Linear regulator
A linear regulator, also known as a linear voltage regulator, is an electronic circuit that maintains a stable, constant output voltage from an unregulated or fluctuating input voltage source by employing a pass element, such as a transistor, to dissipate excess voltage as heat and a feedback loop to adjust for variations in input voltage or load current.[1][2] This regulation ensures the output remains within specified tolerances, typically using an error amplifier that compares the output voltage—scaled via a resistive divider—to a precise reference voltage, thereby controlling the pass element to force the desired voltage at the load.[1][3] Linear regulators are broadly classified into series (with the pass element in series to the load) and shunt (with the pass element in parallel to the load) types; series regulators are further subdivided based on their pass element and dropout characteristics, where dropout voltage is the minimum input-output differential required for regulation. Standard linear regulators often use an NPN Darlington transistor pair, offering dropout voltages of approximately 1.5 V to 2.2 V and supporting high load currents up to 10 A, but they require a significant input-output voltage difference for efficient operation.[1] Low-dropout regulators (LDOs), commonly employing PNP transistors or PMOS FETs, reduce this to 0.6 V to 0.8 V (or as low as 20 mV to 90 mV in advanced designs), making them suitable for battery-powered applications where the input voltage is close to the output.[1][2] Quasi-LDOs and NMOS-based variants provide intermediate performance, with dropout voltages around 0.9 V to 1.5 V and lower ground pin currents compared to full LDOs.[1] One of the primary advantages of linear regulators is their simplicity and low cost, requiring minimal external components, while delivering exceptionally low output noise (e.g., 20 μV RMS) and fast transient response without generating electromagnetic interference (EMI), as there are no switching elements involved.[2] They excel in applications demanding high precision and low ripple, such as analog circuits, sensors, and noise-sensitive systems. However, their efficiency is inherently low, calculated as \eta = \frac{V_{OUT}}{V_{IN}}, often below 30% for large voltage drops (e.g., 12 V to 3.3 V), leading to significant power dissipation given by P_{LOSS} = (V_{IN} - V_{OUT}) \times I_{OUT}, which necessitates thermal management like heat sinks.[2][1] This makes them less suitable for high-power or efficiency-critical scenarios compared to switching regulators.[2] In practice, linear regulators are widely used in point-of-load power supplies within integrated circuits, portable devices, and post-regulation stages following switching converters to filter noise, with design considerations including load regulation (typically 0.5%-1.0% error), line regulation, and stability ensured by compensation networks.[3] Their ground pin current, which can reach 20-45 mA at full load in LDOs, further impacts overall system efficiency but is offset by their reliability in low-to-moderate power environments.[1]Overview
Definition and Purpose
A linear regulator is an analog electronic circuit that employs a linear pass element, such as a transistor or field-effect transistor (FET), to maintain a constant output voltage by varying its effective resistance and dissipating excess input power as heat.[4][2] This regulation occurs through a feedback mechanism that compares the output voltage to a reference and adjusts the pass element accordingly, ensuring stability despite variations in input voltage or load current.[2] The primary purpose of a linear regulator is to deliver a stable direct current (DC) voltage from an unregulated or fluctuating input supply, which is crucial for powering sensitive electronic components such as operational amplifiers, microcontrollers, and analog signal processing circuits that require precise voltage levels to function reliably.[2] Unlike switching regulators, which operate by rapidly toggling power transistors on and off, linear regulators provide inherently low output noise and ripple—often below 20 µV RMS—along with fast transient response times due to their wide-bandwidth feedback loops, making them ideal for applications where minimal electromagnetic interference (EMI) and simplicity are prioritized over high efficiency.[2] Linear regulators are broadly classified into shunt and series types, with the former diverting excess current and the latter dropping voltage in series with the load.[2] Linear regulators emerged in the mid-20th century, initially utilizing vacuum tubes for series-pass regulation in early power supplies during the vacuum tube era, before transitioning to solid-state transistors in the 1960s as semiconductor technology advanced and enabled more compact, reliable designs.[5] This shift coincided with the growing demand for stable power in computing and consumer electronics, where linear regulators became a standard until the late 1970s, when efficiency concerns began favoring switching alternatives.[5]Classification of Linear Regulators
