audible.

Tell me about your project on "Pulse Width Modulated Digital to Analog Converter"

In the project, I delved into the Fourier expansion's foundational principle, which is a classic method for breaking down periodic signals into their basic sine and cosine waves. This approach is like dissecting a symphony to appreciate each instrument's contribution individually. I compared this method to the Fast Fourier Transform, or FFT, which stands out due to its computational speed. FFT is a stark contrast to traditional numerical methods, as it allows for the quick analysis of discrete-time data. A particularly interesting part of my project was applying FFT to an MP3 song. I analyzed the song's periodic nature, turning it into a detailed breakdown of its frequency components. This exercise underscored the practical applications of FFT in audio signal processing and demonstrated its effectiveness in a real-world scenario.

Tell me about the project you design on the "24V brushed DC"

In the project, I set out to design a PCB that would complement an Arduino board by enabling it to control higher power loads. The Arduino's limitation is that it can only supply up to 5 volts and 500 milliamps, which wasn't enough for our needs. To overcome this, I chose the DRV8801 motor driver because it can handle voltages from 8 to 36 volts and currents up to 2.8 amps, which is ideal for driving motors that require more juice. My design included not just the DRV8801 but also additional power sources and protective elements like current sensors and thermal shutdown mechanisms. These features are crucial for preventing damage from power surges or overheating. Another key part of my design was a logic level shifter, which made it possible for the Arduino's 5V signals to communicate with the higher-voltage DRV8801 system. With this setup, I was able to achieve precise control over a motor's speed and direction using PWM signals from the Arduino. This allowed for detailed manipulation of the motor's performance, which is a testament to the DRV8801's capability, particularly its full-bridge output, which is perfect for such applications.

Tell me about the project you design on the "Power Kill Switch"

In this project, my main responsibility was to design a PCB focused on managing power and ensuring safety for a rover. The design challenge was to create a system that could be controlled both on-site and remotely, so I included both a remote and a push switch for versatile power management. To protect the rover's electronics from the initial surge when power is applied, I used an inrush current thermistor. I also added a 12V relay as a safeguard. This relay automatically cuts the power if there's a malfunction or an excessive power surge, which is essential for preventing damage to the rover's systems. For voltage regulation, I chose a buck converter, which efficiently steps down voltage for different components, adapting to their various power needs. The comprehensive approach I took ensures that the rover's electronics are well-protected against power issues and that we can power down safely in the event of system failure. This not only helps in maintaining the rover's operational integrity but also in enhancing its overall reliability.

LDO efficiency analysis. How does output current and dropout voltage impact efficiency?

LDO efficiency is primarily the ratio of output power to input power, which decreases with higher dropout voltage and output current. Efficiency is better when the dropout voltage and output current are minimized because less power is lost as heat.

What happens to any lost power? Why is this bad? (LDO)

Lost power is usually dissipated as heat. This is undesirable because it represents wasted energy, which makes the power supply less economical. Excess heat can also damage components, shorten the life of the power supply, and may necessitate additional cooling systems, increasing the overall cost and complexity.

What is LDO?

Low-Dropout Regulator is a voltage regulator designed to maintain a stable output voltage with a minimal difference between the input and output voltages, which is known as the dropout voltage. The key components of an LDO include a voltage reference, an error amplifier, a pass element, usually a transistor, and input and output capacitors.

What is an LDO?

Low-Dropout Regulator, is a voltage regulator designed to maintain a stable output voltage with a minimal difference between the input and output voltages, which is known as the dropout voltage. The key components of an LDO include a voltage reference, an error amplifier, a pass element, usually a transistor, and input and output capacitors.

How does dropout voltage impact efficiency, ripple, etc? What about output current an input/output voltage?

Lower dropout voltages generally allow for higher efficiency and can also reduce the impact of input voltage variations on the output—critical for noise-sensitive applications. However, as the output current increases, or the difference between input and output voltage rises, the efficiency drops, and the thermal load increases, potentially affecting reliability and necessitating additional components for thermal management. Ripple is generally less of a concern with LDOs as they do not have the switching noise inherent in buck converters.

Explain the concept of noise gain in op-amp circuits?

Noise gain is the gain that an operational amplifier circuit applies to its own internal noise. It's different from the signal gain because it includes the effects of both the signal path and the feedback network on the op-amp's internal noise.

Noise Gain

Noise gain refers to the amplification factor applied to the noise signal within the op-amp's bandwidth.

What is a Noise in a circuit? How does Noise work in Power Electronics? What is Noise in an OPAMP?

Noise in a circuit is the collective term for any unwanted electrical fluctuations in addition to the desired electronic signal. In power electronics, noise is primarily due to switching transients, ripple voltages, and electromagnetic interference caused by high current changes in power devices

What is noise in operational amplifier circuits, and why is it important to analyze it?

Noise in op-amp circuits is usually quantified by measuring the noise voltage or current over a specified bandwidth. Measurement can be performed using a spectrum analyzer.

Why are non-inverting configurations often preferred?

Non-inverting configurations are often preferred because they provide a positive gain while maintaining the same phase between input and output.

Can you explain what PSRR means and why it is important in the context of LDO regulators?

PSRR stands for Power Supply Rejection Ratio. It is a measure of how well an LDO can reject variations in the input voltage and prevent them from affecting the output voltage.

