Input Resistance of Emitter Follower Calculator

An emitter follower is a common amplifier configuration that provides voltage gain while maintaining a low output impedance. Calculating its input resistance is essential for circuit design and analysis. This calculator provides a precise method to determine the input resistance of an emitter follower circuit.

Introduction

The emitter follower is a fundamental amplifier configuration that uses a single transistor to provide voltage gain while maintaining a low output impedance. This configuration is widely used in audio amplifiers, signal buffers, and other applications where impedance matching is important.

One of the key parameters of an emitter follower is its input resistance. The input resistance determines how well the circuit can accept signals from a preceding stage without significant loading effects. Calculating this parameter accurately is crucial for proper circuit design and performance optimization.

Formula

The input resistance of an emitter follower can be calculated using the following formula:

R_in = β × r_π

Where:

R_in = Input resistance (ohms)
β = Current gain of the transistor (h_FE)
r_π = Small-signal input resistance (ohms)

The small-signal input resistance r_π is calculated as:

r_π = V_T / I_B

Where:

V_T = Thermal voltage (approximately 26 mV at room temperature)
I_B = Bias current (amperes)

How to Use the Calculator

Enter the current gain (β) of your transistor in the first field.
Enter the bias current (I_B) in amperes in the second field.
Click the "Calculate" button to compute the input resistance.
The calculator will display the calculated input resistance in ohms.
Use the "Reset" button to clear all fields and start a new calculation.

Note: The calculator assumes room temperature (26 mV thermal voltage). For precise calculations at different temperatures, you may need to adjust the thermal voltage value.

Example Calculation

Let's calculate the input resistance for a transistor with β = 100 and I_B = 10 μA (0.00001 A).

Calculate r_π:

r_π = 0.026 / 0.00001 = 2600 ohms
Calculate R_in:

R_in = 100 × 2600 = 260,000 ohms

The calculated input resistance is 260,000 ohms.

Interpreting Results

The input resistance value indicates how effectively the emitter follower can accept signals from a preceding stage. A higher input resistance generally means better signal acceptance with less loading effect. However, very high input resistance may indicate a problem with the circuit design or transistor parameters.

Typical input resistance values for emitter followers range from several thousand ohms to several hundred thousand ohms, depending on the transistor characteristics and bias current. The exact value should be considered in the context of your specific circuit requirements.

FAQ

What is the difference between input resistance and output resistance in an emitter follower?: The input resistance determines how well the circuit can accept signals from a preceding stage, while the output resistance determines how well it can drive a following stage. An emitter follower typically has high input resistance and low output resistance.
How does the bias current affect the input resistance?: The bias current (I_B) directly affects the small-signal input resistance (r_π), which in turn affects the overall input resistance. Higher bias currents result in lower r_π and consequently lower input resistance.
Can I use this calculator for different transistor types?: Yes, the calculator can be used for any bipolar junction transistor (BJT) as long as you know its current gain (β) and can determine the appropriate bias current for your circuit.
What factors should I consider when designing an emitter follower circuit?: Key factors include selecting an appropriate transistor, determining the correct bias current, ensuring proper load resistance, and considering the thermal characteristics of the transistor. The input resistance calculation is just one aspect of the overall design process.