Linear regulators are broadly classified into two main types: shunt regulators and series regulators, based on the configuration of the regulating element relative to the load. This classification determines their structural differences, efficiency characteristics, and suitability for various applications. Shunt regulators are particularly valued for their simplicity in low-power scenarios, while series regulators excel in handling higher currents with better load regulation.[6][7] In shunt regulators, the regulating element is connected in parallel with the load, meaning the load current flows through a separate path while excess input current is diverted through the shunt element to maintain a stable output voltage. This parallel arrangement results in only a portion of the current passing through the regulating device, making shunt regulators suitable for low-power and precision applications, such as Zener diode-based circuits where the Zener diode acts as the shunt element to clamp the voltage. They are often employed in scenarios requiring minimal components and where the load current is relatively constant and low, typically under 200 mW.[8][6][9] Series regulators, in contrast, place the regulating element in series with the load and the input supply, so the entire load current must pass through this element, which adjusts its resistance to stabilize the output voltage. This configuration commonly uses pass transistors, such as bipolar or MOSFET devices, to handle the current, making series regulators ideal for applications demanding higher output currents, up to several watts in board-level power supplies. They provide superior voltage regulation compared to shunt types, especially under varying load conditions, and are widely used in noise-sensitive circuits like audio amplifiers or RF receivers.[8][6][7] Beyond the shunt-series divide, linear regulators can also be categorized by output polarity and implementation form. Positive-output regulators produce a stable positive voltage relative to ground, while negative-output variants provide a negative voltage, often using complementary circuits or dedicated ICs. For instance, the 78xx series integrated circuits deliver fixed positive voltages (e.g., 5 V from the 7805), whereas the 79xx series handles negative fixed outputs. Additionally, regulators are distinguished as discrete, built from individual components like transistors and diodes for custom designs, or integrated, as monolithic ICs that combine the pass element, error amplifier, and references on a single chip for compact, reliable performance in standard applications.[10][11][10] The following table provides a brief comparison of key attributes between shunt and series linear regulators:| Aspect | Shunt Regulators | Series Regulators |
|---|---|---|
| Configuration | Regulating element in parallel with load | Regulating element in series with load |
| Efficiency | Lower, especially with varying loads; excess current dissipated in shunt | Generally comparable but better for higher, steady loads; dissipation in pass element |
| Dropout Voltage | Typically low; operates without strict headroom requirement | Higher; requires input voltage exceeding output by pass element's minimum drop (e.g., 2 V) |
| Complexity | Simpler design, fewer components | More complex, often includes feedback amplifier |
| Typical Use | Low-power (<200 mW), precision, fixed voltage | Higher current (up to watts), variable loads, better regulation |
Principles of Operation
Shunt Regulator Mechanism
