Simple FET Preamplifier Circuit

This FET preamp circuit is like a supercharger for weak sounds.

It uses a special transistor FET to make quiet sounds much louder.

This particular circuit is really good at its job because it uses two types of transistors together N channel FET and PNP bipolar to keep the sound clear and strong.

This way, you can hear even the faintest sounds nice and clear.

Circuit Working:

Parts List:

This preamplifier design is versatile, as it can utilize general purpose PNP transistors like BC557 or BC558 based on the required gain.

The gain of the preamp is determined by the feedback network and with the specified values of R4 and R5 it is approximately 20.

The preamps input is sensitive and easily interfaces with microphones and ceramic pickups.

Formulas:

The high input impedance and low noise properties of the FET BFW11 make it a popular choice for low noise amplifier circuits.

Below is a general formula:

FET Biasing:

For the FET BFW11 to function, the biasing must be set correctly.

To set the functioning point Q point, this usually entails supplying an appropriate DC voltage V_GS to the gate source terminal.

Determine the desired quiescent current I_DQ flowing through the FET and the suitable biasing resistors R_G using ohms law:

R_G​ = ​V_GS ​​/ I_DQ

Based on the FETs datasheet characteristics and the intended operating circumstances, select V_GS.

RS or source resistor:

For the purpose of adjusting the gain and stabilizing the DC operating point, a resistor R_S is frequently utilized in series with the FET source.

The required voltage gain Av and the FETs transconductance gm may be used to calculate the value of VS

R_S​ = ​1​ / g_m

where,

• g_m​ is the transconductance of the FET.

Capacitors for coupling:

Capacitors are used for coupling signals into and out of the FET stage.

Determine the capacitance values required to transmit AC signals while blocking DC ones based on the frequency response parameters of your circuit.

X_{C ​}= 1​ / 2πfC

where,

• The capacitance is represented by C the frequency by f and the capacitive reactance by X_C.

Notes:

Your particular circuit applications intended gain, frequency responsiveness, and biasing needs will determine the precise values of the resistors and capacitors.

To guarantee correct operation, test the circuit first on a breadboard and modify component values as needed depending on simulation or measurement findings.

This schematic offers a place to start when creating a circuit for a BFW11 FET preamplifier.

Depending on the particular application requirements and performance objectives, adjustments can be required.

How to Build:

To build a Simple FET Preamplifier Circuit you need to follow the below steps:

Connect the Components:

• Connect the gate of the FET to ground through C2 capacitor and music input..
• Connect the source of the FET through resistor R3 and capacitor C3
• Connect the drain of FET through transistor BC557 base and the junction of R2
• Connect R4 15k and R5 680Ω between the collector of the PNP transistor and ground.
• Connect emitter of transistor BC557 to positive of power supply.

Add Capacitors:

• Connect C1 100μF one leg to positive supply and other leg to ground.
• Connect C2 0.22μF between drain of FET through music input and ground.
• Connect C3 10μF between the source of FET through resistor R3 18k

Input and Output:

• The input signal can be connected between the gate of the FET and ground.
• The output signal is taken from the collector of T2 transistor BC557.

Testing:

• Power up the circuit with the appropriate voltage.
• Connect a suitable input signal e.g. microphone or ceramic pickup to test the preamps functionality.
• Adjust the values of R4 and R5 to achieve the desired gain if needed.

Note:

• Ensure you observe proper polarity and connections while building the circuit.

Conclusion:

The FET preamplifier described offers a straightforward yet effective design for audio applications.

By utilizing an N Channel FET and a PNP Bipolar transistor it provides high input impedance low output impedance and stabilized gain.

This makes it suitable for use with microphones and other audio sources, offering a sensitive and stable signal amplification solution.

References:

I don’t quite understand this FET-BJT preamp circuit

Datasheet FET BFW11