Nidhi > Em > Task_4

4. MEASUREMENT OF LOW CURRENTS 4.1. Tasks of the measurement 4.1.1. Measure the forward current of the germanium diode in the low voltage region (20 mV to 100 mV) in five points of the characteristics (see Fig. 4.1): a) using analog micro-ammeter, b) using digital micro-ammeter on more measurement ranges, c) using current-to-voltage converter with an operational amplifier; find the value of the feedback resistor R of the I/U converter to achieve the current-to-voltage conversion factor 105 V/A. Plot the measured curves into common graph. 4.1.2. Evaluate the methodical error caused by non-zero input ammeter resistance in cases 4.1.1a and 4.1.1b. 4.1.3. Evaluate the standard uncertainty of the micro-ammeter reading for cases a and b. For the case c evaluate the expanded uncertainty of current measurement (coverage factor k = 2); consider also input bias current of the op amp. 4.1.4. Evaluate the input resistance of the used micro-ammeters.

4.2. Schematic diagrams 90 Ω

I

+

I A'

A

U1 0 to 1 V

V1

10 Ω

µA

U1'

-

B

B' a), b) R

I A''

+

V2

B'' c) Fig. 4.1 Connection diagram for measurement of low currents

4.3. List of the equipment used V2

- digital voltmeter, type ..., accuracy ± ... % of range ± ... % of reading

µA1

- PMMC micro-ammeter, accuracy class ... , range ...

µA2 R OA U1

- digital micro-ammeter, type ..., accuracy ± ... % of range ± ... % of reading - accurate resistor or decade resistor, accuracy 0.1 % (or 0.2 %) - operational amplifier in a box - variable voltage source with digitally adjustable output value

Theoretical background See the textbook: M. Sedláček, J. Holub, D. Hejtmanová: Laboratory Excercises in Electrical Measurements, CTU Publishing House, Prague, 2005