Pj Problems - Overview
The Number Line
7 Spaces Of Interest - Overview
Triadic Unit Mesh
States Of Matter
COHN - Natures Engineering Of The Human Body
The Human-Body Systems
Faith, Love, Charity
Figure 122.5 is a transistor amplifier circuit designed to supply a Light Emmitting Diode (LED). The output signal from the microcomputer acts as the switch for the LED. The circuit values are as follows:
Microcomputer: output resistance = RB = 1 kΩ; Von = 5 V; Voff = 0 V; current, I = 5 mA.
Transistor: VCC = 5 V; offset voltage, Vγ = VBE = 0.7;
Current gain, β = IC/IB = 95; VCEsaturation =0.2 V.
LED: offset voltage, VγLED = 1.4 V; ILED > 15 mA; Pmax = 100 mW.
(a)Determine collector resistance RC such that the transistor is in the saturation region when the microcomputer outputs 5 V.
(b) Determine the power dissipated by the LED.
S7P5A51 (Physical Change).
Pj Problem of Interest is of type change (physical change). Transistors are primarily used for signal amplification and switching. Both are change problems.
(a)i Computer is off = Voff = 0
So, Base current, IB = 0
So, BJT is in the cuttoff region
(a)ii Computer is on = Von = 5 V
At saturation and assumimg the large signal model we have:
VCC = RCIC + VγLED + VCEsaturation
So, RC = (5 - 1.4 - 0.2)/IC = 3.4/IC
ILEDmin = 15 mA. So lets double this minimum current required to light LED
So, for 30 mA, RC = (3.4/30)103 = 113 Ω
Verifying BJT is in saturation region:
At saturation, β is significantly diminished.
IB = (Von - VBE)/RB
So, IB = (5 - 0.7)/1000 = 4.3 mA
β = 30/4.3 = 6.7.
So, since 6.7 << 95, BJT is operating plausibly in saturation region.
(b) Power disspated by LED = PLED = VγLED(IC) = 42 mW
So, power dissipation of LED is less than power max of LED
So, design is ok.
The point . is a mathematical abstraction. It has negligible size and a great sense of position. Consequently, it is front and center in abstract existential reasoning.
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