Common collector
The common collector amplifier, also known as the emitter follower, is a basic configuration of a bipolar junction transistor (BJT) in which the collector terminal serves as the common connection between the input and output circuits, with the input applied to the base and the output taken from the emitter.[1] This arrangement results in a voltage gain of approximately unity (close to 1), making it ideal for buffering without significant voltage amplification, while providing a current gain of β + 1, where β is the transistor's current gain factor (typically 100 or more).[2] Key characteristics include a high input impedance, often exceeding 4 kΩ and reaching hundreds of kilohms depending on biasing, which minimizes loading on preceding stages, and a very low output impedance, typically 1–10 Ω, enabling it to drive low-impedance loads effectively.[3][2] As one of the three primary BJT amplifier configurations—alongside common emitter and common base—the common collector excels in applications requiring impedance transformation and signal isolation, such as serving as an output stage to connect high-impedance sources to low-impedance loads like speakers or multiple devices.[1] Its unity voltage gain stems from the emitter voltage closely following the base voltage (V_E ≈ V_B - 0.7 V for silicon BJTs), with the small intrinsic emitter resistance r_e (approximately V_T / I_E, where V_T is 26 mV at room temperature) contributing to the low output impedance.[2] The configuration's frequency response is generally broad, with low cutoff frequencies around 100–200 Hz and high cutoffs up to 50–150 kHz in typical designs using components like 0.1 μF coupling capacitors.[3] In practical circuits, the common collector is often biased with a voltage divider at the base and an emitter resistor R_E for stability, using transistors like the 2N3904 or 2N2222, and it is frequently cascaded after a common emitter stage to form a two-stage amplifier with overall voltage gain while benefiting from the low output impedance for improved drive capability.[2][3] This makes it a versatile building block in analog electronics, particularly for audio amplifiers, sensor interfaces, and voltage regulators where preserving signal integrity without attenuation is crucial.[1]Introduction
Definition and Terminology
The common collector amplifier is a configuration of a bipolar junction transistor (BJT) in which the collector terminal is connected to both the input and output circuits, serving as the common electrode, while the input signal is applied to the base and the output is taken from the emitter.[4][5] This setup distinguishes it from other BJT configurations by prioritizing signal transfer through the emitter rather than amplification of voltage across the collector-base junction.[4] Commonly referred to as an emitter follower due to the emitter output closely tracking the base input, this amplifier is also known as a voltage buffer or unity-gain amplifier because of its characteristic voltage gain of approximately one.[5][4] The core terminology emphasizes its role in following the input signal with minimal alteration, leveraging the transistor's inherent properties to maintain signal integrity.[5] At its fundamental level, the common collector operates via a negative feedback mechanism in which the output voltage at the emitter follows the input voltage at the base, offset by the base-emitter voltage drop of approximately 0.7 V for silicon BJTs.[4][5] This feedback ensures stability and results in high current gain—typically β + 1, where β is the transistor's current gain factor—while providing near-unity voltage gain, making it ideal for impedance matching between high-impedance sources and low-impedance loads.[4][5] The common collector configuration is one of the three basic BJT amplifier topologies, which became standard in transistor electronics following the BJT's development in the early 1950s.Comparison to Other Configurations
The common collector (CC) configuration, also known as the emitter follower, is one of the three fundamental bipolar junction transistor (BJT) amplifier topologies, alongside the common emitter (CE) and common base (CB). All three operate the BJT in its active region, where the base-emitter junction is forward-biased and the base-collector junction is reverse-biased, enabling linear amplification without entering saturation or cutoff.[6] Unlike the CE, which provides high voltage gain but inverts the signal with a 180° phase shift, the CC offers unity voltage gain (approximately 1) with no phase inversion (0° shift), making it non-inverting and suitable for applications requiring signal fidelity.[7] The CB configuration, in contrast, also delivers high voltage gain without phase inversion but features a current gain near unity (α ≈ 1), differing from the CC's high current gain of β + 1, where β is the transistor's current gain factor.[6] In terms of impedances, the CC stands out with its high input impedance (typically in the hundreds of kΩ) and low output impedance (often tens of Ω or less), ideal for buffering between stages with mismatched impedances.[7] This contrasts with the CE's moderate input impedance (around 1-2 kΩ) and moderate-to-high output impedance (several kΩ), which suits general amplification but may load prior stages.[7] The CB exhibits the lowest input impedance (tens of Ω) and highest output impedance, making it appropriate for high-frequency applications where low input capacitance is beneficial, though it demands careful impedance matching.[6] The CC's characteristics arise from inherent negative feedback, which stabilizes the output close to the input voltage.[7] Key trade-offs among these configurations highlight the CC's role as a buffer rather than an amplifier: while the CE and CB provide significant voltage amplification (gains >1, often tens to hundreds), the CC sacrifices voltage gain for superior current buffering and impedance transformation, avoiding the signal distortion possible in high-gain CE stages.[6] Additionally, the CB offers the widest bandwidth due to minimized Miller capacitance effects, outperforming the lower bandwidths of CE and CC in high-speed circuits.[6]| Configuration | Voltage Gain | Current Gain | Input Impedance | Output Impedance | Phase Shift | Typical Use Cases |
