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MOSFET Common Source Amplifier - Small Signal Analysis ( Voltage Divider Bias )
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Electrical engineering students and hobbyists looking to understand MOSFET amplifier circuit design and small-signal modeling.
TL;DR
This video explains small-signal analysis for a MOSFET common-source amplifier with voltage divider biasing. It details the step-by-step process of deriving voltage gain, input impedance, and output impedance, including the effects of channel length modulation and source resistance.
Key Takeaways
- The common source configuration is the most popular MOSFET amplifier design because it effectively utilizes the source terminal as a common reference point for both input and output signals.
- Small-signal analysis requires separating DC and AC components, where DC sources are treated as zero and capacitors are modeled as short circuits at the operating frequency.
- In a voltage divider biased common source amplifier, the input impedance is primarily determined by the parallel combination of the bias resistors, R1 and R2.
- The output impedance of a common source amplifier is equal to the drain resistor, Rd, assuming an ideal MOSFET with infinite output resistance.
- Voltage gain is calculated as the product of the transconductance and the drain resistance, with a negative sign indicating a 180-degree phase shift between input and output.
- Increasing the drain resistor can boost voltage gain, but this is limited by the need to keep the MOSFET in the saturation region by maintaining sufficient drain-to-source voltage.
- Accounting for channel length modulation introduces a finite output resistance, which modifies the total output impedance and reduces the overall voltage gain of the amplifier.
- While a bypass capacitor on the source resistor improves AC gain by eliminating negative feedback, omitting it can enhance amplifier stability and frequency response at the cost of reduced gain.
In This Video
- 00:00Introduction to Common Source Amplifiers
This video introduces the common source MOSFET amplifier configuration and outlines the goals of performing small-signal analysis to determine key performance parameters.
- 01:54Small-Signal Analysis Methodology
The process for AC analysis involves treating DC sources as zero and replacing capacitors with short circuits to simplify the MOSFET circuit model.
- 03:56Calculating Impedance and Voltage Gain
We derive the input impedance, output impedance, and voltage gain for the common source amplifier using the small-signal equivalent circuit model.
- 07:42Accounting for Channel Length Modulation
The analysis is refined by incorporating the MOSFET's finite output resistance, which modifies the expressions for both output impedance and voltage gain.
- 08:53Impact of Unbypassed Source Resistors
The video explores how removing the bypass capacitor introduces negative feedback, which improves stability but reduces the overall voltage gain of the amplifier.
Questions & Answers
What is a common source MOSFET amplifier?
It is a popular MOSFET amplifier configuration where the input signal is applied between the gate and source, and the output is measured between the drain and source. The source terminal is common to both the input and output sides for AC signals.
How do you perform small-signal analysis on a MOSFET amplifier?
To perform small-signal analysis, set all DC voltage sources to zero, replace all coupling and bypass capacitors with short circuits, and replace the MOSFET with its small-signal model.
What is the input impedance of a common source amplifier with voltage divider bias?
The input impedance of this configuration is equal to the parallel combination of the two biasing resistors, R1 and R2 (R1 || R2).
What is the formula for voltage gain in a common source amplifier?
Neglecting channel length modulation, the voltage gain is -gm * Rd. When considering the finite output resistance (ro) of the MOSFET, the gain becomes -gm * (Rd || ro).
Why can't the drain resistor (Rd) be increased indefinitely to increase gain?
Increasing Rd increases the voltage drop across it, which reduces the drain-to-source voltage (Vds). If Vds drops too low, the MOSFET may exit the saturation region, which is necessary for proper amplifier operation.
Key Terms
- Small-signal model — A linear circuit representation used to analyze the behavior of a MOSFET when subjected to small AC input signals.
- Voltage divider biasing — A circuit configuration using two resistors to set the DC operating point of the MOSFET gate.
- Channel length modulation — A phenomenon where the MOSFET's output resistance is finite, which affects the gain and output impedance of the amplifier.
- Saturation region — The operating region of a MOSFET where it functions as an amplifier and the drain current is relatively independent of drain-to-source voltage.