Low drift high impedance JFET DC

voltmeter

Submitted by : Mafaz Ahmed 1882/FET/BSEE/F12

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:

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.

Applications for 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

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 potentiometer RV1 is used to balance the circuit.