In a shunt regulator, the regulating element is connected in parallel with the load, diverting excess current from the input supply to ground to maintain a constant output voltage across the load.[12] This configuration ensures that when the input voltage rises or the load current decreases, the shunt element conducts more current, effectively clamping the output voltage at the desired level.[13] A common implementation uses a Zener diode as the regulating element, which operates in its reverse breakdown region to establish a fixed reference voltage, shunting surplus current while the load draws its required amount.[14] The fundamental relationship governing the output is given by the reference voltage of the shunt element, such that the output voltage equals this reference: V_{out} = V_{ref} where V_{ref} is the breakdown voltage for a Zener diode-based design. The shunt current is then the difference between the input and load currents: I_{shunt} = I_{in} - I_{load} This relation highlights how excess input current is dissipated in the shunt path to stabilize V_{out}.[12] In basic Zener diode configurations, regulation relies on the diode's inherent voltage-clamping characteristic without an active feedback loop, providing passive stabilization.[14] More advanced transistor-based shunt regulators, such as those using devices like the TL431, incorporate a feedback mechanism where a voltage divider senses the output voltage and feeds it to an internal error amplifier.[12] The error amplifier compares this sensed voltage against an internal reference (typically 2.5 V) and adjusts the transistor's conduction to modulate the shunt current, ensuring precise regulation even under varying conditions.[12] Shunt regulators offer high output voltage stability across fluctuating loads due to their direct parallel connection, making them suitable for applications requiring low noise and simplicity.[13] However, they exhibit poor efficiency, particularly when the input-output voltage differential is large, as all excess power is dissipated as heat in the shunt element, limiting their use to low-power scenarios.[13]Series Regulator Mechanism
In a series linear regulator, the pass element, typically a transistor or field-effect transistor (FET), is connected in series with the load, forming the primary path for current flow from the input to the output. The control circuitry, consisting of an error amplifier and a stable reference voltage source, monitors the output voltage and modulates the resistance of the pass element to compensate for variations in input voltage or load current, thereby maintaining a constant output voltage. This configuration ensures that excess input voltage is dropped across the pass element as heat, rather than affecting the load.[1] The fundamental relationship governing the output is given by V_{out} = V_{in} - V_{drop} where V_{drop} represents the voltage dissipated in the pass element, which is actively controlled to stabilize V_{out}. The regulation accuracy and transient response are determined by the feedback loop gain, denoted as A \beta, where A is the open-loop gain of the error amplifier and \beta is the feedback factor (typically set by a resistive divider sampling the output). Higher loop gain reduces steady-state errors in line and load regulation, with deviations inversely proportional to A \beta; for transients, the response speed is tied to the loop bandwidth, which increases with gain but requires compensation to ensure stability.[15][16] The feedback mechanism employs a differential error amplifier that compares a scaled version of V_{out} (via the divider) to the reference voltage V_{ref}, generating an error signal that adjusts the base (for bipolar) or gate (for FET) drive of the pass element to minimize the difference. This closed-loop operation enables precise control, with the pass element operating in its linear region to vary conduction dynamically. The dropout voltage, defined as the minimum V_{in} - V_{out} needed for regulation, arises from the pass element's saturation characteristics and is typically 1.5–2 V for standard designs using NPN or Darlington configurations.[1][15]Basic Configurations
Simple Shunt Regulator Circuit
The simple shunt regulator circuit employs a Zener diode connected in parallel with the load to maintain a stable output voltage by shunting excess current from the input supply. This basic discrete configuration consists of an input voltage source, a series current-limiting resistor, the Zener diode (reverse-biased across the load), and the load itself. The Zener diode operates in its breakdown region, where its voltage drop remains nearly constant regardless of current variations, provided the current stays within specified limits.[17][18][19] Component selection begins with choosing the Zener diode based on the desired output voltage, which equals the Zener voltage V_Z; common values include 5.1 V for powering TTL logic circuits or 12 V for general low-voltage applications. The Zener must have a power rating sufficient to handle the dissipated power, typically calculated as P_Z = V_Z \times I_Z, where I_Z is the Zener current, and selected from series like BZX55 (0.5 W) or higher for robustness. The series resistor R_S is then determined to limit current and ensure the Zener operates above its minimum knee current (usually 1-2 mA) for stable regulation; the formula is R_S = \frac{V_{IN} - V_Z}{I_Z}, where I_Z accounts for both no-load (higher) and full-load (lower) conditions to avoid exceeding the Zener's maximum current. For example, with a 12 V input, 5 V Zener, and I_Z of 10 mA, R_S \approx 700 \, \Omega.