BJT & MOSFET Calculator

Test transistors with a multimeter, calculate DC gain (hFE/β), AC voltage gain, and design proper biasing circuits for both BJT and MOSFET transistors.

🔧 How to Test Transistors with Multimeter

Learn to identify good vs bad BJT and MOSFET transistors using a digital multimeter in diode test mode.

Select Transistor Type:

🔍 Testing NPN BJT Transistor

NPN Pinout

✅ Good Transistor Signs

• B→E and B→C show 0.5-0.8V
• Reverse readings show OL
• C↔E both directions OL

❌ Bad Transistor Signs

• Any junction shows 0V (shorted)
• All junctions show OL (open)
• C-E shows low resistance

📊 BJT DC Current Gain (hFE / β) Calculator

Calculate the DC current gain of a BJT transistor from collector and base current measurements.

β = I_C / I_B

DC Current Gain = Collector Current ÷ Base Current

Collector Current (I_C): Measured in mA

Base Current (I_B): Measured in mA (typically µA range)

DC Current Gain (β)

100

Gain Category

Medium

α (IC/IE)

0.99

📖 Understanding DC Gain (hFE)

The DC current gain (β or hFE) indicates how much the transistor amplifies the base current. A transistor with β=100 means 1mA of base current controls 100mA of collector current.

Low Gain (β < 50)

Power transistors, high current applications. Examples: 2N3055, TIP31

Medium Gain (β 50-200)

General purpose transistors. Examples: 2N2222, BC547, 2N3904

High Gain (β > 200)

Darlington pairs, low-signal applications. Examples: MPSA13, BC517

📈 BJT Small-Signal Gain Calculator

Calculate voltage gain and impedance for different BJT amplifier configurations.

Ampliifer Configuration:

Selection Guide: Choose CE for general amplification, CC/Follower for matching high-Z inputs to low-Z outputs, or CB for RF isolation. View Detailed Comparison →

A_v = -R_C / r_e

Where r_e = 26mV / I_E

Collector Resistor (R_C): Collector load resistance in Ω

Emitter Current (I_E): DC emitter current in mA

Emitter Resistor (R_E): Unbypassed emitter resistance in Ω

AC Load (R_L): External load resistance in Ω (default 1MΩ)

Voltage Gain (A_v)

1.0

Input Impedance

50kΩ

Output Impedance

2.2kΩ

⚡ MOSFET Small-Signal Analysis

Calculate transconductance and gain for MOSFET amplifier configurations.

Configuration:

g_m = 2 * √(k * I_D)

A_v = -g_m * R_D

Drain Current (I_D): DC Drain current in mA

K Parameter (mA/V²): MOSFET transconductance param

Drain Resistor (R_D): Drain load in Ω

Source Resistor (R_S): Unbypassed resistance in Ω

g_m (mS)

Voltage Gain (A_v)

-20

Output Z

1kΩ

⚡ Transistor Biasing Calculator

Design stable biasing circuits for BJT and MOSFET amplifiers.

Circuit Configuration:

Design Note: Biasing establishes the Q-Point. It ensures your signal stays centered and stable despite temperature changes. Why biasing matters →

Supply Voltage (V_CC/V_DD): Power supply voltage (V)

Desired I_C (mA): Target collector/drain current

Transistor β (hFE): DC current gain

Desired V_CE (V): Collector-Emitter voltage (≈ VCC/2)

Recommended Component Values:

R₁ (Upper)

47kΩ

R₂ (Lower)

10kΩ

R_C (Collector)

2.2kΩ

R_E (Emitter)

680Ω

📖 Transistor Theory & Selection Guide

Mastering transistors requires understanding not just how they work, but why we use specific circuits and configurations.

⚡ Why do we need Biasing?

Biasing is the process of setting a fixed DC operating point (known as the Q-Point or Quiescent Point) for the transistor. Without proper biasing:

The signal will clip: If the Q-point is too close to the supply voltage (Saturation) or Ground (Cutoff), the amplified signal's peaks will be flattened, causes distortion.
Thermal Runaway: As BJTs heat up, their base-emitter voltage (V_BE) decreases (~2mV/°C), causing more collector current for the same base voltage. This extra current generates more heat, creating a destructive positive feedback loop. A properly biased circuit includes an Emitter Resistor (R_E) which provides negative feedback: as I_C increases → V_E rises → V_BE effectively drops → I_C stabilizes.
Circuit Consistency: No two transistors (even from the same batch) have identical gain. Biasing techniques like the Voltage Divider Bias make the circuit's performance dependent on resistors, not the unpredictable transistor gain.

