Electronics Guide

Key Performance Parameters

Dropout voltage defines the minimum input-to-output differential required for proper regulation. Traditional regulators using NPN pass transistors typically require 2-3 volts of headroom, while modern LDOs using PMOS transistors can operate with dropout voltages below 100 millivolts. This distinction significantly impacts thermal dissipation and overall system efficiency.

Line regulation measures output voltage stability against input voltage variations, typically expressed as millivolts per volt of input change or as a percentage. Excellent line regulation, often below 0.01 percent, ensures that upstream supply fluctuations do not propagate to sensitive loads.

Load regulation characterizes output voltage deviation under varying load currents. Superior load regulation maintains tight voltage tolerance from no-load to full-load conditions, crucial for circuits where operating current varies dynamically.

Power supply rejection ratio (PSRR) quantifies the regulator's ability to attenuate input noise across frequency. A regulator might achieve 80 dB rejection at DC but only 40 dB at 100 kHz, making frequency-dependent PSRR curves essential for applications with switching pre-regulators.

Evaluation Board Techniques

Linear regulator evaluation boards enable rapid assessment of device performance under controlled conditions. Effective evaluation requires attention to board layout, bypass capacitor selection, and measurement methodology.

Input and output capacitor selection critically affects transient response, stability, and noise performance. Ceramic capacitors offer low equivalent series resistance (ESR) but may cause stability issues with certain regulators designed for tantalum or electrolytic capacitors. Always verify capacitor requirements in device documentation and evaluate stability across the full operating temperature range.

Thermal evaluation determines practical current limits and long-term reliability. Most evaluation boards include thermal measurement points near the regulator die. Infrared thermal imaging provides comprehensive temperature distribution data, revealing hotspots that might not appear at single-point measurements.

Reference Architectures

Bandgap references exploit the complementary temperature coefficients of silicon junction voltage and thermal voltage to achieve temperature-stable outputs. First-order bandgap references typically achieve temperature coefficients of 20-100 parts per million per degree Celsius, while curvature-corrected designs reduce this to single-digit ppm/C levels.

Buried Zener references offer superior initial accuracy and long-term stability compared to bandgap designs, though at higher power consumption and cost. These references, often found in precision instrumentation, achieve temperature coefficients below 1 ppm/C with drift specifications under 5 ppm per thousand hours.

Zener diode references provide higher voltage outputs than bandgap references without requiring gain stages. Temperature-compensated Zener references pair positive-temperature-coefficient Zener diodes with negative-coefficient forward-biased diodes to achieve reasonable stability at moderate cost.

Reference Testing and Characterization

Characterizing voltage references demands measurement equipment exceeding reference specifications by substantial margins. A reference claiming 1 ppm accuracy requires measurement systems accurate to 0.1 ppm or better for meaningful evaluation.

Temperature coefficient testing involves controlled thermal chambers and precise temperature monitoring alongside voltage measurement. Sweep rates must be slow enough for thermal equilibrium, and multiple temperature cycles reveal hysteresis effects that single-sweep tests might miss.

Long-term stability testing, while time-consuming, provides essential data for critical applications. Accelerated aging through elevated temperature operation can provide stability projections, though correlation with room-temperature aging requires careful analysis and historical data.

Noise density measurement characterizes reference noise across frequency, identifying both broadband thermal noise and discrete frequency components. Low-noise amplification stages with known gain allow spectrum analyzer measurement of microvolt-level reference noise.

Current Source Topologies

Two-terminal current sources using depletion-mode FETs or current regulator diodes provide simple, self-contained current regulation without external bias supplies. These devices suit applications requiring moderate accuracy with minimal component count, such as LED bias strings or simple sensor excitation.

Op-amp-based current sources offer programmability and high accuracy when designed properly. The classic Howland current source provides bidirectional capability but requires precisely matched resistors to maintain high output impedance. Improved Howland variants reduce sensitivity to resistor matching at the expense of additional components.

Current mirror topologies, implemented discretely or within integrated circuits, enable accurate current replication across multiple loads. Wilson and cascode current mirrors improve output impedance and reduce sensitivity to transistor parameter variations compared to simple two-transistor mirrors.

Electronic Load Design

Current sinks, often called electronic loads, enable power supply testing and battery discharge characterization. Basic electronic loads use power transistors in linear mode with feedback controlling base or gate drive to maintain programmed current.