[17][18][19] Performance limits of the circuit include load regulation, typically specified as the change in output voltage over a load current range, often achieving 1-5% variation for basic setups due to the Zener's dynamic resistance. Input voltage variations are tolerated as long as V_{IN} exceeds V_Z by at least 30% to maintain adequate I_Z, with output ripple minimized but potentially requiring a decoupling capacitor (e.g., 0.1 μF) across the load for noise-sensitive applications. An example of input variation response shows the output voltage remaining stable at V_Z (e.g., 5 V) as V_{IN} fluctuates from 7 V to 15 V, with minor droop below the minimum I_Z threshold, illustrating the circuit's line regulation capability. The design is inefficient for loads above 100 mA, as excess power dissipates as heat in R_S and the Zener, limiting use to low-power scenarios.[17][18][19] Build considerations emphasize verifying the Zener's power rating to prevent thermal runaway; for a 5 V, 0.5 W Zener at 20 mA, the dissipation is 0.1 W, well within limits, but higher currents necessitate heat sinking or a higher-rated device like 1 W or 5 W variants. Use standard resistor values (e.g., E24 series) closest to the calculated R_S and recalculate currents to ensure I_Z stays between 1 mA and the maximum (e.g., 200 mA for a 1 W Zener). Polarization is critical: the Zener cathode connects to the positive input via R_S, with the anode to ground. Testing involves measuring output under no-load and full-load conditions to confirm regulation.[17][18][19]Simple Series Regulator Circuit
The simple series regulator circuit is a discrete topology employing an NPN transistor as the pass element to drop excess input voltage and a Zener diode as the reference voltage source. The collector of the NPN transistor connects to the unregulated input supply V_\text{in}, while the emitter links to the regulated output V_\text{out}, with the load connected between V_\text{out} and ground. A base resistor R_b links V_\text{in} to the transistor base, and the Zener diode connects from the base to ground to set the base voltage at the Zener breakdown value V_z. The output voltage is V_\text{out} \approx V_z - V_\text{BE}, where V_\text{BE} is the transistor base-emitter voltage drop (typically 0.7 V).[20][21][22] The base resistor R_b is sized to deliver adequate base current, calculated as R_b = \frac{V_\text{in} - V_z}{I_b} with I_b \approx \frac{I_\text{load}}{h_\text{FE}} to drive the transistor under maximum load, ensuring h_\text{FE} margin for variations.[20][21][23] In operation under varying loads, the pass transistor remains biased in its active linear region, modulating collector-emitter resistance to counteract disturbances and hold V_\text{out} constant. An increase in load current tends to lower V_\text{out}, which reduces the base-emitter voltage slightly, drawing more base current through the Zener to increase transistor conduction and restore output. Conversely, decreasing load allows the transistor to reduce conduction, with excess input voltage dropped across the collector-emitter path. The Zener maintains base stability, but minimum Zener current (e.g., 1–5 mA) must exceed base needs to avoid reference degradation.[22][20][23] This configuration exhibits limitations compared to integrated regulators, including higher dropout voltage (typically 1.5–2.5 V minimum due to transistor V_\text{BE} and Zener current requirements) due to discrete components, restricting use to applications with sufficient input-output differential. Heatsinking is essential for the pass transistor, as it dissipates heat via P = (V_\text{in} - V_\text{out}) \times I_\text{load}, potentially reaching watts for 10 V excess and 100 mA load, risking thermal runaway without proper design.[21][22][24]Adjustable and Fixed Regulators
Fixed Voltage Regulators
