A common base amplifier is a single-stage bipolar junction transistor (BJT) circuit where the base terminal is grounded (or AC-grounded via a capacitor), the input signal is applied to the emitter, and the amplified output is taken from the collector. In a real circuit, this topology fundamentally changes how your system handles impedance: it transforms a low-impedance current input into a high-impedance voltage output, making it the undisputed champion for matching low-impedance sources like 50-ohm RF antennas, coaxial cables, or moving-coil phono cartridges. Beginners and even intermediate builders commonly confuse it with the common emitter amplifier (which provides both voltage and current gain but chokes at high frequencies due to parasitic capacitance) or its FET cousin, the common gate amplifier.
The Core Mechanics: Voltage Gain Without Current Gain
To understand why the common base (CB) configuration behaves the way it does, you have to look at the current flow. In a BJT, the emitter current ($I_E$) is the sum of the base current ($I_B$) and the collector current ($I_C$). Because the base is grounded, the input signal drives the emitter directly. Almost all of the electrons injected into the emitter are swept across the reverse-biased base-collector junction into the collector.
This means the current gain ($\alpha$) is slightly less than 1 (typically 0.98 to 0.99). You are not amplifying current. However, you are taking that nearly identical current and forcing it through a much higher resistance at the collector ($R_C$) than the resistance seen at the emitter. Think of the emitter as a wide multi-lane highway feeding into a single-lane restriction (the base-emitter junction); the cars (charge carriers) do not multiply in number, but the pressure (voltage) builds up massively as they are forced through the collector's high-resistance path.
Worked Numeric Example: Designing a 50-Ohm RF Input Stage
Let's design a practical front-end amplifier to match a standard 50-ohm RF source using a common 2N2222 NPN transistor. Our goal is an input impedance ($Z_{in}$) of 50 $\Omega$ and a solid voltage gain.
1. Establish the Bias Current and Intrinsic Emitter Resistance
The intrinsic AC emitter resistance ($r_e$) is determined by the thermal voltage ($V_T \approx 26mV$ at room temperature) divided by the DC emitter current ($I_E$). Let's bias the transistor at $I_C \approx 2mA$.
$r_e = \frac{26mV}{2mA} = 13 \Omega$
2. Match the Input Impedance
In a pure common base circuit, $Z_{in} \approx r_e$. Since 13 $\Omega$ is too low for a 50-ohm coaxial source, we need to add an unbypassed external emitter resistor ($R_{E(ac)}$) to make up the difference.
$Z_{in} = r_e + R_{E(ac)}$
$50 \Omega = 13 \Omega + R_{E(ac)}$
$R_{E(ac)} = 37 \Omega$
3. Calculate Voltage Gain
We want a voltage gain ($A_v$) of 20 (which is 26 dB). The formula for CB voltage gain is $A_v = \frac{R_C}{r_e + R_{E(ac)}}$.
$20 = \frac{R_C}{50 \Omega}$
$R_C = 1000 \Omega$ (1k$\Omega$)
4. Verify DC Operating Point
With a 12V supply ($V_{CC}$), the voltage drop across $R_C$ is $I_C \times R_C = 2mA \times 1000\Omega = 2V$. The collector sits at 10V, leaving plenty of headroom for the RF signal to swing without clipping. In reality, the transistor's internal output resistance ($r_o$) sits in parallel with $R_C$, but for a 2N2222 at 2mA, $r_o$ is roughly 50k$\Omega$, making the 1k$\Omega$ $R_C$ the dominant factor.
Where You Meet This in Practice
You will rarely see a standalone common base amplifier in low-frequency audio gear, but it is ubiquitous in specific high-performance niches:
- RF Front Ends: Antennas and coaxial cables are typically 50 or 75 ohms. The CB stage provides a natural, low-noise impedance match without the bandwidth-killing Miller capacitance of a common emitter stage.
- The Cascode Configuration: This is the most common real-world application. A cascode stacks a common emitter transistor (for high input impedance and current gain) directly into a common base transistor (for voltage gain and high-frequency isolation). The CB transistor holds the CE transistor's collector voltage steady, killing the Miller effect entirely.
