Design Guidelines for JFET Audio Preamplifier Circuits
|By Mike Martell|
The Junction Field Effect Transistor (JFET) offers very high input impedance along with very low noise figures. 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.
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 2.5mmhos to 7.5mmhos for the MPF102 transistor. 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.
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:
When designing a JFET
circuit, it is highly recommended to prevent the absolute
maximum current from being exceeded under any
conditions. In design calculations. never use more than
75% of the maximum drain current as specified by the
|R1 = Total Resistance - R2
R1 = 2400 - 600 = 1800 ohms
To prevent oscillations a 10 ohm resistor
and a 100uf capacitor were added to isolate the circuit
from the power supply. A .1uf capacitor was used for
input coupling and a 4.7uf capacitor was used for output
coupling. Slightly larger or smaller capacitor values
will also give acceptable results. The optional 4.7uf
capacitor which bypasses R2 is used to obtain the maximum
amount of gain the transistor will deliver. 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.
|JFET Design Example 2
In the second design example, we will use an MPF102 transistor to add an additional stage of amplification to our circuit. We will make the following assumptions:
R3 = 1Meg
Vcc = 12
Minimum Rds(on) = 0
Ids = 7ma
(Vcc - (Minimum Rds(on) * Ids)) / Ids = Total resistance of R1 and R2
(12 - (0 * .007)) / .007 = 1714 ohms
We will assume R2 to have 25% of the supply voltage.
|R2 = .25 * Total of R1 and R2
R2 = .25 * 1714 = 429 ohms (use 470)
R2 = 470
R1 = Total Resistance - R2
R1 = 1714 - 429 = 1285 ohms (use 1200)
R1 = 1200 ohms
A 4.7uf capacitor was used for input coupling and a 10uf capacitor was used for output coupling. Slightly larger or smaller capacitor values will also give acceptable results. The optional 10uf capacitor which bypasses R2 is used to obtain the maximum amount of gain the transistor will deliver.
By putting our two circuits together we now have a two transistor JFET audio preamplifier with excellent gain and very low distortion. A 10K level control was added to complete the preamplifier circuit. If you decide to use a 2N3819 be aware that the pin-out is different than other JFET transistors
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