TLDR;
This lecture introduces the course on analog circuit design, highlighting three key areas: circuit design, error budgeting, and various techniques used in analog circuits. It emphasizes the blend of art and science in analog design, requiring analytical thinking and experience. The lecture also covers the importance of understanding and computing errors in circuits, as well as knowing various techniques for designing analog circuits. The lecture further discusses the limitations of transistor amplifiers for DC applications due to temperature and supply voltage variations, leading to the development of operational amplifiers (op-amps). The lecture concludes with a historical overview of the op-amp's origins during World War II and its significance in advancing electronics.
- Course focuses on analog circuit design, error budgeting, and design techniques.
- Analog circuit design requires analytical thinking and experience.
- Transistor amplifiers have limitations in DC applications due to temperature and supply voltage variations.
- Operational amplifiers (op-amps) were developed to overcome these limitations and played a crucial role in advancing electronics during World War II.
Course Introduction and Objectives [0:15]
The course will cover three main aspects of analog circuit design: designing analog circuits, error budgeting, and various techniques involved in analog circuits. Analog circuit design involves combining transistors, op-amps, resistors, capacitors, diodes, and other active and passive components to create a functional circuit. This process is both an art and a science, requiring analytical thinking and practical experience. Error budgeting is essential to ensure the desired accuracy of a circuit by understanding and computing various errors and their cumulative effect, considering worst-case scenarios. Knowing various techniques is also crucial, such as advanced methods for capacitance measurement to minimize stray capacitance.
Background Knowledge Required [6:52]
To understand the course, basic knowledge of transistor and operational amplifier (op-amp) operation, as well as the use of resistors, capacitors, and Ohm’s law, is required. The lecture will begin with a review of transistor amplifiers to provide continuity and context for the subsequent topics.
Transistor Amplifier Circuit Analysis [7:54]
The lecture analyzes a typical NPN transistor amplifier circuit, which includes a power supply, biasing resistors (R1 and R2), emitter resistor (RE), collector resistor (RL), and coupling capacitors (C and CL). The goal is to amplify an input AC signal and obtain an amplified output. The transistor is forward-biased at the base-emitter junction and reverse-biased at the base-collector junction. The voltage at the base is determined by the voltage divider formed by R1 and R2. The base-emitter voltage difference is approximately 0.6V for appreciable conduction. The collector current is nearly equal to the emitter current, allowing the voltage across RL to be calculated.
Biasing and Gain Considerations [15:44]
For optimal amplification, the DC voltage at the collector should be set to the midpoint of the power supply voltage to allow for equal swing. The gain of the amplifier is determined by the ratio of RL to RE. For example, a gain of 10 can be achieved by setting RL to 10kΩ and RE to 1kΩ. The values of R1 and R2 are chosen to set the base voltage to the desired level, considering the base current and ensuring it is negligible compared to the current through R1 and R2.
AC Signal Amplification and Inversion [24:29]
An AC voltage is applied to the base through a capacitor, which blocks the DC voltage. The AC signal causes variations in the base voltage, which in turn cause variations in the emitter current and collector current. The amplified AC voltage appears at the collector, but it is inverted (180 degrees out of phase) compared to the input signal. The gain of the circuit is determined by the ratio of RL to RE.
Limitations of Transistor Amplifiers for DC Applications [27:46]
Transistor amplifiers face challenges when used for DC amplification. The output voltage is sensitive to changes in the supply voltage and temperature. The base-emitter voltage changes with temperature at a rate of approximately -2.2 mV per degree Celsius, causing variations in the collector current and output voltage. These variations make the output voltage unreliable for DC amplification, leading to the need for compensation techniques.
The Birth of Operational Amplifiers (Op-Amps) [33:29]
The limitations of transistor amplifiers for DC applications led to the development of operational amplifiers (op-amps). The first significant development was by H. Swartzal in 1941, who patented a circuit using three directly coupled transistor amplifiers with feedback. This circuit, later adapted with vacuum tubes, formed the basis for the summing amplifier.
The Role of Op-Amps in World War II [40:55]
During World War II, the British government sought assistance from the USA to develop a system that could automatically link radar to anti-aircraft guns. MIT was tasked with developing a gun director, which required an analog computing machine. The summing amplifier concept was promising, but early attempts were too slow. Columbia University was then approached to develop an improved operational amplifier.
Development of the First Operational Amplifier [45:30]
A 21-year-old engineer at Columbia University developed the first operational amplifier in just 30 days. This op-amp could perform addition, subtraction, multiplication, division, differentiation, and integration. MIT used 300 of these op-amps, along with assistance from Bell Labs, to create a successful gun director. The gun director significantly improved the hit rate of anti-aircraft guns, leading to a 90% success rate and effectively deterring German aircraft.
Characteristics and Impact of the Operational Amplifier [49:30]
The operational amplifier developed during World War II could amplify both AC and DC voltages. The output voltage was independent of supply voltage and temperature variations, addressing the key limitations of earlier transistor amplifiers. This breakthrough enabled the development of analog computers and significantly advanced electronics, paving the way for digital and power electronics. The lecture concludes by emphasizing the importance of understanding the principles behind op-amp design and their role in solving the drift problem that plagued earlier electronic circuits.
