Understanding the Common Emitter Amplifier: The Complete Guide
Analog electronics relies on fundamental configurations, and the Common Emitter Amplifier is undoubtedly the most essential one to master. Whether you are a student or a hobbyist, understanding this circuit is the key to designing high-performance audio systems and signal processing circuits.
What you'll learn
What is a Common Emitter Amplifier?
The most common amplifier configuration for an NPN transistor is the Common Emitter amplifier circuit. Why this name? Simply because the emitter of the transistor is the common point (usually connected to ground or a reference) for both the input signal and the output signal.
This configuration is famous for offering high voltage gain and current gain, making it a preferred choice for small signal amplification.
The Transistor Amplifier and Biasing
Transistor amplifiers amplify an alternating (AC) input signal that oscillates between a maximum positive value and an opposite negative value. It is therefore necessary to "pre-set" the configuration of a common emitter amplifier circuit so that a Bipolar Junction Transistor (BJT) can operate between these two maximum values. This pre-setting is achieved through a process called biasing.
Biasing is very important in amplifier design because it establishes the correct operating conditions for the transistor amplifier, making it ready to receive signals and thereby reducing any distortion of the output signal.
The Role of the Load Line
Furthermore, the use of a static load line (or DC load line) plotted on the output characteristics curves of an amplifier allows us to visualize all possible operating points of the transistor, from the fully "ON" state (saturation) to the fully "OFF" state (cutoff).
This allows us to determine the quiescent operating point, called the Q-point.. Ideally, this point should be located in the middle of the load line to allow for maximum signal excursion without clipping.
The goal of any small signal amplifier is to amplify the entire input signal with the minimum possible distortion on the output signal; in other words, the output signal must be an exact reproduction of the input signal, but larger (amplified).
Circuit Analysis: Components and Formulas
To stabilize the circuit and ensure proper operation, we typically use voltage divider biasing . Here are the key elements:
1. The Voltage Divider (R1 and R2 )
These two resistors set the voltage at the base of the transistor (VB ). They ensure that the transistor remains conductive (turned on).
VB =VCC×(R2 / (R1 + R2))
2. The Emitter (RE ) (R4) and Collector (RC ) (R3) Resistors
The resistance RC controls the gain and the output voltage. The resistance RE is crucial for the thermal stability of the circuit, preventing the Q-point from drifting if the transistor heats up.
To calculate the emitter current (IE), we use the voltage at the emitter (VE = VB - 0.7V for a silicon transistor):
IE = VE/RE
3. Coupling and Bypass Capacitors
Cin (C2) and Cout (connected to Vout) (Coupling Capacitors): These capacitors block direct current (DC) but allow the alternating signal (AC) to pass. They isolate the amplifier from previous and subsequent stages.
CE (Bypass Capacitor): Placed in parallel with RE, it allows the AC signal to bypass the emitter resistor. This significantly increases the voltage gainfor AC signals while maintaining the DC stability provided by RE.
Gain and Phase Inversion
A unique characteristic of the Common Emitter amplifier is phase inversion. When the input voltage increases, the output voltage decreases, and vice versa. There is a 180-degree (180° or π radians) phase shift between the input and the output.
Voltage Gain Formula (AV)
Voltage gain is the ratio between the output voltage (Vout) and the input voltage (Vin).
If the bypass capacitor CE is present, the gain is approximately the ratio between the collector resistance and the internal emitter resistance (r'e):
AV= - (RC/r'e)
(The minus sign indicates the phase inversion).
Practical Exercises
To validate your knowledge of the Common Emitter amplifier, try solving these two exercises.
Exercise 1: DC Analysis
Data:
- VCC = 12V
- R1 = 10 kΩ, R2 = 2.2 kΩ
- RE = 1 kΩ
- VBE = 0.7V (Silicon Transistor)
Question:
Calculate the base voltage (VB) and the emitter current (IE).
Answer:
Calculate VB (Voltage Divider):
VB = 12V × (2.2kΩ / (10kΩ + 2.2kΩ)) = 12V × 0.18 = 2.16V
Calculate VE:
VE = VB − 0.7V = 2.16V − 0.7V = 1.46V
Calculate IE:
IE = VE / RE = 1.46V / 1000Ω = 1.46 mA
Exercise 2: AC Gain Calculation
Data:
- Using the values from the previous exercise, IE ≈ 1.46 mA.
- Collector resistance RC = 3.3 kΩ.
- Internal dynamic emitter resistance estimated by r′e = 25mV / IE.
Question:
Calculate the voltage gain (Av) of the circuit (with the bypass capacitor present).
Answer:
Calculate r′e:
r′e = 25mV / 1.46mA ≈ 17.1 Ω
Calculate Gain Av:
Av = −RC / r′e = −3300Ω / 17.1Ω ≈ −192.9
The circuit therefore amplifies the input signal approximately 193 times, with a phase inversion.
Summary
The Common Emitter Amplifier is a pillar of analog electronics. Here is what you need to remember for your projects:
- Configuration: The emitter is common to both input and output.
- Biasing: Essential for fixing the Q-Point and avoiding distortion (clipping).
- Gain: It offers high voltage and current gain.
- Phase Shift: The output signal is inverted by 180° relative to the input.
- Key Components: The voltage divider for biasing and the emitter bypass capacitor to maximize gain.
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