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Bias for HF JFET – EDN


Junction field-effect transistors (JFETs) normally require some reverse bias voltage to be utilized to a gate terminal.

In HF and UHF functions, this bias is commonly offered utilizing the voltage throughout the supply resistor Rs (Determine 1).

Determine 1: JFETs usually require some reverse bias throughout the gate terminal and in HF/UHF functions, that is typically offered utilizing the voltage throughout resistor Rs.

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Barring the evident lack of effectivity, such strategy has different shortcomings as properly:

  • The drain present has statistical dispersion, so to get a goal worth of the present some circuit adjustment is required.
  • The drain present might rely upon temperature or energy fluctuations.
  • To realize an appropriate low supply impedance, a number of capacitors Cs have for use.
  • To keep up the identical headroom the next energy voltage is required.
  • The dearth of direct contact with the bottom airplane means worse cooling of the transistor, which is essential for energy functions.

The circuit in Determine 2 is freed from all these. It consists of a management loop which produces management voltage of destructive polarity for n-channel JFET amplifier.

Determine 2: A management loop that produces management voltage of destructive polarity for n-channel JFET amplifier in HF and UHF functions.

The circuit makes use of two infrared LEDs IR333C (diameter = 5 mm) in a self-made photocoupler. Two such LEDs positioned face-to-face in an applicable PVC tube about 12 mm lengthy, that’s all. One such gadget produces 0.81 V @ Iled < 4 mA, which is sort of enough for the HEMT FHX35LG, for instance.

After all, in case you want greater voltage, a number of such gadgets may be merely cascaded.

The primary amplification within the loop is carried out by the JFET itself. Its worth is about gm * R1, the place gm is a transconductance of Q1.

The transistor pair Q2 and Q3 compares the voltage drops on the resistors R1 and R2 making them equal. Therefore, by altering the ratio R2:R3 you possibly can set the working level you want:

Id = Vdd * R2 / ((R2 + R3) * R1)

As we are able to see, the drain present (Id) nonetheless relies on energy voltage (Vdd). To keep away from this dependence, we are able to substitute resistor R2 with a Zener diode, then:

Id = Vz / R1

 Peter Demchenko studied math on the College of Vilnius and has labored in software program growth.

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