Common base
In electronics, the common base (CB) configuration is one of the three fundamental single-stage bipolar junction transistor (BJT) amplifier topologies, in which the base terminal acts as the shared connection point between the input and output signals, excluding DC power supplies.[1] In this setup, the input signal is applied across the emitter-base junction, while the output is taken from the collector-base junction, with the base typically grounded or held at a fixed potential.[2] This configuration results in a non-inverting amplifier where the input and output waveforms are in phase, and it is particularly noted for its use in high-frequency applications due to good stability and isolation between input and output.[1] The common base amplifier exhibits distinct electrical characteristics, including a low input impedance (typically 10–200 Ω) determined largely by the emitter resistance, and a high output impedance approximately equal to the collector load resistance (R_C).[2] Its current gain, denoted as α (the common-base current gain factor), is less than unity—usually ranging from 0.980 to 0.995—and is related to the transistor's common-emitter current gain β by the formula α = β / (β + 1).[1] In contrast, the voltage gain (A_V) is high, often between 100 and 2000, and can be approximated as A_V = R_C / r'e, where r'e is the small-signal emitter resistance given by r'e = 25 mV / I_E (with I_E as the emitter current in mA).[2] The power gain is comparable to the voltage gain, making it suitable for scenarios requiring voltage amplification without current amplification.[1] Despite its strengths, the common base configuration has limitations, such as its low current gain (less than unity), which makes it unsuitable for applications needing significant current amplification, and a voltage gain that can vary unpredictably with DC bias conditions.[1] It offers advantages like excellent high-frequency performance—due to reduced Miller capacitance effects—and effective impedance matching from low to high values, which is beneficial in radio frequency (RF) amplifiers and cascoded stages.[2] Common applications include current buffers in audio and RF circuits, as well as impedance transformation in multi-stage amplifiers where input-output isolation is critical.[2]Fundamentals
Circuit Configuration
The common base configuration of a bipolar junction transistor (BJT) amplifier features the base terminal as the common element, grounded or connected to both the input and output circuits for AC signals. In this topology, the input signal is applied to the emitter terminal, while the output is taken from the collector terminal. This arrangement is applicable to both NPN and PNP transistors, with the primary difference being the polarity of biasing voltages: for an NPN transistor, the emitter is negative relative to the base, and the collector is positive relative to the base; for a PNP transistor, the emitter is positive relative to the base, and the collector is negative relative to the base.[3][4] A typical circuit diagram for a common base NPN BJT amplifier includes DC biasing components to establish the operating point. The base is connected to ground through a bias network, often consisting of two resistors (R1 and R2) forming a voltage divider from the positive supply voltage (VCC) to ground, ensuring the base-emitter junction receives the appropriate DC voltage. An emitter resistor (RE) is placed in series with the emitter terminal to the negative supply or ground, providing negative feedback for stabilization, while a collector resistor (RC) is connected from the collector to VCC. Coupling capacitors are added at the input (between the signal source and emitter) and output (between collector and load) to block DC while passing AC signals. For example, the circuit might use a 2N3904 NPN transistor with VCC = 10 V, R1 = 100 kΩ, R2 = 27 kΩ, RE = 1 kΩ, and RC = 2.2 kΩ, setting a quiescent emitter current of approximately 1.5 mA and collector-emitter voltage of 5 V. The PNP version mirrors this but with reversed polarities and a negative supply.[3][4] The BJT symbol in common base mode uses the standard schematic representation: an NPN transistor is depicted as a circle with an arrow pointing outward from the emitter on the vertical line, while a PNP has the arrow pointing inward; the base lead is connected to the common ground point, the emitter lead receives the input, and the collector lead provides the output. Pinouts follow the transistor's datasheet, such as for the 2N3904 (TO-92 package): pin 1 (emitter), pin 2 (base), pin 3 (collector), with connections routed accordingly to ground the base and interface the emitter and collector ports.[3] In terms of port definitions, the input port is formed between the emitter and base terminals, where the signal voltage drives current into the emitter; the output port is between the collector and base terminals, from which the amplified current is sourced. This setup contrasts with other BJT configurations like common emitter or common collector, which ground different terminals.[4][3] For proper operation, the common base BJT must be biased in the forward-active region, where the base-emitter junction is forward-biased (V_BE ≈ 0.7 V for silicon devices) to allow carrier injection from the emitter, and the base-collector junction is reverse-biased (V_BC < 0 for NPN) to sweep carriers toward the collector without significant recombination. This biasing ensures the transistor functions as a current-controlled device, with the input current at the emitter nearly equaling the output current at the collector under ideal conditions.[5][3]Operating Principles
