Common Base Amplifier
Of the three basic transistor configurations, the common base amplifier is the least commonly encountered in general electronics — but in RF and high-frequency circuit design, it is indispensable. Its defining characteristic is a very low input impedance combined with a very high output impedance and, unlike the common emitter stage, it does not invert the phase of the signal. These properties make the common base configuration the natural choice for RF preamplifiers and cascode circuit topologies used at frequencies from HF through microwave.
Understanding the common base configuration will also deepen your understanding of why certain circuits in your transceiver are designed the way they are. The cascode preamplifier, for example — found in high-quality HF receivers — uses a common emitter input stage stacked on top of a common base stage. The common base portion provides the wide bandwidth and the isolation between input and output that prevents feedback oscillation at high frequencies.
The common base amplifier. The base terminal is at AC ground (bypassed by CB). The input signal enters through the emitter, and the amplified output is taken from the collector. Note that input and output do not share the same ground point — the emitter is the signal input ground and the base is the AC ground for signal purposes.
View LargerThe Common Base Circuit
In the common base configuration, the base terminal of the transistor is common to both the input and output signal paths — that is, the base is connected to AC ground (even if it sits at a DC potential above ground for biasing purposes). A capacitor CB bypasses the base to ground, making it an AC short at the signal frequency. The input signal is injected into the emitter terminal, and the output is taken from the collector terminal. Both the emitter (input) and collector (output) are referenced to the base, which is the common AC ground node.
This is almost the opposite of the common emitter configuration. In the CE stage you drive the base and take the output from the collector. In the CB stage you drive the emitter and take the output from the collector. The transistor is still controlling the same collector current — but the input port has moved from base to emitter, which has major consequences for the circuit's impedance and phase characteristics.
The DC biasing of a common base stage is straightforward. The transistor is biased in the active region exactly as before — you set the base voltage with a voltage divider (or a fixed voltage), set the emitter current with an emitter resistor, and connect a collector load resistor RC to the supply. The base bypass capacitor ensures that the base is at a stable AC ground while the emitter current variations (driven by the input signal) flow through the transistor and produce voltage changes across RC at the output.
How the CB Stage Works
Trace the signal flow: the input AC signal is applied to the emitter. A positive input voltage at the emitter drives the emitter more positive, which reduces the forward bias of the base-emitter junction (remember that VBE = Vbase − Vemitter, and Vbase is fixed at AC ground by CB). Reduced VBE means less collector current. Less collector current means less voltage drop across RC, which means the collector voltage rises. The output goes positive when the input goes positive — no phase inversion.
Alternatively: a negative input at the emitter increases the forward bias, increases collector current, increases the drop across RC, and the collector voltage falls. Input negative → output negative. In-phase response confirmed from both directions. This contrasts directly with the common emitter stage, where input positive → output negative (180° inversion).
The current gain of a common base stage is slightly less than 1. The collector current is slightly less than the emitter current because a small fraction of emitter current (the base current) exits through the base terminal rather than flowing through to the collector. In a transistor with β = 100, the current gain in common base configuration is α = β/(β+1) = 100/101 ≈ 0.99. This means almost all of the emitter current becomes collector current — the current gain is very close to unity but slightly less.
No Phase Inversion
The absence of phase inversion in the common base stage is a direct consequence of the signal path. In a CE stage, the base controls the transistor: more base current → more collector current → more voltage drop across RC → lower collector voltage. Input up, output down — inversion. In a CB stage, the emitter drives the transistor: more emitter current (driven by positive emitter input) → more collector current → more voltage drop across RC → lower collector voltage. But wait — if positive emitter input produces lower collector voltage, that would be an inversion too. The key is that a positive voltage at the emitter actually reduces, not increases, VBE (since VBE = VB − VE and VB is fixed). Reduced VBE means reduced collector current — so positive emitter input → reduced collector current → reduced drop across RC → higher collector output. No inversion.
This is more clearly seen by thinking in terms of current: the input signal is a current injected into the emitter. This emitter current (minus the tiny base current) becomes collector current. The collector current creates a voltage across RC. As emitter current increases (with positive input), collector current increases, and the output voltage (VCC − IC×RC) decreases... wait, that seems like inversion again.
