Electronics Hub

BJT Transistor

NPN SWITCH & AMPLIFIER LAB
PrayogSetu Virtual Instrumentation Laboratory
Common-Emitter NPN Transistor Circuit
ACTIVE REGION β = 100
VCC = 10.0 V RC = 1.00 kΩ VRC = 7.50 V | 56.3 mW LOAD: ACTIVE (75%) IC = 7.50 mA + − Vin = 2.50 V DC CONTROL RB = 24.0 kΩ VRB = 1.80 V | 0.14 mW IB = 75.0 µA B Base C Collector E Emitter NPN VCE VOLTAGE VCE = 2.50 V IE = 7.58 mA 0V (GND) COM
Operating Region: ACTIVE (LINEAR AMPLIFIER)
Saturation Limit (IC,sat): 9.80 mA
Transistor Power (PD): 18.80 mW
VBE Drop: 0.700 V
Transistor DC Load Line & Dynamic Q-Point
DYNAMIC PLOT
Cut-off Region: VCE = VCC, IC ≈ 0 mA
Active Q-Point: Linear Amplification Range
Saturation Region: VCE = 0.2V, IC = IC(sat)
BJT Physics & Mathematical Derivation Solver
LIVE FORMULAS
STEP 1

Base Drive Current (IB)

KVL across Base-Emitter loop with silicon VBE drop (0.7V):

IB = (Vin − VBE) / RB = (2.50V − 0.70V) / 24.0 kΩ = 75.0 µA
STEP 2

Collector Saturation Limit

Maximum current through collector circuit when fully turned ON:

IC(sat) = (VCC − VCE(sat)) / RC = (10.0V − 0.20V) / 1.0 kΩ = 9.80 mA
STEP 3

Operating Region Check

Compare amplified current (β × IB) with physical circuit ceiling:

β × IB = 100 × 75.0 µA = 7.50 mA < IC(sat) (9.80 mA)
Status: ACTIVE REGION (Linear Amplification)
STEP 4

Output Voltage (VCE) & Dissipation

Calculate VCE and total transistor power dissipation:

VCE = VCC − (IC × RC) = 10.0V − (7.50 mA × 1.0 kΩ) = 2.50 V
PD = (VCE × IC) + (VBE × IB) = 18.80 mW

Academic & Industrial Reference Manual

MODULE 01

BJT Architecture & Current Relations

A Bipolar Junction Transistor (BJT) is a current-controlled active device consisting of three doped semiconductor regions: Emitter (E), Base (B), and Collector (C).

IE = IB + IC = (β + 1) × IB

The Base-Emitter PN junction functions as a forward-biased diode requiring approximately 0.7V (for Silicon) before base current begins to flow.

MODULE 02

The Three Operating Regions

  • Cut-off Region (Vin ≤ 0.7V): BE junction unbiased/reverse. IB = 0, IC ≈ 0, VCE = VCC. Transistor acts as an open switch.
  • Active Linear Region: BE forward-biased, BC reverse-biased. IC = β × IB, VCE = VCC − IC·RC > 0.2V. Used for linear signal amplification.
  • Saturation Region: Both BE and BC forward-biased. Collector current reaches ceiling IC(sat) = (VCC − 0.2V) / RC. Transistor acts as a closed switch with VCE ≈ 0.20V.
MODULE 03

DC Load Line & Q-Point (Quiescent Point)

The DC Load Line connects the two circuit operating limits on the IC vs VCE plane:

Cut-off: (VCE = VCC, IC = 0)  ↔  Saturation: (VCE ≈ 0.2V, IC(sat) = [VCC−0.2]/RC)

The operating Q-point is the exact intersection between the transistor's base characteristic curve for IB and the DC load line.

MODULE 04

BJT as an Electronic Switch & Inverter

When used as a digital logic switch:

  • LOW Input (Logic 0, Vin < 0.7V): Transistor OFF → Output Vout = VCE ≈ VCC (HIGH logic 1).
  • HIGH Input (Logic 1, Vin ≥ 2.5V): Transistor in Saturation → Output Vout = VCE ≈ 0.2V (LOW logic 0).

This demonstrates the fundamental NOT Gate (Inverter) action of common-emitter digital circuits.