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Common Drain Stage or Source Follower

Introduction of Field Effect Transistors Distinction between a BJT and FET Junction Field Effect Transistor (JFET) N Channel JFET Summary of JFET Operation Mutual or Transfer Characteristic of n-channel JFET Amplification Factor 𝝁 Definition of Parameters p-Channel JFET

Introduction of Metal Oxide Semiconductor Field Effect Transistor - Mosfet Terminals in MOSFET Physical structure of MOSFET MOSFET Working Operation Summary of Operation MOSFET - N Type MOSFET Operation How does Drain current flow MOSFET operation More about Threshold Voltage Current vs Voltage Characteristics - Derivation Non Ideal Current -Voltage Effects or Second Order Effects Short Channel Effects MOSFET Device Capacitances MOSFET Large and Small signal Model pMOSFET Small SIgnal Model To use MOSFET device as a Capacitor nMOSFET as Capacitor- Working

Single Stage Amplifiers Common Source CS Stage with Diode-Connected Load Circuit to measure the Equivalent Resistance Common Drain Stage or Source Follower Common Gate Amplifier Large Signal Behavior Calculation of Input Resistance of a Common Gate Stage

Inverter with Enhancement n- Type MOS as Load Inverter with Linear Enhancement Type Load

Common Drain Stage or Source Follower

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Common Drain Stage or Source Follower

When the MOSFET is operating in the Common Source stage, for proper amplification it needs a resistor of high value at the output.

In other words the load impedance must be as large as possible.

Now if the common source stage has to drive a low impedance load, then we need a buffer in between so that there is no loss of gain.

Under these conditions the Common Drain stage can be used as a buffer in between. A common drain stage is also popularly called as Source Follower.

Figure 8 shows the circuit diagram of a Source Follower.

Figure 8: Common Drain Stage / Source Follower

We can use the Source follower as a Buffer, figure 9 illustrates the same

Figure 9: Illustration of Source Follower as Buffer

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