Math Problem Statement

Solve this with calculation and final answers

For the transistor with β = 75, analyze the circuit to find the values of RC, all node voltages, and branch currents.

Solution

Let's analyze the circuit step by step to determine the values of RCR_C, all node voltages, and branch currents.

Given Data:

  • β=75\beta = 75 (Current gain of the transistor)
  • IC=0.5mAI_C = 0.5 \, \text{mA} (Collector current provided by the current source)
  • VC=1VV_C = 1 \, \text{V} (Voltage at the collector)
  • Power supply voltages: +5V+5 \, \text{V} and 5V-5 \, \text{V}

1. Calculate IBI_B (Base current):

The transistor's collector current is related to the base current by: IC=βIBI_C = \beta \cdot I_B

Rearranging for IBI_B: IB=ICβI_B = \frac{I_C}{\beta}

Substitute IC=0.5mAI_C = 0.5 \, \text{mA} and β=75\beta = 75: IB=0.575mA=0.00667mA=6.67μAI_B = \frac{0.5}{75} \, \text{mA} = 0.00667 \, \text{mA} = 6.67 \, \mu\text{A}


2. Calculate VBV_B (Base voltage):

The base voltage is determined by the voltage divider formed by the 25kΩ25 \, \text{k}\Omega resistor and the base-emitter junction of the transistor.

For a silicon transistor, the base-emitter voltage VBEV_{BE} is approximately 0.7V0.7 \, \text{V}. Therefore: VB=VE+VBEV_B = V_E + V_{BE}

The emitter current is given by: IE=IC+IBI_E = I_C + I_B

Substitute IC=0.5mAI_C = 0.5 \, \text{mA} and IB=0.00667mAI_B = 0.00667 \, \text{mA}: IE=0.5+0.00667mA=0.50667mAI_E = 0.5 + 0.00667 \, \text{mA} = 0.50667 \, \text{mA}

The voltage across the 25kΩ25 \, \text{k}\Omega resistor is: VE=IE25kΩV_E = I_E \cdot 25 \, \text{k}\Omega

Substitute IE=0.50667mAI_E = 0.50667 \, \text{mA}: VE=0.5066725=12.67VV_E = 0.50667 \cdot 25 = 12.67 \, \text{V}

Thus, the base voltage is: VB=VE+VBE=12.67+0.7=13.37VV_B = V_E + V_{BE} = 12.67 + 0.7 = 13.37 \, \text{V}


3. Calculate RCR_C:

The voltage across RCR_C is: VRC=VCCVCV_{RC} = V_{CC} - V_C

Substitute VCC=5VV_{CC} = 5 \, \text{V} and VC=1VV_C = 1 \, \text{V}: VRC=51=4VV_{RC} = 5 - 1 = 4 \, \text{V}

The collector current IC=0.5mAI_C = 0.5 \, \text{mA}. Using Ohm's law, the value of RCR_C is: RC=VRCICR_C = \frac{V_{RC}}{I_C}

Substitute VRC=4VV_{RC} = 4 \, \text{V} and IC=0.5mAI_C = 0.5 \, \text{mA}: RC=40.5×103=8kΩR_C = \frac{4}{0.5 \times 10^{-3}} = 8 \, \text{k}\Omega


4. Summary of Results:

  1. RC=8kΩR_C = 8 \, \text{k}\Omega
  2. Base current: IB=6.67μAI_B = 6.67 \, \mu\text{A}
  3. Emitter current: IE=0.50667mAI_E = 0.50667 \, \text{mA}
  4. Node voltages:
    • VB=13.37VV_B = 13.37 \, \text{V}
    • VC=1VV_C = 1 \, \text{V}
    • VE=12.67VV_E = 12.67 \, \text{V}

Let me know if you would like further clarification or additional steps explained in detail. Here are some follow-up questions to deepen your understanding:

  1. How does changing β\beta affect the values of IBI_B and RCR_C?
  2. Why is the VBEV_{BE} drop approximately 0.7V0.7 \, \text{V} for silicon transistors?
  3. How would the circuit change if the current source was increased to 1mA1 \, \text{mA}?
  4. What would happen to RCR_C if VCV_C was set to 2V2 \, \text{V} instead?
  5. How does the voltage divider impact the transistor's base voltage in this circuit?

Tip: Always verify the power dissipation across resistors to ensure they operate within their rated limits.

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Math Problem Analysis

Mathematical Concepts

Ohm's Law
Transistor Amplifier Analysis
Voltage and Current Relationships

Formulas

IC = β * IB
IB = IC / β
VRC = VCC - VC
RC = VRC / IC
IE = IC + IB
VB = VE + VBE
VE = IE * R

Theorems

Kirchhoff's Voltage Law (KVL)
Basic Current Amplification in BJTs

Suitable Grade Level

Undergraduate Electrical Engineering