Emitter Follower Symbol
Definition: The Emitter Follower symbol represents a common-collector bipolar junction transistor (BJT) amplifier stage in which the output is taken from the emitter terminal, providing unity voltage gain, high input impedance, and low output impedance, as depicted in IEEE 315-1975 / ANSI Y32.2 using the standard NPN or PNP BJT symbol with the output connected at the emitter node.
Also known as: common-collector amplifier, voltage follower (BJT), BJT buffer, unity-gain buffer, emitter follower buffer, CC amplifier.
What the Emitter Follower symbol means
The Emitter Follower symbol denotes a BJT configured in the common-collector topology: the input signal is applied to the Base, the collector is connected to the supply (VCC), and the output is taken from the Emitter through a load resistor to ground. The output voltage 'follows' the input voltage with a gain of approximately 1 (unity), shifted down by one VBE diode drop (~0.7 V for silicon NPN).
In schematic diagrams, this symbol or block appears wherever a high-impedance signal source must drive a low-impedance load without significant voltage attenuation. The emitter follower acts as an impedance transformer, isolating the source from the load and delivering significant current gain (β) even though voltage gain is near unity. It is one of the most fundamental analogue building blocks in transistor circuit design.
How to identify the Emitter Follower symbol
The Emitter Follower block symbol is drawn as a rectangle labelled 'Emitter Follower' or 'Common Collector' with three pins: Base (left side, input), VCC (top right, collector supply), and Emitter (bottom right, output). In a full transistor-level schematic, the NPN BJT symbol is used: a vertical base line, a diagonal collector line pointing up-right, and a diagonal emitter line pointing down-right with an outward-pointing arrow (NPN convention). The collector connects to VCC and the emitter drives the output load.
Function in a circuit
The emitter follower provides current amplification (current gain = β+1) with near-unity voltage gain. When a signal is applied to the Base, the transistor regulates collector-to-emitter current so that the emitter voltage tracks the base voltage minus VBE (~0.7 V). The high input impedance (βRE) prevents loading the signal source, and the low output impedance (RS/β + re, typically tens of ohms) allows the stage to drive relatively large currents into low-impedance loads such as loudspeakers, transmission lines, or subsequent low-resistance stages.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-05 defines the NPN BJT symbol (a circle with base, collector, and emitter lines; the emitter carries an outward arrow). The emitter follower topology is not given a separate symbol in IEC 60617 — it is identified by the circuit connection at the collector node tied to supply. |
|---|---|
| ANSI/IEEE 315 | IEEE 315-1975 / ANSI Y32.2 defines the NPN BJT symbol with an outward-pointing emitter arrow and the collector line pointing upward. The common-collector (emitter follower) configuration is indicated by circuit connection, not a distinct symbol. The block symbol for the emitter follower stage is a functional representation used in system-level schematics. |
| Key difference | IEC and ANSI/IEEE use essentially the same BJT transistor symbol conventions (outward emitter arrow for NPN, inward for PNP). There is no meaningful graphical difference between IEC and ANSI for the BJT itself; regional preference for block-level representations varies. |
Terminals / pins
| Pin | Name |
|---|---|
| base | Base |
| vcc | VCC |
| emitter | Emitter |
Typical values
Voltage gain: approximately 1 (unity, typically 0.95–0.99). Current gain: β+1 (common-emitter β typically 50–300). Input impedance: β × RE (high, kΩ to MΩ range). Output impedance: RS/β + re (low, typically 10–200 Ω). VBE offset: ~0.6–0.7 V (silicon NPN at room temperature). Bandwidth: extends to high frequencies due to low output impedance.
Where the Emitter Follower symbol is used
- Audio preamplifier output stages: emitter followers buffer the preamplifier output to drive cable capacitance and power amplifier input impedances
- Microcontroller GPIO driver stages: emitter followers boost the current capacity of a 3.3 V or 5 V GPIO pin to drive LEDs, relays, or motor drivers
- Impedance-matching stages in RF and instrumentation circuits where a high-impedance sensor signal must drive a 50 Ω or 75 Ω transmission line
- Voltage regulator pass stages: the emitter follower is the output element in series linear regulators (e.g., LM317 equivalent discrete design)
- Class-AB push-pull audio output stages: complementary NPN/PNP emitter follower pairs form the output stage of nearly all discrete and IC audio power amplifiers
- Base drive for Darlington transistor pairs, where a first emitter follower stage drives the base of a second to achieve very high current gain
Example
In a microphone preamplifier circuit, a 2N3904 NPN transistor is configured as an emitter follower: the Base pin receives the high-impedance microphone output (10 kΩ source impedance), the VCC pin connects to +9 V, and the Emitter pin drives a 1 kΩ load resistor to ground. The emitter output replicates the microphone signal at low impedance (~50 Ω), allowing a 1 m shielded cable to carry the signal to a mixer without high-frequency rolloff caused by cable capacitance.
