Every device has slightly different characteristics which must be accounted for in your circuit design. The pSpice simulator encapsulates device measurements such as these in a numerical model, which may comprise many parameters. In the results and plots shown below, we are relying on a numerical model to predict how circuits will work. The schematic is just for the convenience of a human designer. Fortunately, it is fairly easy to convert a schematic diagram into data that pSpice can accept. DC V2 0
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It is very suitable for extremely low level audio applications as in audio preamplifiers. The JFET is more expensive than conventional bipolar transistors but offers superior overall performance. Unlike bipolar transistors, current can flow through the drain and source in any direction equally.
Often the drain and source can be reversed in a circuit with almost no effect on circuit operation. 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.
It is indicated as Mhos or Siemens and is typically 2. Because of the high input impedance, the gate is considered an open circuit and draws no power from the source. Although voltage gain appears low in a JFET, power gain is almost infinite.
Drain Characteristics Even though no voltage appears at the gate, a substantial amount of current will flow from the drain to the source. In fact, the JFET does not actually turn off until the gate goes several volts negative. This zero gate voltage current through the drain to the source is how the bias is set in the JFET.
Resistor R3, which is listed in the above diagram, merely sets the input impedance and insures zero volts appears across the gate with no signal. Resistor R3 does almost nothing for the actual biasing voltages of the circuit. When the gate voltage goes positive, drain current will increase until the minimum drain to source resistance is obtained and is indicated below: Minimum Rds on or On State Resistance The above value can be determined by reading specification sheets for the selected transistor.
In cases where it is not known, it is safe to assume it is zero. The other important characteristic is the absolute maximum drain current. Listed below are absolute maximum drain currents for some common N-channel transistors: MPF - 20ma 2N - 22ma 2N - 15ma When designing a JFET circuit, it is highly recommended to prevent the absolute maximum current from being exceeded under any conditions. In design calculations. We will allow no more than 5 ma of drain current under any circumstances.
For resistor R3, the gate resistor, we will use 1 Meg for a very high impedance across the gate. The gate resistor is normally anywhere from 1 Meg to K. 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.
Sometimes the value of this resistor needs to be adjusted for impedance matching depending on the type of signal source involved.
Because we will only allow 5 ma of current through the drain to source, we will calculate the total resistance for resistors R1 and R2. We will assume the Minimum Rds on to be zero.
Cross reference 2N3819
Simulating a FET Amplifier with pSpice
2N3819 Fairchild Semiconductor, 2N3819 Datasheet