Common Collector Amplifier
The common collector amplifier — universally known as the emitter follower — is the third and final basic BJT configuration, and in some ways it is the most unusual. Unlike the common emitter stage, it does not amplify voltage at all: the output voltage is almost exactly equal to the input voltage, giving a voltage gain of approximately 1 (0 dB). If voltage gain is not the goal, why use it?
The emitter follower's value lies not in voltage amplification but in impedance transformation. It presents a very high impedance to the signal source (loading it minimally) while delivering that signal into a very low output impedance (driving a low-impedance load with ease). This makes it the ideal buffer between a high-impedance source and a low-impedance load — exactly the situation that arises constantly in amplifier chain design. Understanding the emitter follower will explain a large number of design choices you encounter in radio schematics.
The common collector (emitter follower) amplifier. The collector is connected directly to VCC — it is at AC ground (VCC is a fixed supply, which is AC ground for signal purposes). Input enters at the base; the output is taken from the emitter. The emitter voltage "follows" the base voltage.
View LargerThe Common Collector Circuit
In the common collector configuration, the collector terminal is common to both the input and output signal paths. In practice, this means the collector is connected directly to the supply voltage VCC (with no collector resistor). Since VCC is a constant DC voltage, the collector is at AC ground — it does not move with the signal. The input signal is applied to the base (through a bias network and coupling capacitor), and the output is taken from the emitter (through an emitter resistor RE and output coupling capacitor).
Notice what is absent compared to a common emitter stage: there is no collector resistor RC. This is the key structural difference. Without RC there is no mechanism to convert collector current variations into collector voltage variations. The collector voltage is fixed at VCC. The output is not at the collector — it is at the emitter, which means the output follows whatever the emitter is doing in response to the input at the base.
The biasing of the common collector stage is straightforward. R1 and R2 form a voltage divider to set the base bias, exactly as in the common emitter circuit. The emitter resistor RE serves as both the current-setting element and the load — the output signal appears across RE. There is no separate load resistor at the collector because nothing useful happens at the collector in this configuration.
Why the Gain is Unity
Consider what happens when the input voltage at the base rises by a small amount ΔV. The base-emitter voltage VBE = Vbase − Vemitter. For the transistor to remain in its active region, VBE must stay at approximately 0.6–0.7 V (for silicon). If the base rises by ΔV, the emitter must also rise by ΔV to maintain this constant VBE relationship. The emitter follows the base almost exactly, one VBE below it at all times.
This is where the name "emitter follower" comes from: the emitter voltage follows the base voltage. The output (at the emitter) tracks the input (at the base) with almost no voltage difference — the only difference is the fixed DC offset of VBE. For AC signals (which are separated from DC by the coupling capacitor), the output is essentially identical to the input in amplitude and phase.
Av ≈ RE / (RE + re) ≈ 1 (since RE >> re in most practical circuits)
Input impedance:
Zin = R1 ‖ R2 ‖ β(RE + re) ≈ R1 ‖ R2 ‖ β×RE (very high)
Output impedance:
Zout ≈ re + Rsource/β (very low — typically a few ohms to tens of ohms)
The gain is slightly less than 1 because re is in series with RE in the emitter path. In most practical circuits RE is much greater than re, making RE/(RE + re) very close to 1. For example, with RE = 1 kΩ and re = 26 Ω: Av = 1000/(1000 + 26) = 0.975 — less than 2.5% below unity. This is called "approximately unity" voltage gain. There is no phase inversion: the output is in phase with the input.
Impedance Transformation
The real power of the emitter follower lies in what happens to impedance. The input impedance is very high because the transistor's current gain β magnifies the effective emitter resistance as seen at the base. The output impedance is very low because re and the source resistance divided by β appear as the output impedance at the emitter.
re = 26/1 = 26 Ω
Transistor input impedance (base to emitter) = β × (RE + re) = 150 × 1026 = 153,900 Ω = 154 kΩ
Zin = R1 ‖ R2 ‖ 154 kΩ (if R1 = 100 kΩ, R2 = 22 kΩ: bias network parallel = 18 kΩ)
Zin ≈ 18 kΩ ‖ 154 kΩ = 15.9 kΩ
Zout = re + Rsource/β = 26 + 10,000/150 = 26 + 67 = 93 Ω
This stage transforms a 10 kΩ source impedance to approximately 93 Ω output impedance. The voltage gain is 0.97. A subsequent common emitter stage seeing 93 Ω source will suffer much less loading loss than if directly driven from the 10 kΩ source.
