# UNLV PHYS 483 - Review the Basics (3 pages)

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## Review the Basics

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## Review the Basics

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Phys 483 - Special Topics in Physics
##### Special Topics in Physics Documents
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Review the Basics 12V R1 V IR What is the current in R1 R2 and the voltage at point A I 12V 3K 4mA VA 4mA 2K 8V 1K A R2 2K GND 12V R2 Transistors FETs 100 B If A 5V what s the current in R1 R1 IR1 5V 0 7V 1K 4 3mA A Q1 2N3904 If hfe 50 what s the current in R2 and 1K the voltage at B GND hfe Ib 50 4 3mA 215mA If Q1 is fully on i e VB 0 2V then IR2 12V 100 120mA With the given voltage and resistance it s impossible to put more than 120mA through R2 so IR2 120mA and VB 0 2V note It s OK to assume VB 0 for the current calculation If hfe 10 what s the current in R2 and the voltage at B hfe Ib 10 4 3mA 43mA 43mA 100 4 3V VB 12V 4 3V 7 7V 12V Use the ID VS VDS graph for the FET questions R2 If A 2 5V what s the current in R2 and the voltage at B 3 B From graph IDmax with VGS 2 5 is about 2 7A IR2 2 7A 2 7A 3 8 1V VB 12 8 1V 3 9V R1 Q1 A IRL2910 If A 15V what s the current in R2 and the voltage at B Note RDS ON 1K 0 026 GND With VGS 15V the graph shows an increasing ID with VDS i e the max current is limited by the switch on resistance which at VGS 15V is 0 026 Since RDS 3 ID 4A VB 4 0 026 0 104V This makes sense because the graph shows that when ID 4A VDS 0 1V Another words the FET is fully on and the full 12V is across the 3 resistor Would the current in R2 or the voltage at B change if R1 was 100K explain NO R1 controls how fast the gate capacitance charges and discharges Doesn t affect steady state The switch is initially closed short circuit and the circuit is allowed to come to steady state i e wait a few seconds The switch is then opened open circuit What is the current in R2 and the voltage at B about one second after the switch is opened The gate will stay charged to 9V for quite a while so the FET will remain on The max current when VGS 9V is well above the 120mA max 12V 100 so VB 0 ID 120mA 12V R2 100 B B1 9V R1 Q1 IRL2910 1K GND Same circuit but with R3 added The switch is initially closed and the circuit is allowed to come to steady state The switch is then

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