|---|---|---|---|---|---|---|
| Common Emitter (CE) | High (>1) | High (β) | Moderate (kΩ) | Moderate to High (kΩ) | 180° | General amplification, signal inversion[7] |
| Common Base (CB) | High (>1) | ≈1 (α) | Low (tens of Ω) | High (kΩ) | 0° | High-frequency amplification, impedance matching[6] |
| Common Collector (CC) | ≈1 | High (β + 1) | High (hundreds of kΩ) | Low (tens of Ω) | 0° | Buffering, voltage following, impedance transformation[7] |
Circuit Configuration
Basic Circuit Diagram
The basic common collector circuit, also known as an emitter follower, utilizes an NPN bipolar junction transistor (BJT) as the core active device. In the standard schematic, the collector terminal of the NPN BJT is directly connected to the positive supply voltage V_{CC}, forming the common point shared by input, output, and power supply. The base serves as the input terminal, where the AC signal is applied through an input coupling capacitor C_{in} to block DC while passing the signal; the base is also biased via a voltage divider network consisting of resistors R_1 (connected to V_{CC}) and R_2 (connected to ground). The emitter terminal provides the output, connected to an emitter resistor R_E that leads to ground, with the AC output signal extracted across R_E via an output coupling capacitor C_{out} to isolate the load from DC bias. A bypass capacitor C_E is often placed in parallel with R_E to shunt AC signals around the resistor for enhanced AC performance.[8][9] The signal flow in the diagram follows the path from the base input, through the BJT, to the emitter output, while DC biasing paths are established separately: from V_{CC} through R_1 to the base, and from the emitter through R_E to ground, ensuring stable quiescent operating points. Key components include the NPN BJT (e.g., 2N3904), biasing resistors R_1 and R_2 (typically in the kΩ range for voltage division), R_E (for current setting), and coupling capacitors C_{in} and C_{out} (chosen based on frequency response needs).[8][5] A variation of the circuit employs a PNP BJT for applications requiring inverted polarity, where the emitter connects to the negative supply, the collector to ground (or common), and biasing adjusted accordingly, while maintaining the same topological input at the base and output at the emitter.[5][10]Biasing and Operation
The common collector amplifier, also known as an emitter follower, employs voltage divider biasing at the base to establish a stable DC operating point in the active region of the bipolar junction transistor (BJT). This method uses two resistors, R1 connected from the collector supply voltage V_CC to the base and R2 from the base to ground, creating a Thevenin equivalent voltage V_B = V_CC \cdot \frac{R_2}{R_1 + R_2} that forward-biases the base-emitter junction while ensuring the collector-emitter voltage V_CE exceeds the base-emitter voltage V_BE (typically around 0.7 V for silicon BJTs), with collector current I_C approximately equal to emitter current I_E.[11][12] For DC analysis, the quiescent operating point is determined by solving the emitter current as I_E = \frac{V_B - V_{BE}}{R_E}, where R_E is the emitter resistor connected to ground or a negative supply, assuming the base current is negligible due to high current gain β. This setup positions the transistor's Q-point centrally in the active region to allow maximum signal swing without saturation or cutoff, with the collector typically tied directly to V_CC for unity voltage gain in the DC sense.[13][12] In AC operation, the input signal is superimposed on the DC bias at the base, and the emitter voltage follows the base voltage with a small offset due to V_BE, providing a voltage gain near unity while offering high current gain for impedance matching. The emitter resistor R_E plays a key role in stabilization by introducing negative feedback (degeneration), which counteracts variations in transistor parameters.[11][13] Practically, R_E is chosen such that it supports the desired load current while providing negative feedback (degeneration) without a bypass capacitor to minimize thermal runaway, where rising temperature increases I_E and risks device failure; this feedback reduces the temperature coefficient of the bias point by opposing current increases.[13][12]Performance Characteristics
Gains and Impedances
The common collector amplifier provides a voltage gain A_v approximately equal to 1, making it a non-inverting configuration where the output voltage closely follows the input signal. This unity gain arises because the emitter voltage tracks the base voltage minus the base-emitter drop (V_{BE} \approx 0.7 V), resulting in a gain slightly less than 1 in practice.[14][5] The current gain A_i is approximately equal to the transistor's current gain \beta, typically ranging from 100 to 300 for common bipolar junction transistors, though it is more precisely \beta + 1. This high current gain allows the amplifier to provide significant drive capability to loads without drawing excessive current from the input source.[8][5] Input impedance Z_{in} is high, approximated as Z_{in} \approx \beta (R_E \parallel R_L), where R_E is the emitter resistance and R_L is the load resistance; this characteristic makes the common collector suitable as a buffer stage following amplifiers with lower output impedance. Output impedance Z_{out} is low, approximated as Z_{out} \approx r_e + (R_{source} / \beta), with r_e being the small-signal emitter resistance, enabling effective driving of low-impedance loads such as speakers or transmission lines.[8][14]| Parameter | Approximate Value/Expression | Typical Range/Implication |
|---|---|---|
| Voltage Gain (A_v) | \approx 1 (non-inverting) | Slightly <1 due to V_{BE} drop; unity follower action |
| Current Gain (A_i) | \approx \beta | 100–300; high drive for loads |
| Input Impedance (Z_{in}) | \approx \beta (R_E \parallel R_L) | High (kΩ range); buffers prior stages |
| Output Impedance (Z_{out}) | \approx r_e + (R_{source} / \beta) | Low (<50 Ω); drives low-impedance loads |