The emitter follower as an impedance buffer. A high-impedance source (left) drives the base with minimal loading. The emitter presents a low output impedance that can drive a low-impedance load without signal loss.
View LargerCurrent Gain and Power Gain
Although the emitter follower has unity voltage gain, it has significant current gain and therefore power gain. The input current is the small base current Ib, while the output current is the emitter current Ie ≈ β × Ib. A stage with β = 100 can source 100 times more current at its output than flows into its input. This is the fundamental power gain of the emitter follower.
Power gain = Vout × Iout / Vin × Iin. Since Vout ≈ Vin (unity voltage gain) and Iout = β × Iin, the power gain is approximately β. For a typical transistor with β = 100, this is 20 dB of power gain. The emitter follower does not amplify voltage — it amplifies current, and because power is voltage × current, it amplifies power.
This is the crucial insight: the emitter follower is a power amplifier even though it is a voltage buffer. It enables a low-power signal source (perhaps a low-current oscillator or filter output) to drive a low-impedance load (a 50 Ω cable, an audio loudspeaker, an antenna tuner input) without the signal being loaded down.
Using the Emitter Follower as a Buffer
The emitter follower's role as a buffer between stages is one of the most important concepts in amplifier chain design. Here is a concrete problem it solves:
Suppose you have designed a high-Q resonant filter with a resonant impedance of 50 kΩ, and you need to drive a 50 Ω coaxial cable from its output. Connecting the cable directly loads the filter — the 50 Ω load is in parallel with the 50 kΩ source impedance of the filter. The parallel combination is approximately 50 Ω, which is only 0.1% of the filter's resonant impedance. This loading dramatically de-Qs the filter and drops the signal level by nearly 1000:1 (60 dB). The filter no longer works properly.
Insert an emitter follower between the filter and the cable: the emitter follower presents perhaps 40 kΩ input impedance to the filter (only slight loading), and delivers the signal into the 50 Ω cable from its own low output impedance of perhaps 50 Ω. The filter sees a light load and retains its Q; the cable sees a proper source impedance and receives the full signal.
This buffering function appears everywhere in radio equipment:
- After crystal or LC oscillator tanks to prevent loading from altering the oscillation frequency
- Between driver stages and power output stages to prevent the high-current demand of the final stage from disturbing the driver's operating point
- Between audio filter networks and loudspeaker circuits
- In VFO (variable frequency oscillator) circuits where the output must be stable against load variations
- In signal distribution circuits where one oscillator must drive multiple loads simultaneously without the impedance interaction between loads affecting each other
Ham Radio Applications
VFO buffer: Every variable frequency oscillator in a transceiver needs to be buffered from the loads it drives. A VFO that is frequency-stable when driving one stage may shift frequency when an additional load is connected, because the additional current draw changes the transistor's operating point. An emitter follower buffer after the VFO tank circuit presents a stable, consistent load to the oscillator and drives all downstream stages from its own low output impedance, regardless of how many loads are connected.
Microphone preamp output: The output impedance of a condenser microphone is typically 1 kΩ or higher. The input of a speech processor or audio mixer may be only 600 Ω. An emitter follower (or FET source follower, which operates identically) provides the impedance transformation to deliver maximum signal with minimum loss.
Driver-to-final interface: In a multi-stage RF power amplifier, the driver stage may have an output impedance of several kilohms while the final transistor requires a low source impedance (a few ohms or tens of ohms) for optimum power transfer. An emitter follower or a common collector stage with an impedance-matching transformer between driver and final solves this interface mismatch.
