Transistor Amplifier Supply For A LED
Strings (SiPjAjk) = S7P5A51 Base Sequence = 12735 String Sequence = 12735 - 5 - 51
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.
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