Common Emitter Amplifier: DC and Small Signal Analysis

The common emitter amplifier operates in the active region to amplify signals effectively.

### Common Emitter Amplifier Analysis: DC and Small Signal Insights
* 🔌 In this week’s lecture, we discussed the **common emitter amplifier configuration**, where the emitter terminal serves as the **common point for both input and output signals**.
* 📊 We applied Kirchhoff's Voltage Law (KVL) to analyze the circuit, leading to equations that relate collector current (IC), base current (IB), and emitter current (IE), with the relationship expressed as **IE = (1 + β)IB**, where β is the transistor's **current gain**.
* 🔧 The base-emitter voltage (VBE) for silicon transistors is typically around **0.7V**, and using voltage divider resistors, we calculated the base voltage (VB) as **VCC * R2 / (R1 + R2)**.
* ⚡ The collector current (IC) can be derived from the base current (IB) using the equation **IC = β * IB**, which is crucial for determining the **output characteristics of the amplifier**.
* 📈 During the small signal analysis, we emphasized that the transistor must operate in the **forward active region**, where the **emitter-base junction is forward-biased** and the **collector-base junction is reverse-biased**, allowing the transistor to function as an **amplifier**.
* 🛠 We noted that the gain of the common emitter amplifier is represented by the product of **transconductance (gm)** and the **load resistance (RC)**, with the output voltage being **Vout = VDC - gm * RC * VBE**.
* 🔄 Additionally, the role of coupling capacitors was highlighted, as they **allow AC signals to pass while blocking DC components**, ensuring that the transistor remains in the **active region during signal amplification**.
* 📉 Finally, we concluded that **understanding these principles is essential** for analyzing and designing effective amplifier circuits, paving the way for **further exploration of current mirror circuits** in the upcoming sessions.

### Understanding Common Emitter Amplifiers and Current Mirrors
* 🔌 Capacitors behave as **open circuits for DC** and pass **AC**, which is crucial for coupling capacitors connected at the **base of transistors** in common emitter amplifiers.
* ⚡ The coupling capacitor ensures that the **DC part does not affect the operating point** of the transistor, allowing it to **function properly in the active region**.
* 🔄 If a small base voltage is directly applied, the transistor may not operate in the **active region**, leading to **incorrect biasing** of the collector-base junction.
* 📊 The design of the common emitter amplifier includes coupling capacitors (CB and CC) and a bypass capacitor (CE) to **establish AC ground** and **eliminate DC components** at the output.
* 🔍 The DC analysis involves calculating the **base-emitter voltage (0.7V for silicon transistors)** and establishing a proper **voltage divider for biasing**.
* 📈 The small signal analysis allows for the incorporation of AC signals, where the relationship between **emitter current and base current** is expressed as **IE = (1 + β)IB**.
* 🔗 The introduction of current mirrors aims to create a **constant current source** that is **independent of the emitter resistor (RE)**, enhancing circuit stability.
* 🔧 A diode-connected transistor is used to establish this **constant current**, which can be mirrored to other stages in the circuit, facilitating the design of **more complex amplifier configurations**.

Week 08 Session 01 - Live session

EEC Course Admin

The discussion focuses on applying Kirchhoff's Voltage Law (KVL) in the collector-emitter loop of a transistor circuit.

The base current (IB), collector current (IC), and emitter current (IE) are related through the transistor's beta value.

The base voltage (VB) for a silicon transistor is typically 0.

7V, and the voltage divider resistors are used to calculate the base voltage and resistance.

Common Emitter Configuration and Analysis

The common emitter configuration of a transistor has the emitter terminal common between the input and output.

The DC analysis involves calculating the collector-emitter voltage (VCE) using known values of VCC, collector current (IC), and resistor (RC).

The transistor's operation is influenced by the base-emitter voltage (VI), with the output voltage (VNOT) changing in response to the input, ultimately creating an inverter configuration.

Small Signal Analysis and Amplifier Design

Small signal analysis is used to build an amplifier, with a focus on the inverting and non-inverting configurations.

The transistor must operate in the forward active region to function as an amplifier, with the emitter-base junction forward biased and the collector-base junction reverse biased.

The labeling convention for DC and AC parameters is important, and the amplitude of the AC signal to be amplified should be less than the thermal voltage.

Small Signal Analysis and Common Emitter Amplifier

The analysis involves both DC and AC components, with the DC current determined by VCC and RC.

The small signal voltage (VBE) is amplified by the transconductance (GM), which is the ratio of the DC collector current (IC) to the thermal voltage (VT).

The common emitter amplifier provides an inverting gain, with a phase shift between the input and output signals.

Building a Common Emitter Amplifier Circuit

The common emitter amplifier circuit is designed using a single power supply, with resistors RC, RE, R1, and R2 for voltage division.

A bypass capacitor is added in parallel to establish AC ground and amplify the signal.

The coupling capacitor at the base ensures that DC is not affected while passing the AC signal, allowing the transistor to operate in the active region.

The emitter junction is forward biased, while the collector-base junction is reverse biased, resulting in the collector voltage being less than the base voltage.

Coupling capacitors are used to eliminate DC components at the output, while the bypass capacitor establishes AC ground.

The circuit analysis involves applying KVL and KCL, with the emitter current related to the base current through the transistor's beta value.

Understanding the Common Emitter Amplifier and Current Mirror

The common emitter amplifier's gain is determined by GM and RC, with coupling and bypass capacitors playing crucial roles.

The current mirror is introduced to establish a constant DC current, independent of the emitter resistance.

The constant current source is then set up using a diode, with the possibility of constructing it from a transistor by shorting its terminals.

Diode-Connected Transistor and Current Source Design

A transistor can be transformed into a two-terminal device by shorting its base and collector terminals, creating a diode-connected transistor.

This configuration allows for the establishment of a constant current source, independent of the emitter resistor.

The current is mirrored using two matched transistors, Q1 and Q2, with Q1 being diode-connected, to establish the desired current flow.

Current Mirror and Differential Amplifier Concepts

The DC current in the current mirror is independent of the resistor, allowing for current multiplication through cascading stages.

The session will transition to discussing differential amplifiers, covering common mode, differential mode, and common mode rejection ratio (CMRR).

Reference materials and PDF questions for upcoming quizzes will be provided, with a focus on diode circuits and additional topics.

The first recommended book is "Microelectronic Circuits" by Sedra and Smith.

Other notable books include "Electronic Device and Circuit Theory" by Boylestad and Nashelsky, and "Electronic Principles" by Malvino.

The course admin also offers to discuss remaining topics and provide assistance with quizzes on Monday.