In the common base configuration of a bipolar junction transistor (BJT), particularly for an NPN type, the principle of current amplification arises from the injection of charge carriers at the emitter, their transport across the base, and collection at the collector. With the base-emitter junction forward-biased and the base-collector junction reverse-biased, electrons are injected from the heavily doped n-type emitter into the p-type base, where they act as minority carriers. These electrons diffuse across the thin, lightly doped base region toward the collector, swept by the electric field in the reverse-biased base-collector depletion region. Due to the narrow base width (typically on the order of 0.1–1 μm) and minimal recombination—facilitated by a long minority carrier lifetime and asymmetric doping that favors electron injection over hole injection—the vast majority of injected electrons reach the collector without recombining with holes in the base. This results in the common-base current gain, denoted α (alpha), defined as the ratio of collector current to emitter current (α = I_C / I_E), approaching unity (typically 0.98–0.999), indicating near-complete transfer of emitter current to the collector.[6][7] The input signal voltage, applied between the emitter and the grounded base, modulates the base-emitter forward bias voltage (V_BE), exponentially controlling the rate of electron injection and thus the emitter current (I_E) according to the Ebers-Moll model, where I_E ≈ I_S (exp(V_BE / V_T) - 1) and I_S is the saturation current. For operation in the active region, the transistor requires V_BE ≈ 0.6–0.7 V (for silicon) to forward-bias the base-emitter junction and V_BC < 0 (or V_CE > V_BE) to maintain reverse bias at the base-collector junction, ensuring carrier injection without saturation or cutoff. The DC collector current is then given by I_C = α I_E, while the small base current supplies the recombination needs and is expressed as I_B = (1 - α) I_E, highlighting that only a small fraction of emitter current is lost in the base.[8][9] Qualitatively, the input impedance in the common base setup is low because the signal is applied directly to the emitter, where the dynamic resistance is dominated by the small intrinsic emitter resistance r_e ≈ V_T / I_E (typically 10–100 Ω), akin to emitter degeneration that stabilizes current but requires a low-impedance source to drive effectively. Conversely, the output impedance is high, as the collector current remains relatively independent of the collector-emitter voltage (V_CE) in the active region, behaving nearly like a current source with resistance on the order of tens to hundreds of kΩ. However, the Early effect introduces a finite slope in the I_C-V_CE output characteristics, arising from base-width modulation: as V_CE increases, the base-collector depletion region widens into the base, effectively narrowing the neutral base and increasing the minority carrier concentration gradient, which boosts I_C slightly (modeled as I_C ≈ I_S exp(V_BE / V_T) (1 + V_CE / V_A), where V_A is the Early voltage, 15–150 V). This modulation causes a small positive output conductance but does not significantly degrade the high-impedance behavior for most applications.[4][8][9]Small-Signal Characteristics
Input and Output Parameters
In the common base configuration, the small-signal input impedance r_{in} is low and approximated as the dynamic emitter resistance r_e = \frac{V_T}{I_E}, where V_T is the thermal voltage (about 26 mV at room temperature) and I_E is the quiescent emitter current; this results in typical values of 10 to 100 ohms, making it suitable for driving from low-impedance sources. The low impedance stems from the emitter terminal acting as the input port with the base AC-grounded, effectively presenting the intrinsic emitter resistance to the signal.[2] The small-signal output impedance r_{out} is high, approximately equal to the transistor's output resistance r_o = \frac{V_A}{I_C}, where V_A is the Early voltage (typically 50 to 100 V) and I_C is the quiescent collector current; this yields values from hundreds of kilohms to several megohms, reflecting the collector's behavior as a high-impedance current source.[10] This characteristic arises because variations in collector voltage have minimal impact on collector current due to the Early effect, enhanced by the fixed base potential.[11] The reverse voltage gain parameter h_{rc} \approx \frac{1}{1 + g_m r_o} is small (often less than 0.01), where g_m = \frac{I_C}{V_T} is the transconductance, owing to the high loop gain in the feedback path at low frequencies. The forward current transfer ratio h_{fb} = -\alpha \approx -[1](/page/1), with \alpha being the common-base current gain nearly equal to unity, indicates that the collector current closely mirrors the negative of the emitter current.[11] This \alpha relates to the common-emitter current gain \beta by \alpha = \frac{\beta}{1 + \beta}, typically yielding \alpha > 0.98 for \beta in the range of 50 to 200.[12] For context, the following table compares the approximate input and output impedances across BJT configurations, highlighting the common base's unique low-input, high-output profile:| Configuration | Input Impedance | Output Impedance |
|---|---|---|
| Common Base | Low (\approx r_e \approx 10{-}100 \, \Omega) | High (\approx r_o \approx 100 \, \mathrm{k}\Omega{-}1 \, \mathrm{M}\Omega) |
| Common Emitter | Medium (\approx \beta r_e \approx 1{-}10 \, \mathrm{k}\Omega) | Medium (\approx r_o \parallel R_C \approx 10{-}100 \, \mathrm{k}\Omega) |
| Common Collector | High (\approx \beta (r_e + R_E) > 100 \, \mathrm{k}\Omega) | Low (\approx \frac{r_e}{\beta + 1} \approx 10{-}100 \, \Omega) |