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Transcript
PHYS - 321 ELEMENTARY ELECTRONiCS
• RESISTORS
• KIRKOFFS LAW and LOOP-MESH METHOD
• VOLTAGE DIVIDER
• INTERNAL RESISTANCE and
OUTPUT IMPEDANCE
• HOW TO MEASURE OUTPUT IMPEDANCE OF A
DEVICE
RESISTORS
•Materials have different resistivities ρ
• R = ρ (L/A) = resistance
L
ρ
A
σ=1/ρ
Kirchoffs Laws(1)
(1) Let I run CW in loops.
(2) Add the emfs.
(3) Subtract the voltage drops.
V
V
i1
i
R1
R
i2
R2
+V-iR = 0
+V-i1 R1 +i2 R1 = 0
0 -i2 R2 -i2 R1 +i1 R1 = 0
V = i1 R1 -i2 R2
0 = -i1 R1 +i2 (R1+R2)
Voltage Divider
V
R1
Vout
i
R2
V-I R1 -I R2 = 0
V=I(R1+R2)
I= V/(R1+R2)
Vout = I R2
= {R2/(R1+R2)} V
Voltage drop on ith resistor is proportional to ratio
of Ri to Rtot!
Vi = {Ri/Rtot} V
ΔV1 ={R1/(R1+R2)} V
ΔV2 ={R2/(R1+R2)} V
Output Impedance
•
•
•
Every Source of Emf has some small internal resistance.
A signal generator has an internal resistance called output
Impedance z. ( z~50 Ohm ).
A voltage divider circuit can be used to measure r and z.
Adjust R until ΔVR=1/2V ! Then R = z !
battery
Sine Generator
r
V
i
R
-
z
+
R
Input Impedance
• All input devices has some small internal resistance to the current
flowing into it.
• A voltmeter has a high input impedance to limit the current
flowing in to the measuring device.
• An ammeter wants to divert all the current into it and therefore
has a very low input impedance.
V
A
1MΩ
+
f
u
s
e
1Ω
-
+
-
Thevenin's Theorem(1)
Thevenin's Theorem(2)