SAME JFET, DIFFERENT JOB: SOURCE FOLLOWER VS. COMMON-SOURCE AMPLIFIERS
A JFET can serve very different purposes depending on how it is connected.
Place the output at the drain, and the device can provide voltage gain. Take the output from the source, and the same JFET can buffer a signal while presenting a lower impedance to the next stage.
These two fundamental configurations—the common-source amplifier and the source follower—appear throughout low-noise preamplifiers, sensor interfaces, audio circuits, instrumentation and other precision analog systems.
Understanding the differences between them helps engineers choose the right configuration, bias point and JFET for the job.
WHAT DOES “COMMON” MEAN?
The terms common-source and common-drain describe which JFET terminal is effectively common to both the input and output signal paths.
In a common-source amplifier:
The signal is applied to the gate.
The output is taken from the drain.
The source is the common reference point for the signal.
In a source follower, also called a common-drain stage:
The signal is applied to the gate.
The output is taken from the source.
The drain is effectively common to the input and output signal paths.
The JFET may be the same, but changing the output terminal changes the circuit’s gain, phase relationship, output impedance and overall function.
THE COMMON-SOURCE AMPLIFIER
The common-source configuration is used primarily for voltage amplification.
A small change in gate-to-source voltage changes the JFET’s drain current. That changing current develops a larger voltage variation across the drain resistor or active load, producing voltage gain at the drain.
The output signal is inverted relative to the input. When the gate voltage moves in a direction that increases drain current, the voltage drop across the drain resistance increases and the drain voltage falls. The result is an output that is approximately 180 degrees out of phase with the input.
Key characteristics of a common-source stage generally include:
Voltage gain
An inverted output
High input impedance
Moderate-to-high output impedance
Greater sensitivity to device transconductance and drain loading
Potential bandwidth reduction from the Miller effect
The actual gain depends on the JFET’s transconductance, the drain resistance, the source resistance, the load and the device’s internal output resistance.
A simplified gain relationship is:
Av ≈ −gm × effective drain resistance
The negative sign represents the phase inversion.
This relationship is only an approximation. Source degeneration, loading, finite device output resistance and parasitic capacitances all influence the actual result.
SOURCE DEGENERATION AND GAIN
Many common-source amplifiers include a resistor between the source and ground.
This source resistor introduces local negative feedback, often called source degeneration. As drain current increases, the voltage across the source resistor rises. This reduces the effective change in gate-to-source voltage and counteracts the original increase in current.
Source degeneration can:
Improve bias stability
Improve linearity
Reduce distortion
Reduce sensitivity to device variation
Lower voltage gain
A bypass capacitor may be placed across the source resistor to reduce AC degeneration over a selected frequency range. This can increase AC gain while retaining the resistor’s DC bias stabilization.
However, the capacitor value and the impedances surrounding it affect the amplifier’s low-frequency response. It should therefore be selected as part of the complete circuit design rather than treated as an ideal short circuit.
THE SOURCE FOLLOWER
A source follower takes its output from the JFET’s source terminal.
As the gate voltage changes, the source voltage moves in the same direction. The source therefore “follows” the input signal.
Unlike the common-source amplifier, the source follower does not normally provide voltage gain greater than one. Its voltage gain is typically slightly below unity.
The source follower’s value comes from impedance transformation rather than voltage amplification.
It presents a high impedance to the signal source while providing a lower output impedance to the following stage. This makes it useful for buffering high-impedance sensors, driving cables, isolating circuit stages and preventing a load from disturbing a sensitive signal source.
Key characteristics of a source follower generally include:
Voltage gain slightly below one
A non-inverting output
High input impedance
Lower output impedance
Local negative feedback
Improved ability to drive the following stage
The source follower does not create additional signal voltage. Instead, it helps preserve the voltage already present while making the signal more usable by the next circuit stage.
WHY THE SOURCE FOLLOWER HAS LOWER OUTPUT IMPEDANCE
The source follower contains strong local feedback.
If the output voltage at the source is pulled away from the voltage established by the gate, the resulting change in gate-to-source voltage changes the JFET current in a direction that opposes the disturbance.
