Calculating Bias Resistorsemitter Follower
'/%3E%3Ccircle cx='48' cy='39' r='19' fill='%23f2c9a5'/%3E%3Cpath d='M27 35c3-16 39-17 42 2-8-8-27-9-42-2z' fill='%23374151'/%3E%3Cpath d='M21 86c5-24 49-28 56 0z' fill='%232563eb'/%3E%3Ccircle cx='41' cy='41' r='2' fill='%23111827'/%3E%3Ccircle cx='55' cy='41' r='2' fill='%23111827'/%3E%3Cpath d='M42 52c4 3 9 3 13 0' stroke='%2392400e' stroke-width='3' fill='none' stroke-linecap='round'/%3E%3C/svg%3E)
Reviewed by Calculator Editorial Team
An emitter follower is a common amplifier configuration in bipolar junction transistor (BJT) circuits. Proper bias resistor selection is critical for stable operation. This guide explains how to calculate bias resistors for emitter follower circuits, including the formulas, assumptions, and practical considerations.
Introduction
An emitter follower is a class A amplifier configuration that provides high input impedance and low output impedance. It's commonly used as a buffer stage between other amplifier stages. The key components of an emitter follower circuit are the transistor, bias resistors, and coupling capacitors.
Proper bias resistor selection ensures the transistor operates in its active region, provides adequate gain, and maintains stable DC operating points. The bias resistors establish the DC bias current through the transistor while allowing the AC signal to pass through the coupling capacitors.
Formula
The bias resistors for an emitter follower circuit can be calculated using the following formulas:
Collector Bias Resistor (RC)
RC = (VCC - VCE) / IC
Where:
- VCC = Supply voltage
- VCE = Collector-emitter voltage (typically 0.7V for silicon transistors)
- IC = Collector current
Emitter Bias Resistor (RE)
RE = VBE / IE
Where:
- VBE = Base-emitter voltage (typically 0.7V for silicon transistors)
- IE = Emitter current
Note: The collector and emitter currents are related by the transistor's current gain (β). Typically, IC = IE / β.
Example Calculation
Let's calculate the bias resistors for an emitter follower circuit with the following parameters:
- VCC = 12V
- VCE = 0.7V
- VBE = 0.7V
- IC = 1mA
- β (current gain) = 100
Step 1: Calculate Collector Bias Resistor (RC)
RC = (VCC - VCE) / IC = (12V - 0.7V) / 1mA = 11.3V / 0.001A = 11,300Ω
Step 2: Calculate Emitter Current (IE)
IE = IC × β = 1mA × 100 = 100mA
Step 3: Calculate Emitter Bias Resistor (RE)
RE = VBE / IE = 0.7V / 0.1A = 7Ω
The calculated bias resistors are RC = 11,300Ω and RE = 7Ω.
Interpreting Results
The calculated bias resistors ensure the transistor operates in its active region with the desired collector current. The emitter resistor provides additional stabilization by reducing the effect of β variations.
When selecting actual resistor values, use standard E96 or E24 series values that are close to the calculated values. The actual resistor values may differ slightly from the calculated values, but they should be within a reasonable range to maintain stable operation.
FAQ
- What is the purpose of bias resistors in an emitter follower circuit?
- The bias resistors establish the DC operating point for the transistor, ensuring it operates in its active region while allowing AC signals to pass through the coupling capacitors.
- How do I choose the correct bias resistor values?
- Use the formulas provided in this guide to calculate the required resistor values based on your circuit parameters. Then select standard resistor values that are close to the calculated values.
- What happens if the bias resistors are too large or too small?
- If the resistors are too large, the transistor may not have enough current to operate properly. If they are too small, the transistor may be overloaded and may not function correctly.
- Can I use the same resistor value for both the collector and emitter?
- No, the collector and emitter resistors serve different purposes. The collector resistor sets the DC operating point, while the emitter resistor provides additional stabilization.
- How does the transistor's current gain (β) affect the bias resistor calculation?
- The current gain (β) affects the emitter current calculation. A higher β means the emitter current will be larger for the same collector current, which affects the emitter resistor calculation.