Common Collector
Part of The Transistor
The emitter follower configuration — unity voltage gain with high current drive capability and impedance transformation.
Why This Matters
The common collector (emitter follower) configuration is the standard solution to a common problem: you have a signal at high impedance that needs to drive a low-impedance load without significant voltage loss. A microphone output driving a long cable, a sensor output driving a meter, an amplifier output driving a speaker — all these benefit from the emitter follower’s impedance transformation.
The emitter follower outputs essentially the same voltage as its input but draws very little current from the source (high input impedance) while supplying large current to the load (low output impedance). Current gain up to hFE+1 makes it a powerful buffer between impedance-mismatched stages.
For a rebuilding civilization, the emitter follower is used in nearly every non-trivial circuit: at the input to each amplifier stage to prevent loading, at the output to drive loads, and as power stage drivers. Understanding it thoroughly enables you to place it correctly and calculate its parameters without trial and error.
Circuit Configuration
In common collector, the collector is connected to the supply rail (or through a small decoupling resistor). The input signal is applied to the base. The output is taken from the emitter through a load resistor to ground.
The name “common collector” indicates that the collector is the common terminal between input and output signal paths (both are referenced against the collector rail).
Schematic (NPN, single supply):
Vcc ─────────┬──── Collector
│
R1 ───┤
│ Base ← Input (via Cin)
R2 ───┤
│
Ground ──────┘
│ Emitter ──→ Output (via Cout or direct)
│
RE
│
Ground ──────┘
Bias is provided by R1/R2 voltage divider. RE is the emitter load. Input coupling capacitor Cin blocks DC from source. Output can be direct (for DC-coupled applications) or through coupling capacitor for AC.
Gain and Impedance Analysis
Voltage gain: Av = RE / (RE + re) where re = VT/IC = 26 mV / IC (intrinsic emitter resistance).
For RE = 1 kΩ and IC = 1 mA: re = 26 Ω. Av = 1000 / (1000 + 26) = 0.974 ≈ 1.
The gain is less than 1 (typically 0.97-0.99 for RE >> re) but non-inverting. As RE increases, gain approaches 1 more closely. As IC increases (re decreases), gain also approaches 1. For most practical calculations, Av ≈ 1 is sufficient.
Input impedance: Zin = R1 || R2 || [hFE × (RE || r_load + re)]
The transistor’s contribution to input impedance: hFE × (RE + re). For hFE = 100, RE = 1 kΩ: contribution = 100 × 1026 = 102 kΩ. Combined with R1 || R2 (typically 10-100 kΩ for voltage divider bias), overall Zin is typically 10-50 kΩ — much higher than the emitter resistor alone.
This is the emitter follower’s key advantage: looking into the base, the emitter resistor appears hFE times larger than its actual value. This is why it buffers high-impedance sources — the high-impedance source sees a high-impedance load (the emitter follower input) rather than the actual low-impedance load.
Output impedance: Zout = RE || [(Rs + rπ) / hFE + re]
where Rs is the source impedance driving the base, rπ = hFE × VT / IC (base-emitter resistance from small-signal model).
For Rs = 10 kΩ, rπ = 2.6 kΩ, hFE = 100, re = 26 Ω: Zout = RE || [(10000 + 2600)/100 + 26] = RE || [126 + 26] = RE || 152 Ω
For RE = 1 kΩ: Zout ≈ 132 Ω — much lower than the source impedance of 10 kΩ.
The general rule: output impedance of the emitter follower is approximately the source impedance divided by hFE. A 10 kΩ source driving an hFE=100 emitter follower produces ~100 Ω output impedance. This makes it an effective impedance divider of ~100:1 — precisely what is needed to drive low-impedance loads from high-impedance sources.
Current gain: Iout / Iin = (hFE + 1) ≈ hFE for large hFE. The emitter follower provides near-unity voltage gain but high current gain.
Design Procedure
Design an emitter follower buffer for audio signal:
- Source: microphone with 10 kΩ impedance, 50 mV signal
- Load: 600 Ω line input
- Supply: 9V
- Choose IC = 2 mA (moderate current for good re and drive capability)
- re = 26 mV / 2 mA = 13 Ω (low, for near-unity gain)
- RE: choose for VE = 4V (half supply for maximum swing). RE = 4V / 2 mA = 2 kΩ
- Maximum output swing: ±(VE - VCEsat - VLoad_min) ≈ ±3.5V — more than adequate for 50 mV input
- Voltage gain: Av = (RE || 600) / (RE || 600 + re) = (462) / (475) = 0.97
- Output impedance: ≈ (10000 + rπ) / hFE + re ≈ (10000 + 1300) / 100 + 13 ≈ 126 Ω
- Bias: VB = VE + VBE = 4 + 0.7 = 4.7V. Bias resistors for stability.
The 600 Ω load is now driven from 126 Ω source — minimal loading loss, clean signal transfer.
Practical Applications
Microphone preamp input: Electret microphones have ~2 kΩ internal impedance and small signal level. An emitter follower as the first stage presents >50 kΩ input impedance, preventing loading, then feeds a common-emitter amplifier with low source impedance.
Tone arm/sensor buffer: High-impedance sensors (thermocouples, some pH electrodes, piezo sensors) need to drive amplifier inputs over long cable lengths. Cable capacitance with high source impedance creates an RC low-pass filter that attenuates high frequencies. An emitter follower placed close to the sensor reduces effective source impedance from kiloohms to hundreds of ohms, eliminating the problem.
Meter driver: Moving-coil meters have 50-1000 Ω internal resistance and need 100 µA to 1 mA for full-scale deflection. A transistor amplifier’s common-emitter output impedance is typically 5-10 kΩ — too high to drive a meter directly. An emitter follower drives the meter with low output impedance, making the meter reading independent of the amplifier’s output impedance.
Push-pull output stage complementary emitter followers: Two emitter followers, one NPN conducting positive half-cycles and one PNP conducting negative half-cycles, together form a push-pull output stage. Each is a common-collector circuit. Together they provide rail-to-rail drive capability with low output impedance. This is the standard audio output stage. Small bias current through both transistors (class AB) eliminates the crossover distortion.
Bootstrap: In some circuits, the bias network (R1, R2) loads the input. To increase effective input impedance further, “bootstrapping” connects the junction of R1 and R2 to the output (emitter) through a capacitor. Since the emitter follows the base (voltage gain ≈ 1), the voltage across R1 is nearly constant — no current flows through it. The input impedance appears infinite to signal frequencies. This technique raises input impedance to the MΩ range for transistors with hFE = 100-200.