This document describes a low drift, high impedance DC voltmeter circuit that uses JFETs. The circuit uses a pair of matched JFETs configured in a balanced bridge source-follower amplifier. Very high value resistors are used in the voltage divider to take advantage of the high input impedance of the JFET preamplifier. This allows the circuit to have both low drift and high impedance.

1. Low drift high impedance
JFET DC voltmeter
Submitted by:
Mafaz Ahmed 1882/FET/BSEE/F12

2. INTRODUCTION:
The voltmeter shown in the
schematic diagram below has very high impedance. The
range selector uses conventional voltage divider resistors,
but since the following pre amplifier uses JFET, the divider
can be designed using very high resistance series resistors.
This DC voltmeter circuit also featured with low drift. This
circuit uses a pair JFETs, which is configured in a balanced-
bridge source-follower amplifier circuit. The Q2 and Q1
must be matched within 10% for Idss that will maintain
bridge balance over temperature. Here is the schematic
diagram of the circuit:

3. What is a JFET?
A JFET, or junction gate field-effect transistor, is a type of field-effect
transistor. A JFET can be used as a voltage-controlled resistance or as an
electronically-controlled switch. A p-type JFET consists of a channel of
semiconductor material containing a large amount of positive charge
carriers or holes, whereas an n-type JFET consists of a channel of
semiconductor material containing a large amount of negative charge
carriers or holes. At each end of the JFET, ohmic contacts form the
source and drain. Electric charge flows through the channel between the
source and drain. Electric current can be impeded or switched off by
applying a reverse bias voltage to a gate.
Applicationsfor JFETs:
JFETs can be used for several applications including:
 • High Input Impedance Amplifier
 • Low-Noise Amplifier
 • Differential Amplifier
 • Constant Current Source
 • Analog Switch or Gate
 • Voltage Controlled Resistor
Components
 Sensor
 Voltmeter
 JFET
 Resisters 22M,6.28M,2.2M,628K,
 313K,100K,5K,4.7K

4. Simulation:
As shown in the simulation above the JFET used is of very high input
impedance. For that high resistances are used in voltage divider circuit. The
potentiometerRV1 is used to balance the circuit.