The subtlety: in the CB configuration the signal polarity at the emitter is referenced to ground, not to the base. When the emitter terminal is driven positive with respect to ground, since the base is held at a fixed positive DC bias, the emitter is actually being driven toward the base potential, which means VBE is being reduced. This reduction in VBE reduces IC. The output rises. In-phase.
In practice, there is an easier way to remember this: the CB stage is essentially taking the signal current that flows into the emitter and passing it (almost unchanged) to the collector and through the collector load. The output voltage across the load increases when more current flows into the emitter from the signal source. This is a non-inverting relationship.
Gain and Impedance
The voltage gain of a common base stage is almost identical to the bypassed common emitter stage:
Av = RC / re (no phase inversion — positive value)
Common base input impedance:
Zin ≈ re (very low — typically 10–50 Ω)
Common base output impedance:
Zout ≈ RC (in parallel with transistor output resistance, usually very high, so ≈ RC)
where re = 26 mV / Ic (as before)
The voltage gain is the same as the CE stage (bypassed), which makes sense: the same transistor controlling the same collector current through the same RC — the gain mechanism is identical. What differs is the impedance seen at the input. In a CE stage, the input impedance is β × re (the transistor's base-to-emitter resistance scaled by beta). In a CB stage, the input impedance is just re — the emitter resistance alone, without the beta multiplication. This is a very low impedance, typically in the range of 10 to 50 Ω.
re = 26 mV / 5 mA = 5.2 Ω
Voltage gain: Av = RC / re = 1000 / 5.2 = 192 (no phase inversion)
Input impedance: Zin ≈ re = 5.2 Ω
Output impedance: Zout ≈ RC = 1 kΩ
The gain is very high (192) but the input impedance is only 5.2 Ω. This means the signal source must be able to drive a very low impedance. A 50 Ω antenna cable would present a slight impedance mismatch (50 Ω source driving 5.2 Ω load) causing signal attenuation at the CB input — which is why CB stages in RF designs often include an input matching network.
Impedance comparison between common emitter and common base stages. The CE stage has Zin ≈ β × re (relatively high); the CB stage has Zin ≈ re (very low). Both have high output impedance at the collector.
View LargerFrequency Response Advantage
The common base configuration's most important practical advantage is its superior high-frequency performance. To understand why, you need to understand the Miller effect, which is the main factor that limits the high-frequency gain of the common emitter stage.
In a CE amplifier, there is a small capacitance — typically a few picofarads — between the collector and the base of the transistor. This is the collector-base junction capacitance (Cbc, also called Cob or Crss on datasheets). In a CE stage, this capacitance connects two nodes that are moving in opposite directions (the input at the base moves one way, the output at the collector moves the other way by a factor of Av). The effective capacitance at the input is multiplied by (1 + Av) — this is the Miller effect. For a stage with gain of 100, a 5 pF collector-base capacitance becomes effectively 505 pF at the input, dramatically limiting bandwidth.
In a common base stage, the collector-base capacitance connects the output (collector) to the common node (base, which is at AC ground). The output node moves, but the base is grounded — there is no Miller multiplication. The capacitance appears simply as a shunt at the output, not as an amplified input capacitance. This is why common base stages are used at high frequencies: they avoid the Miller effect entirely, allowing much wider bandwidth for the same transistor.
A transistor datasheet will specify fT — the transition frequency, at which the common emitter current gain falls to unity. For a given transistor, the common base configuration can be used at frequencies much closer to fT than the common emitter configuration can, because the Miller effect does not limit its bandwidth in the same way. This is the fundamental reason why CB stages appear in VHF and UHF preamplifiers.
The Cascode Configuration
The cascode circuit is one of the most important configurations in RF amplifier design. It stacks a common base stage directly on top of a common emitter stage, exploiting the best properties of both:
- The CE stage at the input provides high input impedance and voltage gain.
- The CB stage at the output provides high output impedance and, critically, eliminates the Miller effect by presenting a low-impedance load to the CE collector — reducing the collector voltage swing of the CE stage to nearly zero, which eliminates Miller capacitance multiplication.
The result is an amplifier with the high gain and high input impedance of a CE stage, the wide bandwidth of a CB stage, and better isolation between input and output than either stage alone. Cascode amplifiers are standard in high-quality HF and VHF receivers, in spectrum analysers, and in any application where gain and bandwidth must both be maximised simultaneously.
