Emitter resistance is the total opposition to current flow in a bipolar junction transistor's (BJT) emitter leg, consisting of both the internal dynamic junction resistance (re) and any external physical resistor (RE) you solder into the circuit to stabilize bias and control gain. In a real circuit, adding external emitter resistance introduces local negative feedback (emitter degeneration), which deliberately trades raw voltage gain for vastly improved signal linearity, wider bandwidth, and thermal stability. The most common point of confusion for hobbyists and students is conflating the temperature-dependent internal re (which you calculate) with the fixed external RE (which you physically select and install).
The Two Faces of Emitter Resistance: Internal vs. External
To design a reliable common-emitter amplifier, you must treat emitter resistance as two distinct variables that interact on the workbench.
This is the small-signal AC resistance of the base-emitter PN junction itself. You cannot buy this component; it is a physical property of the silicon. It is inversely proportional to the DC emitter current and is calculated as re ≈ 26mV / IE at room temperature (25°C). If your transistor gets hot, that 26mV thermal voltage (VT) rises, shifting your gain.
This is the physical carbon-film or metal-film resistor you place between the emitter pin and ground. Its primary DC job is to prevent thermal runaway: as the transistor heats up and conducts more current, the voltage drop across RE increases, which reduces the base-emitter bias voltage (VBE) and throttles the current back down. Its AC job is to set your closed-loop voltage gain.
Think of an unbypassed external emitter resistor like a stiff shock absorber on a truck: it limits the maximum suspension travel (voltage gain) but keeps the chassis level and stable over rough terrain (signal linearity and thermal drift).
Worked Numeric Example: Biasing a 2N3904 Common-Emitter Amp
Let's design the emitter leg for a general-purpose audio preamp using a standard 2N3904 NPN transistor. We want a quiescent collector current (IC) of 2mA from a 12V supply to keep the noise floor low while maintaining adequate headroom.
Step 1: Sizing the External RE for DC Stability
For good thermal stability and symmetrical voltage swing, we typically drop about 10% to 15% of VCC across the emitter resistor. Let's target VE = 1.2V.
- RE = VE / IE (Assuming IE ≈ IC = 2mA)
- RE = 1.2V / 0.002A = 600Ω
- Bench Pick: Use a standard 620Ω 1/4W metal-film resistor.
Step 2: Calculating Internal re
- re = 26mV / 2mA = 13Ω
Step 3: The Gain Trade-Off (Bypassed vs. Unbypassed)
Assume our collector resistor (RC) is 2.2kΩ. The AC voltage gain (Av) is roughly -RC divided by the total AC emitter resistance.
- Unbypassed (No capacitor): Total AC resistance is RE + re = 620 + 13 = 633Ω.
Av = -2200 / 633 = -3.47. (Highly linear, very stable, low gain). - Fully Bypassed (Capacitor across RE): The capacitor shorts out RE for AC signals. Total AC resistance is just re = 13Ω.
Av = -2200 / 13 = -169. (Massive gain, but highly prone to clipping and thermal drift).
Where You Meet Emitter Resistance in Practice
You will encounter emitter degeneration in almost every linear analog circuit that relies on discrete BJTs. According to foundational texts like the All About Circuits Semiconductor volume, mastering this concept is the bridge between making a transistor act as a simple digital switch and making it act as a precision analog amplifier.
- Audio Preamplifiers: Microphone preamps use unbypassed or partially bypassed emitter resistors to keep Total Harmonic Distortion (THD) below 0.1%. Fully bypassing the emitter in an audio stage usually results in harsh, asymmetric clipping on loud transients.
- RF Oscillators (e.g., Colpitts): In high-frequency circuits, a small unbypassed RE (often 10Ω to 47Ω) is used to linearize the transistor's transconductance (gm), which reduces phase noise and prevents the oscillator from drifting as the transistor heats up.
- Constant Current Sinks / LED Drivers: Here, RE isn't used for gain at all. It acts as a shunt sense resistor. If you want a precise 20mA LED driver, you set the base voltage to 1.2V and use a 60Ω emitter resistor. The transistor will automatically adjust its internal resistance to maintain exactly 20mA through the LED, regardless of the LED's forward voltage.
Decision Tree: Selecting and Bypassing Your Emitter Resistor
Do not default to fully bypassing your emitter resistor just to get maximum gain. Use this decision matrix to choose the right configuration for your specific build.
| Circuit Goal | Configuration | Concrete Bench Pick (for 2mA IC) |
|---|---|---|
| Maximum AC voltage gain (e.g., RF detector, simple sensor trigger) | Fully Bypassed | 620Ω RE with a 10µF electrolytic capacitor in parallel. Warning: High distortion. |
| Maximum linearity and lowest distortion (e.g., studio mic preamp, instrumentation) | Unbypassed | 620Ω RE with NO capacitor. Gain will be low (~3.5); you may need a second gain stage. |
| Best of both worlds: Good gain + Good linearity (General Audio/Sensor) | Split Emitter (Partial Bypass) | Use two resistors: 100Ω (unbypassed) + 510Ω (bypassed with 47µF cap). Total DC = 610Ω. |
Frequently Asked Questions
Why does my fully bypassed emitter amplifier sound terrible on bass frequencies?
The emitter bypass capacitor (CE) forms a high-pass filter with the emitter resistance. If CE is too small, the AC gain rolls off at low frequencies. To ensure full 20Hz audio bandwidth, size CE so its reactance is at least 10 times smaller than RE at your lowest target frequency. For a 620Ω resistor at 20Hz, you need a minimum of 13µF. Always round up to a standard 47µF or 100µF electrolytic capacitor to be safe.
Does the internal re change if I use a different transistor like a 2N2222?
No. The formula re ≈ 26mV / IE is derived from the Shockley diode equation and applies to the silicon PN junction itself, regardless of whether it's inside a 2N3904, a 2N2222, or a power transistor like a TIP31. The only things that change re are the DC emitter current and the physical temperature of the silicon die.
My unbypassed emitter amp is oscillating at high frequencies. How do I fix it?
Unbypassed emitter stages have wider bandwidth, which can expose you to parasitic RF oscillation caused by lead inductance and stray capacitance. To fix this, add a 47Ω to 100Ω carbon composition base stopper resistor directly at the base pin of the transistor. This kills the RF Q-factor without affecting your audio or DC bias.