- Moving Coil (MC) Phono Preamplifiers: MC cartridges output incredibly low voltages with very low source impedance (often 10 to 40 ohms). A CB input stage provides the exact low-impedance load the cartridge wants to see while stepping up the voltage with minimal added noise.
- Current Buffers: Because the current gain is unity, CB stages are used to isolate a high-impedance current source from a low-impedance load, acting as a current buffer.
Common Base vs. Common Emitter: Quick Comparison Matrix
Choosing between the standard common emitter (CE) and the common base (CB) depends entirely on your source impedance and frequency requirements. For a deeper dive into BJT topologies, the Electronics Tutorials BJT guide provides excellent baseline schematics.
| Parameter | Common Emitter (CE) | Common Base (CB) |
|---|---|---|
| Input Impedance | Medium to High (1k$\Omega$ - 10k$\Omega$) | Very Low (10$\Omega$ - 100$\Omega$) |
| Output Impedance | Medium to High (10k$\Omega$ - 100k$\Omega$) | High (10k$\Omega$ - 1M$\Omega$) |
| Voltage Gain | High | High |
| Current Gain | High ($\beta$, typically 100+) | Unity ($\alpha \approx 1$) |
| Phase Shift | 180$^\circ$ (Inverting) | 0$^\circ$ (Non-inverting) |
| High-Frequency Response | Poor (Limited by Miller Effect) | Excellent (No Miller Effect) |
| Primary Use Case | General purpose audio/signal amplification | RF amplifiers, Cascode stages, impedance matching |
Common Base Amplifier FAQ
Why does a common base amplifier have such low input impedance?
The input is applied directly to the forward-biased base-emitter junction. In any bipolar transistor, a small change in base-emitter voltage ($V_{BE}$) causes a massive exponential change in emitter current. Because impedance is the ratio of voltage change to current change ($Z = \frac{\Delta V}{\Delta I}$), a junction that allows a flood of current for a tiny voltage push inherently presents a very low resistance to the source driving it. Mathematically, it is bounded by the intrinsic emitter resistance $r_e$, which is usually under 20 ohms at typical operating currents.
How does a common base amplifier compare to a common gate MOSFET circuit?
They are functional equivalents across the BJT and FET families. Both offer low input impedance, high output impedance, high voltage gain, and zero phase shift. However, the common gate (CG) MOSFET has an inherently higher input impedance than a BJT common base stage because a MOSFET's transconductance ($g_m$) is generally lower than a BJT's for a given bias current. If you need an input impedance of exactly 50 ohms, a BJT common base is often easier to bias predictably, whereas a CG MOSFET might require wider, more power-hungry devices to pull the impedance down to 50 ohms without external degeneration resistors.
Why is the common base configuration rarely used for audio amplifiers?
Audio sources (like microphones, DACs, and guitar pickups) typically have medium-to-high output impedances and require an amplifier with a high input impedance to prevent signal loading and high-frequency roll-off. The common base amplifier's extremely low input impedance would severely load down a standard audio source, resulting in massive signal attenuation and a terrible signal-to-noise ratio before the gain stage even begins. Audio designers rely on common emitter or common collector (emitter follower) topologies to bridge these impedance gaps. For further reading on audio impedance bridging, All About Circuits covers the loading effects in their semiconductor textbook.
How does a cascode circuit solve the common emitter bandwidth problem?
In a standard common emitter amplifier, the parasitic capacitance between the base and collector ($C_{bc}$) is multiplied by the stage's voltage gain due to the Miller effect, creating a massive low-pass filter that kills high-frequency response. In a cascode circuit, the common emitter transistor's collector is tied directly to the emitter of a common base transistor. The common base transistor acts as a current buffer that holds the CE transistor's collector voltage virtually constant (at AC ground). Because the voltage at the CE collector doesn't swing, the Miller multiplication factor drops to near zero, allowing the circuit to retain the high input impedance of the CE stage while achieving the massive bandwidth of the CB stage.