What do you mean by "phase response" in a filter, and why is it important?

Phase response describes the change in phase of the output signal compared to the input as a function of frequency. It's critical in applications where the timing of signal components must be preserved, like in data communications.

What happens to any power losses in an LDO?

Power losses in an LDO are dissipated as heat through the pass element, which is why LDOs can run hot and may require heatsinks or other thermal management strategies, especially at higher power levels.

What is quiescent current?

Quiescent current (IQ) is the current drawn by a system in standby mode with light or no load. It is essential for applications that need to continue to run in sleep mode.

Voltage Gain

Ratio of the output voltage to the input voltage this is usually the closed-loop voltage gain.

What is the difference between the Fourier Transform and Fourier Series?

The Fourier Series is used to represent a periodic signal as a sum of sine and cosine functions, while the Fourier Transform is a more general form that converts a time-domain signal into a continuous spectrum of frequencies

What is the Fourier Transform?

The Fourier Transform is a mathematical operation that transforms a time-domain signal into its frequency-domain representation.

Open-Loop Gain

The amplification factor when the op-amp is used without any feedback. It is typically very high, with values in the range of 10^4 to 10^6 or more, which makes the op-amp impractical for use without feedback due to stability and control issues.

What do you understand by the terms "cut-off frequency," "roll-off rate," and "passband" in filter design?

The cut-off frequency is the frequency at which the filter starts to attenuate the signal. The roll-off rate is the speed at which the filter attenuates the signal past the cutoff frequency.

What is a Dropout Voltage

The dropout voltage is the minimum difference between the input voltage and the output voltage at which the regulator can still maintain the output voltage within its specified range.

What is meant by "filter order" and how does increasing the order affect a filter's performance?

The filter order indicates the number of reactive components (like capacitors or inductors) in the filter. A higher order means more steepness in the roll-off rate, leading to a closer approximation of the ideal filter response.

Can you list and describe the four basic types of filters used in electronic circuits?

The four basic types are low-pass, high-pass, band-pass, and band-reject filters. Low-pass filters allow frequencies below a certain cutoff to pass, while attenuating higher frequencies. High-pass filters do the opposite, blocking frequencies below the cutoff. Band-pass filters allow a certain frequency range to pass, and band-reject (or notch) filters block a specific frequency range.

Closed-Loop Gain

The gain when feedback is used to control the overall gain of the amplifier. The closed-loop gain is determined by the network of resistors (and sometimes capacitors) in the feedback path. This gain can be accurately controlled and is less sensitive to op-amp parameter

Tell me about your project on "7-Segment Display Controller"

The goal of my project was to create a module that counts both upwards and downwards, using a series of three T-flip-flops on the Nexys A7 FPGA board. To make this work, I started by mapping out a transition table for the T-flip-flops. This table helped me figure out the logic needed to drive the flips between states, using Karnaugh maps to simplify the logic functions. I chose T-flip-flops because they have a simpler control logic than D flip-flops, which made them better for this particular task.

What do the magnitude and phase plots in a Bode plot signify?

The magnitude plot illustrates the gain (in dB) as a function of frequency and The phase plot shows the phase difference between the input and output signals as a function of frequency.

What is the difference between the time domain and the frequency domain?

The time domain represents how a signal changes over time, while the frequency domain represents the same signal in terms of its constituent frequencies.

How does the bandwidth of an operational amplifier affect its noise characteristics?

The wider the bandwidth, the more noise is integrated into the signal path. This is why many precision applications may use narrow bandwidths to keep the noise levels low.

If I want to measure a sine wave, what should be the sample rate?

To measure a sine wave without aliasing, you should sample at a rate that is at least twice the highest frequency component of the sine wave, known as the Nyquist rate.

Tell me about the project you design on the "Encoding and Decoding Touch-Tone (Dual Tone Multiple Frequency) Signals"

When building this project, I started by designing a Dual Tone Multi-Frequency (DTMF) phone dialer. This device generates a sound by mixing two different frequencies, which are selected from a standard touch tone table. To simulate and test the signals, I used MATLAB's 'soundsc' function, which scales an audio signal to a range between -1 and 1, playing it back through the speakers at a sampling rate of 8192 Hz. After the dialer was functioning, I moved on to create a decoder. My design used eight bandpass filters to process the input signal. Each signal coming in would be filtered and then broken down into segments. I then applied a scoring function to these segments to accurately extract the frequency components. This step was crucial for the decoder to interpret which keys were pressed, based on the tones generated by the dialer.

How do the poles and zeros of a transfer function affect the shape of its Bode plot?

Zeros contribute to an increase in the system's gain, and poles contribute to a decrease. At each pole or zero, the slope of the magnitude plot changes by 20 dB/decade for each order of the pole or zero.

What are the three rules of an ideal opamp?

a. Infinite voltage gain. b. Infinite input impedance, meaning it draws no current. c. Zero output impedance, allowing it to supply as much current as needed.

What does stability refer to in a buck?

stability refers to the system's ability to maintain a steady output voltage despite variations in input voltage, output load, or temperature.

What is Bandwith in an opamp?

the range of frequencies over which the op-amp can operate effectively and provide amplification without significant attenuation of the signal.