T/R relay driver: A logic signal from a microcontroller may be able to source only 5 mA, but driving a relay coil requires 50 mA or more. An emitter follower (or more often, a common emitter switch) provides the current gain needed to activate the relay from a weak logic signal. The emitter follower's high current gain (β) allows a tiny input current to control a much larger output current.
⚖ Experiment: Emitter Follower as a Buffer
This experiment demonstrates the emitter follower's impedance transformation by showing how it protects a signal source from a low-impedance load that would otherwise attenuate the signal severely.
- One 2N3904 or BC547 NPN transistor
- Resistors: 47 kΩ, 10 kΩ, 1 kΩ, 100 Ω (all ¼ W)
- Capacitors: 10 µF electrolytic × 2
- 9 V battery
- Breadboard
- Multimeter (AC millivolts or AC volts range) and signal source (function generator, audio oscillator, or mobile phone headphone output)
- Build a simple voltage divider using two 47 kΩ resistors as a "high-impedance signal source simulation": connect the signal generator through a 47 kΩ series resistor to the base of the circuit. This simulates a high-impedance signal source.
- Apply a 1 kHz sine wave, 1 V peak-to-peak from the generator. Measure the voltage at the source end of the 47 kΩ resistor (at the generator output) and at the circuit input end of the 47 kΩ resistor. Record both.
- First, connect the low-impedance load (100 Ω resistor) directly across the signal source through the 47 kΩ series resistor (with no transistor). The 100 Ω load in parallel with any other impedance will load down the source heavily. Measure the voltage across the 100 Ω load — it will be very low compared to the source voltage (voltage divider action: 100 Ω / (47,000 + 100) ≈ 0.2%).
- Now build the emitter follower: R1 = 47 kΩ (VCC to base), R2 = 10 kΩ (base to ground), RE = 1 kΩ (emitter to ground). Couple the signal into the base through a 10 µF capacitor. Connect the 100 Ω load to the emitter through a second 10 µF coupling cap. Power from 9 V battery.
- Drive the emitter follower's base with the signal through the 47 kΩ source resistor. Measure the voltage at the base and at the emitter output across the 100 Ω load. Compare the output voltage to the source voltage.
Step 3 (direct connection): with a 47 kΩ source driving a 100 Ω load directly, the signal is attenuated to about 0.2% of its original level — a 54 dB loss. Step 5 (with emitter follower): the output at the 100 Ω load is close to the original signal level, perhaps 80–90% of it. The emitter follower's high input impedance loads the 47 kΩ source only slightly, while its low output impedance drives the 100 Ω load effectively. This is the buffer function in action: same signal, same source, same load — but the intermediate transistor transforms the impedance and preserves the signal.
Frequently Asked Questions
If the emitter follower has unity voltage gain, how is it useful?
Voltage gain is only one measure of an amplifier's usefulness. The emitter follower provides significant current gain (approximately β) and therefore power gain (approximately β, or 20 dB for β = 100). It also provides impedance transformation: a high-impedance source can drive a low-impedance load through the emitter follower without signal loss. In many circuits what matters is not amplifying the voltage but driving a low-impedance load from a high-impedance source — and the emitter follower does this excellently. It is an essential building block wherever impedance matching or current drive capacity is needed.
Why is there no collector resistor in the emitter follower?
Because the output is taken from the emitter, not the collector. In a CE stage, the collector resistor converts current changes into voltage changes at the collector. In the emitter follower, the emitter resistor RE serves as the output load — the output voltage appears across RE. Since nothing useful is taken from the collector, there is no need for a collector resistor. The collector connects directly to VCC. This is the defining structural feature of the common collector configuration.
Does the emitter follower work with FETs too?
Yes. The FET equivalent of the emitter follower is the source follower (common drain configuration). It operates identically: the gate is the input, the source is the output, and the drain is connected to the supply. The source voltage follows the gate voltage, one Vgs below it. FET source followers have an even higher input impedance than BJT emitter followers (essentially infinite gate input impedance for MOSFETs) and a similar low output impedance. They are widely used in high-impedance measurement probes, radio input stages, and audio preamplifiers where minimal loading of the signal source is critical.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.