2N3819 FET PDF

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We will allow no more than 5 ma of drain current under any circumstances.

Designing JFET Audio PreAmplifiers

We will make the following assumptions: The JFET is more expensive than conventional bipolar transistors but offers superior overall performance. Resistor R3, which is listed in the above diagram, merely sets the input impedance and insures zero feg appears across the gate with no signal. Minimum R ds on or On State Resistance. The higher values allow the JFET to amplify very weak signals but require measures to prevent oscillations.

The lower values enhance stability but tend to decrease gain. The optional 10uf capacitor which bypasses R2 is used to obtain the maximum amount of gain the transistor will deliver.

The above value can be determined by reading specification sheets for the selected transistor. This zero gate voltage current through the drain to the source is how the bias is set in the JFET.


23819 The addition of this capacitor may introduce a small amount of unwanted white noise and should only be used when an absolutely quiet preamplifier is not required. It is very suitable for extremely low level audio applications as in audio preamplifiers. In cases where it is not known, it is safe to assume it is zero.

To prevent oscillations a 10 ohm resistor and a uf capacitor were added to isolate the 23819 from the power supply. A 10K level control was added to complete the preamplifier circuit. When the gate voltage goes positive, drain current will increase until the minimum drain to source resistance cet obtained and is indicated below: Because of the high input impedance, the gate is considered an open circuit and draws no power from the source.

Resistor R3 does almost nothing for the actual biasing voltages of the circuit. Listed below are absolute maximum drain currents for some common N-channel transistors: Often the drain and source can be reversed in a circuit with almost no effect on circuit operation.

Drain Characteristics Even though no 2n3891 appears at the gate, a substantial amount of current will flow from the drain to the source.

Although voltage gain appears low in a JFET, power gain is almost infinite. We will assume the Minimum R ds on to be zero.

2N3819 N Channel FET

Back to Projects Page! Transconductance The ability of a JFET to amplify is described as trans-conductance and is merely the change in drain current divided by the change in gate voltage. Slightly larger or smaller capacitor values will also give acceptable results. The gate resistor feg normally anywhere from 1 Meg to K.


When designing a JFET circuit, it is highly recommended to prevent the absolute maximum current from being exceeded under any conditions. Sometimes the value of this resistor needs to be adjusted for impedance matching depending on the type of signal source involved. For resistor R3, the gate resistor, vet will use 1 Meg for vet very high impedance across the gate. Minimum R ds on or On State Resistance The above value can be determined by reading specification sheets for the selected transistor.

Unlike bipolar transistors, current can flow through the drain and source in any direction equally. Because we will only allow 5 2m3819 of current through the drain to source, we will calculate the total resistance for resistors R1 and R2. In fact, the JFET does not actually turn off until the gate goes several volts negative.

It is indicated as Mhos or Siemens and is typically 2.