Fixed voltage regulators are integrated circuits designed to deliver a predetermined output voltage, such as +5 V or +12 V for positive supplies and -5 V or -12 V for negative supplies, using an internal voltage reference, error amplifier, and pass transistor to ensure stable regulation regardless of input fluctuations or load changes.[11][25] These three-terminal devices simplify circuit design by eliminating the need for external voltage-setting components, making them suitable for straightforward power supply applications.[26] Prominent examples include the 78xx series for positive voltages and the 79xx series for negative voltages, both featuring a standard pinout with input, ground, and output terminals.[11][26] The 78xx series, such as the 7805 (5 V output) and 7812 (12 V output), supports output voltages from 5 V to 24 V, while the 79xx series, like the 7905 (-5 V) and 7912 (-12 V), covers -5 V to -24 V.[25] These regulators typically handle output currents up to 1.5 A, with a representative dropout voltage of 2 V for the 78xx at 1 A load (1.1 V for 79xx).[11][26] Key performance specifications include line regulation, which measures output stability against input voltage changes, and load regulation, which assesses response to varying output currents; for instance, the 7805 exhibits a maximum line regulation of 50 mV (over 7.5 V to 25 V input) and load regulation of 100 mV (from 5 mA to 1.5 A load).[11] These metrics ensure reliable operation, with percentages typically under 1-2% for both line and load variations across the series.[25] In practice, fixed voltage regulators like the 78xx series are widely employed to supply clean power to digital logic circuits, such as microcontrollers and TTL devices, where stable voltage is critical.[11] For optimal performance and to prevent oscillations, external bypass capacitors—typically 0.33 µF on the input and 0.1 µF on the output—are recommended in series configurations.[25]Adjusting Fixed Regulators for Variable Output
Fixed linear regulators in the 78xx series, such as the µA7805, can be modified for variable output voltages above their nominal fixed value by incorporating external resistors to form a feedback network at the common pin. This approach taps the internal reference voltage through a resistor divider, allowing the output to be set higher than the regulator's designed V_ref while retaining the device's built-in protections. The method is described in manufacturer datasheets as a way to extend the utility of fixed regulators without switching to dedicated adjustable ICs.[27] The typical circuit connects the regulator's input to an unregulated DC supply (at least 3V above the desired output for proper operation), the output directly to the load, and the common pin to the junction between resistors R1 (from common to ground) and R2 (from common to output). Stability is ensured by adding a 0.33 µF capacitor across the input and a 0.1 µF capacitor across the output relative to ground. The quiescent current flowing into the common pin introduces a small offset that must be considered in resistor selection.[27] The output voltage is calculated using the formula: V_{out} = V_{ref} \left(1 + \frac{R_1}{R_2}\right) + I_Q R_1 where V_{ref} is the regulator's nominal voltage (e.g., 5V for µA7805), R_1 and R_2 are the external resistors (with R_1 > R_2 for outputs above V_{ref}), and I_Q is the ground pin quiescent current (typically 3–5 mA, varying with temperature and load). Resistor values are chosen to achieve the target V_{out}, often starting with low values (e.g., 50–500 Ω) to minimize the I_Q term's impact while ensuring the divider draws sufficient current for stable regulation.[27] Example Calculation for 8V Output from 5V Fixed RegulatorTo derive 8V from a µA7805 (V_{ref} = 5 V, I_Q \approx 4 mA), first approximate ignoring I_Q: V_{out} \approx 5 \left(1 + \frac{R_1}{R_2}\right) = 8 \implies 1 + \frac{R_1}{R_2} = 1.6 \implies \frac{R_1}{R_2} = 0.6 Select R_2 = 100 Ω, then R_1 = 60 Ω. Including I_Q: V_{out} = 5 \left(1 + \frac{60}{100}\right) + (0.004)(60) = 5(1.6) + 0.24 = 8.24 \ \text{V} To compensate, reduce R_1 to 56 Ω: V_{out} = 5 \left(1 + \frac{56}{100}\right) + (0.004)(56) = 5(1.56) + 0.224 = 7.8 + 0.224 = 8.024 \ \text{V} This yields approximately 8V; fine-tuning may be needed based on exact I_Q measurement or simulation. Power dissipation in the resistors is minimal at low currents but should be verified for the application.[27] This configuration offers simplicity over fully variable ICs like the LM317, requiring only two resistors and no additional active components, while providing outputs up to 20–30V depending on input voltage and thermal limits. However, the range is restricted to voltages above V_{ref}, and precision is moderate due to I_Q variability (up to 50% over temperature) and resistor tolerances, resulting in line/load regulation of 1–5% compared to <1% for dedicated adjustable regulators. It is best suited for non-critical applications where exact matching to the fixed value is not required.[27]