Source Follower Power Calculations

A source follower is a common circuit configuration in analog electronics that provides voltage buffering between a high-impedance source and a low-impedance load. Calculating the power dissipation in a source follower is essential for proper circuit design and thermal management.

What is a Source Follower?

A source follower is a basic amplifier configuration that uses a field-effect transistor (FET) as the amplifying device. It's called a "source follower" because the output is taken from the source terminal of the FET, which follows the input voltage with some gain.

The key characteristics of a source follower include:

Voltage gain less than 1 (typically 0.7-0.9 for MOSFETs)
High input impedance
Low output impedance
Inversion of the input signal (output is 180° out of phase with input)

Source followers are commonly used in analog circuits where buffering is needed between high-impedance sources and low-impedance loads, such as in operational amplifiers and voltage regulators.

Power Calculation

The power dissipated in a source follower can be calculated using the following formula:

P = V_DS × I_D

Where:

P = Power dissipation (Watts)
V_DS = Drain-to-source voltage (Volts)
I_D = Drain current (Amperes)

The drain current can be calculated using the FET's transconductance parameter (k) and the overdrive voltage (V_GS - V_th):

I_D = k × (V_GS - V_th)²

Where:

k = Transconductance parameter (A/V²)
V_GS = Gate-to-source voltage (Volts)
V_th = Threshold voltage (Volts)

Example Calculation

Consider a MOSFET with k = 0.001 A/V², V_th = 2V, and V_GS = 5V. The drain-to-source voltage is 10V.

Calculate the overdrive voltage: V_GS - V_th = 5V - 2V = 3V
Calculate the drain current: I_D = 0.001 × (3)² = 0.009A (9mA)
Calculate the power dissipation: P = 10V × 0.009A = 0.09W (90mW)

Practical Applications

Source followers have several practical applications in electronic circuits:

Buffering between high-impedance sources and low-impedance loads
Level shifting in digital circuits
Impedance matching in RF circuits
Voltage regulation in power supply circuits
Signal conditioning in measurement systems

Comparison of Source Follower and Common Drain Amplifier
Parameter	Source Follower	Common Drain Amplifier
Voltage Gain	Less than 1	Less than 1
Input Impedance	High	High
Output Impedance	Low	Low
Signal Inversion	Yes	No
Common Application	Buffering	Voltage regulation

Limitations

While source followers are useful, they have several limitations that designers should be aware of:

Signal inversion can be problematic in some applications
Voltage gain is typically less than 1, which may not be sufficient for some applications
Power dissipation can be significant at higher currents
Frequency response may be limited by the FET's capacitances

For applications requiring higher gain or non-inverting operation, a common source amplifier might be a better choice.

FAQ

What is the difference between a source follower and a common drain amplifier?

A source follower and a common drain amplifier are essentially the same circuit configuration. The terms are often used interchangeably, though "common drain" is more commonly used in bipolar junction transistor (BJT) circuits.

How do I choose the right FET for a source follower circuit?

When selecting a FET for a source follower, consider parameters such as transconductance (k), threshold voltage (Vth), and power handling capabilities. The FET should be able to handle the expected drain current and voltage while maintaining adequate gain.

What is the typical voltage gain of a source follower?

The voltage gain of a source follower is typically between 0.7 and 0.9 for MOSFETs. This is because the output follows the input voltage with some attenuation due to the FET's characteristics.