This feedback reduces the effective output impedance.
As a first-order approximation, the intrinsic output resistance contributed by the device is related to:
Zout ≈ 1/gm
The actual circuit output impedance also depends on the source resistor, load, bias arrangement and the JFET’s other small-signal parameters.
A device with higher transconductance can generally provide a lower follower output impedance under comparable operating conditions.
INPUT IMPEDANCE IS NOT DETERMINED BY THE JFET ALONE
JFET gates draw extremely little current when the gate-channel junction is properly reverse biased. This allows both configurations to offer very high intrinsic input impedance.
However, the input impedance of the finished circuit is not determined by the JFET alone.
It may also be limited by:
Gate-bias resistors
Protection components
Leakage across the circuit board
Input capacitance
Connector or cable leakage
Contamination and moisture
Sensor impedance
Frequency
A JFET with picoampere-level gate leakage cannot deliver picoampere-level system performance if the surrounding board, bias network or protection circuitry introduces substantially greater leakage.
The complete input path must be evaluated.
THE MILLER EFFECT IN A COMMON-SOURCE STAGE
The capacitance between the gate and drain can have a much greater effect in an inverting common-source amplifier than its datasheet value might initially suggest.
Because the drain signal moves in the opposite direction from the gate signal, the voltage across the gate-to-drain capacitance changes by more than the input voltage alone. From the input’s perspective, this can make the capacitance appear multiplied by the stage gain.
This phenomenon is known as the Miller effect.
The increased effective input capacitance can interact with a high source impedance to reduce bandwidth, slow signal transitions and affect stability.
The Miller effect becomes especially important when:
The common-source stage has high voltage gain
The signal source has high impedance
The application requires wide bandwidth
The JFET has significant gate-to-drain capacitance
Several stages are cascaded
A device with lower capacitance may therefore provide better high-frequency performance even if another device has a lower headline noise specification.
CAPACITANCE IN A SOURCE FOLLOWER
A source follower behaves differently because the source moves in the same direction as the gate.
Since the gate and source voltages change together, the AC voltage developed across the gate-to-source capacitance can be reduced. This bootstrapping effect can lower the apparent influence of that capacitance at the input.
This is one reason source followers are useful in high-impedance and capacitance-sensitive applications.
The effect is not perfect. The follower’s gain is below one, and the circuit still includes gate-to-drain capacitance, stray capacitance, board capacitance and the input capacitance of the following stage.
Nevertheless, a properly designed source follower can isolate a sensitive, high-impedance source from a more demanding load.
HEADROOM AND SIGNAL SWING
Neither configuration can be evaluated by gain and impedance alone.
The JFET must be biased so that it remains in its intended operating region throughout the required signal swing.
In a common-source amplifier, the drain voltage must have enough room to move upward and downward without driving the device into cutoff or an unsuitable low-voltage operating region.
In a source follower, the source sits at a voltage determined by the gate voltage, gate-to-source voltage and bias current. The output cannot simply follow the input to either supply rail.
Designers should consider:
Supply voltage
Drain current
Drain-to-source voltage
Gate-to-source voltage
Expected input amplitude
Required output swing
Load current
Device IDSS range
Temperature variation
A source follower may offer excellent buffering while still clipping if its bias point or available headroom is inadequate.
COMMON-SOURCE VS. SOURCE FOLLOWER
Common-Source
Primary function: Voltage amplification
Voltage gain: Often greater than one
Output phase: Inverted
Input impedance: High, but influenced by the bias network and capacitance
Output impedance: Generally higher than a source follower
Bandwidth consideration: Miller multiplication of gate-to-drain capacitance
Typical uses: Preamplifiers, gain stages and signal conditioning
Source Follower
Primary function: Buffering and impedance transformation
Voltage gain: Slightly below one
Output phase: Non-inverting
Input impedance: High, but influenced by the bias network and capacitance
Output impedance: Lower than a common-source stage
Bandwidth consideration: Reduced effect of gate-to-source capacitance through bootstrapping
Typical uses: Sensor buffers, cable drivers, stage isolation and impedance matching
WHICH CONFIGURATION SHOULD YOU USE?
