Common Drain Amplifier

The common drain amplifier, also called a source follower, is the MOSFET version of the emitter follower. It sits at the output of amplifier chains to drive low-impedance loads without losing voltage.

Core Concept

The common drain amplifier (source follower) has the drain connected directly to VDD, making it the common terminal. The input goes to the gate and the output is taken from the source. The source voltage follows the gate voltage closely, hence the name source follower.

The voltage gain is slightly less than 1 because VGS must remain greater than Vth for the transistor to stay on. As Vin rises, VGS tends to rise, which increases ID, which raises VS, which partially cancels the VGS increase. This negative feedback loop is what keeps Vout nearly equal to Vin. The gain approaches 1 as gm*RS becomes large.

The key strength is the output impedance. The source follower presents a low output resistance of approximately 1/gm, typically 100 to 500 Ω, while its input resistance is the parallel combination of the bias resistors, which can be in the megaohm range. This impedance transformation makes it an ideal buffer stage between a high-impedance signal source and a low-impedance load.

Key Equations

Voltage gain: Av = gm*RS / (1 + gm*RS) (always less than 1, positive, non-inverting)

When gm*RS >> 1: Av ≈ 1

Input resistance: Rin = RG1 || RG2 (high, set by bias resistors)

Output resistance: Rout = (1/gm) || RS (low, approximately 1/gm)

Transconductance: gm = 2*sqrt(K*IDQ)

Given:
  VDD  = 12 V
  RS   = 2.2 kΩ
  IDQ  = 1.5 mA
  K    = 0.5 mA/V^2
  RG1  = 100 kΩ
  RG2  = 47 kΩ

Why this formula:
  Common drain; gain = gm*RS / (1 + gm*RS).

Step 1 - Find gm:
  gm = 2 * sqrt(K * IDQ)
     = 2 * sqrt(0.5e-3 * 1.5e-3)
     = 2 * sqrt(0.75e-6)
     = 2 * 0.866e-3
     = 1.732 mA/V

Step 2 - Find gm*RS:
  gm * RS = 1.732e-3 * 2200 = 3.81

Step 3 - Voltage gain:
  Av = 3.81 / (1 + 3.81) = 3.81 / 4.81 = 0.792

Step 4 - Output resistance:
  Rout = (1/gm) || RS
       = (1/1.732e-3) || 2200
       = 577 || 2200
       = (577 * 2200) / (577 + 2200)
       = 1,269,400 / 2777
       = 457 Ω

Final Answer:
  Av = 0.792, Rout = 457 Ω

Exam Tip: GATE problems on source followers test whether students know Av is less than 1 and Rout is approximately 1/gm. A common trap is setting Av = 1 exactly. The correct formula is Av = gm*RS/(1+gm*RS). Also, the output resistance is (1/gm)||RS, not just RS. Students who write Rout = RS miss the 1/gm term and overestimate the output impedance.

Key Properties

• Voltage gain is always less than 1 (typically 0.8 to 0.99) and positive; the output follows the input.
• No phase inversion: output is in phase with input.
• High input resistance set by bias resistors RG1 and RG2 (can exceed 1 MΩ with 100 kΩ resistors).
• Low output resistance approximately equal to 1/gm, typically 100 to 600 Ω.
• Used as buffer between stages to prevent loading of a high-impedance source by a low-impedance load.
• The drain is at AC ground (connected to VDD through decoupling capacitor or directly), so no gain from drain side.
• Gain increases toward 1 as RS or gm increases; gm can be raised by increasing IDQ.

Quick Revision

• Input: gate. Output: source. Common terminal: drain (connected to VDD).
• Phase: non-inverting. Gain: less than 1.
• Voltage gain: Av = gm*RS / (1 + gm*RS).
• Input resistance: high (RG1||RG2, megaohm range).
• Output resistance: low, (1/gm)||RS.
• Purpose: impedance transformation and buffering.
• Increasing RS or IDQ pushes Av closer to 1.
• Exam trap: Students write Av = 1 for the source follower instead of Av = gm*RS/(1+gm*RS), and write Rout = RS instead of (1/gm)||RS.