📐 Choosing BJT Configurations

The "Common" pin refers to the terminal used for both input and output signals:

Common Emitter (CE): The "All-Rounder". Provides both high voltage and current gain. Use for: General-purpose amplifiers. (Note: It inverts the signal).
Common Collector (CC): Also called an Emitter Follower. Voltage gain is ~1, but current gain is high. Use as: A Buffer to drive low-resistance loads (like speakers).
Common Base (CB): Has low input impedance and high output impedance. Use for: VHF/UHF radio frequency circuits as it eliminates the Miller capacitance effect.

⚖️ BJT vs. MOSFET Selection

Selecting the right tech depends on your load and switching speed:

Property	BJT	MOSFET
Control	Current-Based	Voltage-Based
Input Z	Low/Mid	Near Infinite
Heat Loss	Static (Vce)	Resistive (Rds)
Switching	Fast	Ultra-Fast

🌳 Transistor Selection Decision Tree

1. What is the Load?

• Sensitive Signal/Audio: Use BJT (Better linearity, lower noise).
• High Power/PWM: Use MOSFET (Voltage controlled, low heat).

2. Do you need Gain?

• Yes (Voltage): Use Common Emitter (BJT) or Common Source (MOSFET).
• No (Buffering): Use Emitter Follower (CC) or Source Follower (CD).

3. Direct MCU Drive?

• 3.3V / 5V GPIO: Use a Logic-Level MOSFET to ensure the transistor fully turns on without high current drain.

🧮 Practical Bias & Application Examples

Example 1: BJT Common Emitter Q-Point

Setup: V_CC = 12V, R₁ = 47kΩ, R₂ = 10kΩ, R_C = 2.2kΩ, R_E = 1kΩ, β = 100

R₁, R₂ form a voltage divider to set V_B
R_C is the collector load resistor
R_E provides thermal stability via negative feedback

                                    Step 1: VB = VCC × (R2 / (R1 +
                                    R2)) = 12 × (10k / 57k) = 2.1V

                                    Step 2: VE = VB − 0.7V = 1.4V

                                    Step 3: IE = VE / RE = 1.4V / 1kΩ =
                                    1.4mA

                                    Step 4: IC ≈ IE = 1.4mA

                                    Step 5: VC = VCC − (IC × RC) = 12
                                    − 3.08 = 8.9V

                                    Step 6: VCE = VC − VE = 8.9 − 1.4 =
                                    7.5V
                                

BJT Output Characteristics with Load Line

✓ Q-Point: (V_CE = 7.5V, I_C = 1.4mA) → Output can swing ±4V without clipping.

⚠️ Important: The Q-Point MUST sit on the DC Load Line! The load line is defined by Kirchhoff's Voltage Law: V_CC = I_C×R_C + V_CE + I_E×R_E. Every possible operating point for your circuit lies on this line. The Q-point is where this line intersects the transistor's I_B characteristic curve for your chosen base current.

Example 2: MOSFET Motor Driver (Switching Application)

Goal: Control a 10A DC Motor using a 3.3V ESP32 GPIO.

Q = N-Channel MOSFET (low-side switch)
D₁ = Flyback diode (reverse-biased normally; conducts back-EMF when motor turns off)
10kΩ = Gate pull-down (ensures OFF when MCU floats)

                                    Step 1: Choose a Logic-Level N-MOSFET (e.g., IRLZ44N, VGS(th) ≤
                                    2V)

                                    Step 2: Gate Drive = 3.3V → VGS > Vth(max) ✓

                                    Step 3: Power Loss: P = I² × RDS(on) = 10² × 0.022Ω =
                                    2.2W

                                    Step 4: Add 10kΩ Gate-Source pull-down resistor

                                    Step 5: Add 1N4007 flyback diode across motor terminals

💡 Tip: 2.2W dissipation needs a small heatsink or wide PCB copper pour for cooling.

Example 3: BJT Emitter Follower (CC) — Impedance Buffer

Problem: High-Z guitar pickup (1MΩ) driving a low-Z amplifier input (10kΩ).