Use a common-source stage when the circuit needs:
Voltage gain
Amplification of a small sensor signal
An inverting stage
A discrete low-noise voltage amplifier
Signal conditioning before a later stage
Use a source follower when the circuit needs:
High input impedance
Lower output impedance
Isolation between stages
Reduced loading of a sensor or signal source
Improved drive for a cable, capacitor or following amplifier
Near-unity voltage transfer without phase inversion
Many practical circuits use both.
A common-source stage may provide the required voltage gain, followed by a source follower that prevents the load from reducing that gain or degrading the signal.
SELECTING A JFET FOR EACH CONFIGURATION
The best JFET depends on the circuit and application.
Important parameters may include:
Voltage-noise density
Current noise and gate leakage
Transconductance
Input capacitance
Gate-to-drain capacitance
IDSS range
VGS(off)
Drain-to-source voltage rating
Matching and thermal tracking
Package and board layout
Production testing and guaranteed limits
For a common-source amplifier, transconductance, noise and gate-to-drain capacitance can be especially important because they directly affect gain and bandwidth.
For a source follower, transconductance influences output impedance and drive capability, while capacitance and leakage remain important when buffering a high-impedance source.
No single parameter identifies the best device. Noise, capacitance, bias current, available headroom and source impedance must be considered together.
LINEAR SYSTEMS JFET OPTIONS
Linear Systems offers several JFETs that can be evaluated for common-source and source-follower designs.
LSK170
The LSK170 is a low-noise, single N-channel JFET suited for low-level amplification and buffering in audio, instrumentation and sensor applications.
LSK389
The LSK389 is a monolithic dual N-channel JFET designed for applications requiring low noise, close matching and thermal tracking. It is well suited for differential input stages and other precision circuits in which the relationship between two devices matters.
LSK489
The LSK489 is a lower-capacitance monolithic dual N-channel JFET. Its combination of matching, noise performance and reduced capacitance can benefit higher-speed or capacitance-sensitive front ends.
LSK189
The LSK189 is a low-capacitance N-channel JFET that can be useful in high-impedance sensor interfaces and source-follower applications where input capacitance is a major consideration.
The appropriate device and IDSS grade should be selected using the current datasheet and the circuit’s required bias current, noise, capacitance, voltage and signal-swing conditions.
COMMON MISCONCEPTIONS
“A source follower has no gain.”
A source follower normally has no voltage gain greater than one, but it can provide current and power gain while transforming impedance. That can be more valuable than voltage gain in many applications.
“A common-source amplifier always has high gain.”
Its gain depends on transconductance, drain resistance, source degeneration, loading and operating point. A heavily loaded or strongly degenerated stage may have modest gain.
“A JFET automatically gives the circuit extremely high input impedance.”
The gate itself may have extremely high resistance, but the circuit’s bias network, contamination, leakage and capacitance determine the input impedance engineers actually obtain.
“The lowest-noise JFET is always the best choice.”
The circuit may also require low capacitance, a particular IDSS range, sufficient transconductance, matching, voltage capability or a specific package. The best device is the one whose complete set of characteristics fits the application.
CONNECTING THE LEARNING SERIES TO PRACTICAL DESIGN
The Linear Systems Learning Series explains the device principles behind precision analog design: how JFET parameters behave, why test conditions matter and how circuit configuration changes device performance.
Our weekly JFET Design Spotlight complements that foundation by examining practical circuits and design approaches. Together, the two series connect datasheet specifications and device physics to the decisions engineers make at the schematic and bench.
The Learning Series explains the “why.” The JFET Design Spotlight shows the “how.”
ENGINEERING TAKEAWAY
The same JFET can perform two very different jobs.
A common-source stage uses the JFET’s transconductance to produce voltage gain, with an output that is inverted relative to the input.
A source follower trades voltage gain for high input impedance, lower output impedance and improved isolation between circuit stages.
The correct choice depends on what the signal needs next: amplification or buffering.
Understanding that distinction helps engineers make better decisions about biasing, impedance, bandwidth, noise, capacitance and device selection.
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