Source Follower Amplifier: Circuit Theory, Biasing Math, and Practical Uses

A source follower amplifier is a common-drain field-effect transistor (FET) circuit that provides near-unity voltage gain while acting as an impedance buffer, transforming a high-impedance input signal into a low-impedance output. In a real circuit or installation, it changes how a signal drives a load rather than how large the signal gets; it sacrifices voltage amplification to deliver current, preventing high-impedance sources like piezo sensors or guitar pickups from being loaded down and attenuated by downstream stages. Beginners frequently confuse it with the common-source amplifier (which actually multiplies voltage) or the BJT emitter follower (which suffers from a fixed ~0.7V $V_{BE}$ drop, whereas the FET source follower's DC offset depends on the threshold voltage and bias current).

The Core Mechanism: Voltage Follower, Current Booster

To understand the source follower, you have to stop thinking about voltage gain and start thinking about current delivery. The input signal is applied to the gate, and the output is taken from the source terminal. The drain is tied directly to the supply voltage ($V_{DD}$), making it "common" to both input and output AC grounds.

Think of a high-impedance signal source like a tall, narrow water tower feeding a wide municipal pipe. If you open the valve (connect a low-impedance load), the pressure (voltage) instantly collapses because the narrow tower pipe can't supply enough flow (current). A source follower acts like a sensor on the tower that drives a massive local pump; it matches the tower's exact water pressure at the output, but can supply virtually unlimited flow to the municipal pipe.

Mathematically, the AC voltage gain ($A_v$) of a source follower is governed by the transconductance ($g_m$) of the FET and the source resistor ($R_S$):

Because $g_m R_S$ is always a positive number, the gain is always strictly less than 1. However, with a reasonably high $g_m$ and $R_S$, the gain typically lands between 0.85 and 0.98. The real magic happens at the impedance boundaries. The input impedance is essentially the gate leakage resistance (often >1GΩ for JFETs and MOSFETs), while the output impedance drops to roughly $1/g_m$, which is often just a few hundred ohms.

Worked Numeric Example: Biasing a 2N5457 JFET Source Follower

Let's build a buffer for an acoustic guitar piezo pickup using a classic 2N5457 N-channel JFET. JFETs are preferred here over MOSFETs for their lower gate leakage current and softer clipping characteristics in audio applications.

Given Datasheet Typical Values:

• $I_{DSS}$ (Zero-gate voltage drain current) = 3.0 mA
• $V_P$ (Gate-source cutoff voltage) = -2.0 V
• Target Drain Current ($I_D$) = 1.5 mA (half of $I_{DSS}$ for maximum symmetrical swing)

Step 1: Calculate the Required Gate-Source Voltage ($V_{GS}$)

We use the Shockley equation for JFETs, rearranged to solve for $V_{GS}$:

$V_{GS} = V_P \times (1 - \sqrt{\frac{I_D}{I_{DSS}}})$

$V_{GS} = -2.0 \times (1 - \sqrt{\frac{1.5}{3.0}})$

$V_{GS} = -2.0 \times (1 - 0.7071) = -2.0 \times 0.2929 = -0.586\text{ V}$

Step 2: Calculate the Source Resistor ($R_S$)

Since the gate is tied to ground via a 1MΩ resistor (to provide a DC return path without loading the piezo), $V_G = 0\text{ V}$. Therefore, the source voltage $V_S$ must be $+0.586\text{ V}$ to maintain that $-0.586\text{ V}$ difference.

$R_S = \frac{V_S}{I_D} = \frac{0.586\text{ V}}{0.0015\text{ A}} = 390.6\ \Omega$

Step 3: Select Standard Components

We select a standard 5% 390Ω resistor for $R_S$. This will shift our $I_D$ slightly, but well within the safe operating area. To prevent the source resistor from degenerating our AC signal (which would lower the gain and raise the output impedance), we bypass $R_S$ with a 10μF electrolytic capacitor to ground. Wait—in a true source follower, the output is taken across the unbypassed $R_S$. If we bypass it, the AC output drops to zero! Therefore, in a source follower, never bypass the source resistor. The AC signal develops across $R_S$ and the load.

Where You Meet This In Practice

You won't often see source followers used to amplify RF or audio signals directly, but you will find them everywhere as interface stages. According to standard MOSFET amplifier design principles, the common-drain topology is the undisputed king of impedance matching.

• Piezo and Electret Microphone Preamps: Piezo elements have massive internal impedance (often >5MΩ). A source follower buffers this signal so it can drive a standard 10kΩ mixer input without losing high-frequency content to cable capacitance.
• Oscilloscope Probe Buffers: Active FET probes use a source follower right at the probe tip to isolate the high-impedance DUT from the 50Ω or 1MΩ coaxial cable running to the scope.
• Sample-and-Hold Circuits: In ADC front-ends, a MOSFET source follower is used to quickly charge a hold capacitor without drawing continuous current from the multiplexer.
• Driving Long Coaxial Cables: When a 3.3V microcontroller GPIO needs to drive a 75Ω video line, a discrete MOSFET (like a BS170) in a source-follower config provides the necessary current sourcing without an op-amp.

Source Follower vs. Common Source vs. Emitter Follower

Choosing the wrong topology is the most common reason a DIY sensor board fails on the bench. Here is how the source follower stacks up against its siblings.

Frequently Asked Questions