Common Emitter Introduction
The common emitter amplifier configuration is widely used due to its ability to provide high voltage gain and moderate input and output impedance, making it versatile for various applications.
Voltage Gain Basics
Voltage gain in a common emitter amplifier is the ratio of the output voltage change to the input voltage change. It's heavily dependent on the transistor's characteristics and the external resistor values.
General Gain Formula
The general formula for voltage gain (Av) is Av = -Rc/re, where Rc is the collector resistor and re is the intrinsic emitter resistance, embodying the transconductance of the transistor.
Factors Affecting Gain
Factors such as temperature, biasing conditions, and the AC load line significantly impact the voltage gain. The negative sign in the gain formula represents an inverted output signal.
Intrinsic Emitter Resistance
The intrinsic emitter resistance (re) is not a physical resistor but a dynamic resistance representing the transistor's base-emitter diode's response to AC signals, typically calculated as 26mV/Ie for small signals.
Maximizing Voltage Gain
To maximize voltage gain, designers often increase Rc or decrease re. However, there's a trade-off with stability and linearity. Also, maximizing gain might require changes in the biasing circuit.
Real-world Gain Limitations
In practice, the calculated voltage gain is an idealization. Parasitic capacitances, non-linearities, and the Early effect can all reduce the actual gain experienced in a real amplifier circuit.
What does a common emitter provide?
Input impedance only
High voltage gain
Constant output impedance
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