Input: High-Z signal to Base
Output: Low-Z from Emitter
R_E sets the DC operating point
C₁ blocks DC, passes AC signal

                                    Voltage Gain: Av ≈ 1 (Unity gain, no amplification)

                                    Input Z: Zin ≈ β × RE = 200 × 10kΩ = 2MΩ

                                    Output Z: Zout ≈ re || (RS / β) → Very
                                        Low

✓ Result: Signal preserved. No "tone suck" effect when connecting to low-Z loads.

Example 4: MOSFET Common Source (CS) — Voltage Amplifier

Setup: V_DD = 15V, R_D = 470Ω, V_GS = 4V, V_th = 2V, k = 5mA/V²

R₁, R₂ set gate bias voltage
R_D = Drain load (output taken here)
R_S = Source degeneration (stability)

                                    Step 1: ID = k × (VGS − Vth)² = 5mA/V² ×
                                    (4−2)² = 20mA

                                    Step 2: VD = VDD − (ID × RD) = 15
                                    − 9.4 = 5.6V

                                    Step 3: gm = 2 × k × (VGS − Vth) = 2 × 5m ×
                                    2 = 20mS

                                    Step 4: Av = −gm × RD = −20mS × 470Ω =
                                    −9.4
                                

💡 Application: High input impedance amplifier for capacitive sensors or photodiodes.

Example 5: BJT Common Base (CB) — RF/VHF Application

Key Feature: No Miller Effect → Enables high-frequency operation.

Base is at AC ground (via C_B)
Input applied to Emitter (low-Z)
Output taken from Collector (high-Z)
No Miller capacitance → Fast!

                                    Current Gain: α = β / (β + 1) ≈ 1

                                    Input Z: Zin = re = (26mV / IE) = 26 / 1mA =
                                    26Ω (Very Low)

                                    Output Z: High (Same as RC)

💡 Use Case: Antenna impedance matching, VHF/UHF pre-amplifiers, high-speed switching.

Example 6: MOSFET Source Follower (CD) — Power Buffer

Goal: Drive a 100Ω buzzer from a weak microcontroller GPIO.

Drain connects to V_DD
Gate receives input signal
Source outputs to load
R_LOAD = Load being driven (buzzer/speaker)
Gain ≈ 1, but high current drive

                                    Output: VOUT = VIN − VGS ≈ VIN −
                                    3.5V

                                    Input Z: Effectively infinite (>100MΩ)

                                    Output Z: ≈ 1/gm → Very Low

✓ Benefit: Zero current drain from MCU pin while delivering power to low-Z load.

Example 7: MOSFET Voltage Divider Bias — Stable Q-Point

Setup: V_DD = 20V, R₁ = 1MΩ, R₂ = 1MΩ, R_D = 2kΩ, R_S = 1kΩ, V_th = 4V, k = 2mA/V²

R₁, R₂ set V_G = 10V
R_D = Drain load resistor
R_S = Provides thermal stability via negative feedback

                                    Step 1: VG = VDD × R2 / (R1 +
                                    R2) = 20 × 0.5 = 10V

                                    Step 2: VGS = VG − VS = VG −
                                    (ID × RS)

                                    Step 3: Solve: ID = k × (VGS − Vth)² →
                                    ID ≈ 2.8mA

                                    Step 4: VS = 2.8mA × 1kΩ = 2.8V

                                    Step 5: VD = 20 − (2.8mA × 2kΩ) = 14.4V

                                    Step 6: VDS = VD − VS = 14.4 − 2.8 =
                                    11.6V
                                

MOSFET Output Characteristics with Load Line

✓ Q-Point: (V_DS = 11.6V, I_D = 2.8mA) — Thermally stable due to R_S negative feedback.

💡 Quick Pro-Tips

Inductive Loads: If using a MOSFET or BJT to drive a relay or motor, always use a flyback diode.
Heat Dissipation: If your MOSFET is getting hot in a switching circuit, your Gate voltage might be too low. Use a Logic-Level MOSFET for 3.3V/5V microcontrollers.
High Power DC: MOSFETs are almost always superior for high-power DC switching due to their ultra-low Rds(on) resistance.

📚 Common Transistors

2N2222 (NPN) β≈100

BC547 (NPN) β≈200

2N3904 (NPN) β≈150

IRF540 (NMOS) V_th≈4V