Try simple FET Preamplifier circuit (Very high impedance)

If you need a very high impedance preamplifier. We may see many circuits using transistors or ICs. But if we need the circuit that is small, easy, and economical. I think the FET Preamplifier circuit might be something to look for.

Why should we use a preamplifier?

Try Simple FET Preamplifier circuit (Very high impedance)

Note:
This circuit is one of 4 transistor preamplifier circuits. All circuits have PCB so easy to learn and having fun.
  1. Low impedance input Preamplifier
  2. The medium Impedance Preamplifier circuit
  3. High Impedance Preamplifier circuit
  4. Simple Preamplifier using FET

You should read all 4 articles for more understanding.

How does the circuit work

The circuit using a field-effect transistor or FET is shown in the figure below. First of all, The DC power supply powers via a Decoupling system consisting of R4, C3 like other circuits.

FET Preamplfier high impedance input

Then, the positive voltage enters the drain (D) While the gate (G) requires Bias as a negative electric potential. So, it takes R1 from the gate to the voltage 0 volts. But a direct connection causes uncertain bias control.

Learn more: how FET works (later)

So, We should use the improved bias method by connecting R1 to R2 / R3. Which they are the resistor on the source pin (S) of FET.

The voltage across the R3 is constant at about 3 volts. While the emitter pins of the transistor are generally equal to half the power supply.

Connecting the R1 to the gate like this. It is a built-in positive feedback system, also known as bootstrapping. To increase the efficiency of the resistance Causes the input impedance of this circuit, to increase to about 6 magma ohms. The output impedance when the signal comes to around 500 ohms.

What is this circuit suitable for?

This FET preamplifier circuit is suitable for sending signals from the preamp to the amplifier circuit that uses long wiring.

We can change the resistance of R3 from 1.5K to 4.7K when using low voltage power supply.

How to build

If you want to try creating this circuit It is very easy. If we compared to general transistor circuits. You may solder equipment on the multi-purpose PCB. Or if using the same PCB as the original circuit. See the layout below. You will see that it is the same on all 4 circuits.

Actual-size of Single-sided Copper PCB layout
Actual-size of Single-sided Copper PCB layout
components layout of FET preamplifier circuit very high impedance
But the devices are positioned differently.

Parts you will need

0.25W Resistors, tolerance: 5%

  • R1: 2.2M
  • R2: 1K
  • R3: 1.8K or see text
  • R4: 100 ohms

Capacitors

  • C1: 0.1uF 50V Ceramic
  • C2: 10uF 25V Electrolytic
  • C3: 100uF 25V Electrolytic

Semiconductors

  • Q1: 2N3819 or equivalent, N-channel, Rf/VHF/UHF JFET

 

Here are a few related Circuits you may want to see:

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