Headphone Amplifier using JFET

A headphone amplifier is electronics circuit that amplifies the low level signal from signal source such as radio or PC to adequate high level signal to drive the speaker inside headphone. Here it is shown how to build inexpensive and simple headphone amplifier using JFET. It is a common source amplifier utilizing a JFET, designed to amplify signals for high-impedance headphones, such as piezoelectric earphones that are widely available. The headphone amplifier circuit diagram is shown below.

headphone JFET amplifier circuit diagram
The amplifier circuit is designed using common source self bias method. The JFET transistor 2N3819 is used here. The input is applied to the gate via the coupling capacitor C1. To operate a JFET the gate must be negatively biased. In self bias, the source resistor provides the necessary negative gate to source voltage at the gate to reverse bias the gate to source junction of the JFET. The input impedance of a JFET is very high. This amplifier incorporates a volume control potentiometer in the drain circuit instead of a fixed resistor. Capacitor C2 is included to prevent any direct current (DC) interference with the headphones. A general rule of thumb for selecting the coupling capacitors is to choose their reactance value to be equal to the impedance of the load at the lowest desired frequency of operation.

Self bias method is the easiest method to bias JFET amplifier. The voltage drop across the source resistor is fed back to the gate and thus reverse biasing the gate to source voltage. One should note that JFET is normally on device and thus negative gate voltage has to be applied to make current flow from drain to source. The transconductance of the above amplifier is around 22mS. To bias the JFET transistor we need the gate shorted drain current(Idss) and gate to source cutoff voltage(Vgs). These data can be obtained from the JFET datasheet. Here the gate shorted drain current(Idss) is 10mA and gate to source cutoff voltage(Vgs) is -1.26V. We can calculate the component values using the online JFET  amplifier design calculator.

See the following video demonstration of JFET headphone amplifier.

References and Further Readings

[1] Self Biased JFET Colpitts Oscillator

[2] JFET Biasing Worked Out Example Calculation

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