Key facts
- The Emitter Follower is an NPN (or PNP) BJT in the common-collector configuration: input at Base, output at Emitter, collector connected to supply (VCC); this arrangement yields voltage gain ≈1 and current gain ≈β+1.
- The three pins in the block symbol are Base (input, left), VCC (collector supply, top), and Emitter (output, bottom right); the emitter arrow points outward for NPN per IEC 60617-05 and IEEE 315-1975.
- Voltage gain is slightly less than unity (typically 0.95–0.99) because the output is VIN − VBE; the 0.6–0.7 V VBE offset means the emitter sits approximately 0.7 V below the base for an NPN stage.
- Input impedance of the emitter follower is high (≈β × RE), preventing the stage from loading the signal source; output impedance is low (≈RS/β + re), enabling the stage to drive low-resistance loads efficiently.
- The NPN emitter arrow in the transistor symbol points outward (away from the base), indicating conventional current flows out of the emitter; a PNP emitter arrow points inward, and the emitter follower output would sit above the input by |VBE|.
- Also called a voltage follower (BJT variant), unity-gain buffer, or common-collector amplifier; the MOSFET equivalent configuration is the source follower (common-drain).
- The emitter follower has no voltage gain but provides power gain: it can deliver significant output current (IC = β × IB) from a small base drive current, making it essential wherever current amplification without voltage inversion is needed.
Frequently asked questions
What does the emitter follower symbol mean in a circuit diagram?
The emitter follower symbol represents a common-collector BJT amplifier stage that provides unity voltage gain, high input impedance, and low output impedance. It appears as a block with Base (input), VCC (supply), and Emitter (output) pins, or as a full NPN/PNP transistor symbol with the collector tied to supply and the output taken from the emitter.
What does an emitter follower look like in a schematic?
In a transistor-level schematic, an emitter follower is an NPN BJT with its collector connected to VCC (supply), its base driven by the input signal, and its emitter connected through a resistor to ground where the output is taken. The NPN symbol shows a base line, a collector line angled up, and an emitter line angled down with an outward-pointing arrow.
What is the voltage gain of an emitter follower?
The voltage gain of an emitter follower is approximately 1 (unity), typically 0.95–0.99. The output follows the input shifted down by approximately 0.7 V (the base-emitter voltage VBE of a silicon NPN transistor). Despite near-unity voltage gain, the stage provides current gain of β+1.
What is the difference between an emitter follower and a common-emitter amplifier?
An emitter follower (common-collector) takes its output from the emitter and has voltage gain ≈1 with no phase inversion and low output impedance. A common-emitter amplifier takes its output from the collector, provides high voltage gain (−gmRC), inverts the signal by 180°, and has relatively high output impedance. The emitter follower is used for impedance matching; the common-emitter is used for voltage amplification.
Why is the emitter follower used as a buffer?
The emitter follower is used as a buffer because it presents high input impedance (β × RE) to the signal source — preventing loading — and low output impedance (RS/β + re, typically tens of ohms) to the load. This impedance transformation allows a weak, high-impedance signal source to drive a heavy, low-impedance load without significant voltage loss.
What standard defines the emitter follower transistor symbol?
The BJT transistor symbol used in emitter follower schematics is defined in IEC 60617-05 (semiconductor devices) and IEEE 315-1975 / ANSI Y32.2. Both standards use the same NPN symbol: a circle with a vertical base line, an upward-angled collector line, and a downward-angled emitter line with an outward-pointing arrow indicating the NPN type.
What is the MOSFET equivalent of an emitter follower?
The MOSFET equivalent of the emitter follower is the source follower (common-drain configuration). In the source follower, the gate is the input, the drain connects to supply, and the output is taken from the source through a load resistor. Both circuits provide near-unity voltage gain with high input impedance and low output impedance.
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