The cascode amplifier. Q1 is a common emitter stage; Q2 is a common base stage. The CB stage loads the CE collector with a very low impedance (its input impedance = re), which eliminates Miller multiplication and dramatically extends bandwidth.
View LargerHam Radio Applications
The common base configuration and its derivative the cascode appear in several specific places in amateur radio equipment:
HF receiver front-end preamplifiers: Many high-performance HF transceivers use a cascode preamplifier immediately after the antenna bandpass filter. The low noise transistor pair (often BF998 dual-gate MOSFET or discrete BJT cascode) provides gain with excellent dynamic range and wide bandwidth. The common base portion eliminates the Miller effect and provides reverse isolation between the antenna and the mixer, preventing the mixer's local oscillator from leaking backward and radiating from the antenna.
VHF/UHF preamps: At 144 MHz, 432 MHz, and above, the Miller capacitance problem of common emitter stages becomes severe. Common base stages using low-capacitance microwave transistors (BFR91, BFG591, ATF-54143 GaAs HEMT) provide the wide bandwidth needed at these frequencies. Dedicated receive preamplifiers for weak-signal VHF work (such as those used for EME — Earth-Moon-Earth — moonbounce contacts) almost invariably use common base or cascode topologies.
RF power stages: At high power levels the common base configuration is sometimes used for power stages because its low input impedance naturally provides a good match to the low optimum source impedance of a power transistor at high frequencies. The BLY90, 2SC1969, and similar RF power transistors are often used in common base circuits for their VHF and UHF power amplifier applications.
| Property | Common Emitter | Common Base | Common Collector |
|---|---|---|---|
| Input terminal | Base | Emitter | Base |
| Output terminal | Collector | Collector | Emitter |
| Phase shift | 180° (inverting) | 0° (non-inverting) | 0° (non-inverting) |
| Voltage gain | RC/re (high) | RC/re (high) | ≈ 1 (unity) |
| Current gain | β (high) | α ≈ 1 | β+1 (high) |
| Input impedance | β × re (medium) | re (very low) | β × (re+RE) (very high) |
| Output impedance | RC (medium-high) | RC (high) | re (very low) |
| Miller effect | Severe | None | Minimal |
| Primary use | General amplification | RF, VHF/UHF, cascode | Buffer, impedance matching |
Frequently Asked Questions
Why doesn't the CB stage have phase inversion?
In the common base configuration, a positive signal at the emitter (input) drives the emitter voltage up toward the base voltage. Since VBE = Vbase − Vemitter, and the base is at a fixed DC potential (bypassed to AC ground), increasing the emitter voltage reduces VBE. Reduced VBE means reduced collector current. Reduced collector current means less voltage drop across RC. Less voltage drop means higher collector voltage — so the output goes positive when the input goes positive. No inversion. This contrasts with the CE stage, where positive input at the base increases VBE, increases collector current, increases drop across RC, and lowers the collector voltage.
If the CB stage has very low input impedance, doesn't it load down the source?
Yes, and this is a real design constraint. A common base stage with re = 10 Ω presents a 10 Ω load to the signal source. If the source is a 50 Ω coaxial line, there is an impedance mismatch that wastes signal. In practice, this is handled with an input matching network (often an LC transformer or balun) that transforms the 50 Ω source impedance down to match the re of the transistor. This also provides a noise match for minimum noise figure. Alternatively, in a cascode topology, the preceding CE stage provides the driving source for the CB stage — and the CE stage's output impedance (approximately equal to RC of the CE stage) is typically well above re, so a matching network is still sometimes needed.
What is the Miller effect and why does CB avoid it?
The Miller effect is the apparent multiplication of the collector-to-base capacitance Cbc when seen at the input. In a CE stage, Cbc connects a node that moves in one direction (collector, inverted output) to a node that moves in the other direction (base, non-inverted input). The effective capacitance at the input is Cbc × (1 + Av). For a gain of 100 and Cbc = 5 pF, this creates 505 pF at the input — severely limiting bandwidth. In a CB stage, the base is at AC ground. Cbc connects the output (collector) to a grounded node. There is no bootstrapping effect. Cbc simply shunts the output to ground, appearing as a normal output capacitance without multiplication. This is why common base stages remain useful at much higher frequencies than equivalent common emitter stages using the same transistor.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.