Thermal management dominates electronic load design, as the load must dissipate full input power minus minimal losses. Heat sink selection, forced air cooling, and safe operating area (SOA) compliance require careful attention, particularly for loads handling rapid transients where instantaneous power far exceeds average dissipation.

Dynamic load capability distinguishes advanced electronic loads from basic designs. High slew rate current changes test power supply transient response, requiring load designs with wide bandwidth control loops and minimal parasitic inductance in current sense paths.

Voltage and Current Control

Dual-loop control systems regulate both voltage and current, with the active loop determined by load conditions. In constant voltage (CV) mode, the voltage loop dominates while current remains below the setpoint. When load demands exceed the current limit, the supply transitions to constant current (CC) mode, allowing voltage to fall as needed to maintain programmed current.

Reference sources for voltage and current setpoints determine overall accuracy and stability. Multi-turn potentiometers with precision voltage dividers provide analog adjustment, while modern designs increasingly use digital-to-analog converters driven by microcontrollers for programmability and recall of stored settings.

Output stage design balances response speed, efficiency, and operating range. Simple series-pass designs offer excellent transient response but suffer efficiency penalties at high current with large input-output differentials. Pre-regulation using switching converters improves efficiency while linear post-regulation maintains low noise output.

Protection and Safety Features

Overcurrent protection prevents damage from short circuits or excessive loads. Foldback current limiting reduces short-circuit dissipation by decreasing current as voltage falls, though this can cause problematic latching behavior with certain loads. Constant current limiting maintains safe operation without latch-up concerns.

Overvoltage protection guards sensitive equipment against output voltage excursions due to pass transistor failure or control loop malfunction. Crowbar circuits using silicon-controlled rectifiers provide fast protection but require main supply fusing for successful crowbar operation.

Thermal protection prevents catastrophic failure from excessive internal heating. Temperature sensors near power devices trigger current reduction or complete shutdown when safe operating temperatures are exceeded, allowing recovery after cool-down without component damage.

Noise Requirements for Audio

Audio power supplies must achieve noise floors compatible with system dynamic range requirements. A preamplifier with 100 dB dynamic range needs power supply noise below signal levels by this margin, potentially requiring submicrovolt noise densities in critical frequency ranges.

Power supply noise coupling into audio circuits occurs through multiple mechanisms: direct conduction through supply rails, magnetic coupling from transformers and inductors, and electric field coupling from high-voltage nodes. Effective audio power supply design addresses all coupling paths.

Hum, typically at power line frequency and harmonics, represents the most common audible power supply artifact. Traditional linear supplies with 50 or 60 Hz transformers require careful attention to rectifier smoothing and regulation to reduce hum to acceptable levels. Switching supplies introduce higher-frequency noise that, while often above audible range, may intermodulate with audio signals or remain audible to sensitive listeners.

Dual-Rail Supply Design

Bipolar audio circuits require positive and negative supply rails, typically of equal magnitude. Dual-rail power supply design must maintain symmetry between rails to prevent DC offset at audio outputs that could damage speakers or create audible artifacts.

Tracking regulators ensure positive and negative rails maintain equal magnitude despite load imbalances. Precision tracking prevents supply-induced distortion in push-pull output stages where mismatched rails shift quiescent operating points.

Star grounding configurations minimize ground loop noise by ensuring signal return currents take known paths without mixing with high-current power supply returns. Ground plane design in printed circuit boards requires particular care to maintain star topology benefits while providing low-impedance returns at high frequencies.

Precision Amplifier Selection

Buffer and gain amplifiers in precision voltage sources must not degrade reference accuracy. Critical amplifier parameters include input offset voltage, offset drift with temperature, input bias current, and open-loop gain. Modern chopper-stabilized amplifiers achieve microvolt-level offset with near-zero drift, though their chopping action may introduce noise at chopping frequency and harmonics.

Gain-setting resistor networks determine voltage source accuracy when output differs from reference voltage. Bulk metal foil resistors offer the best combination of initial tolerance, temperature coefficient, and long-term stability, though at significant cost premium over standard film types. Resistor networks from the same manufacturing batch improve tracking, reducing ratio errors from temperature changes.

Kelvin Output Connections

Precision voltage sources often include four-wire Kelvin output connections that separate current-carrying conductors from sense leads. This arrangement eliminates output lead resistance from the regulation loop, maintaining accuracy regardless of cable length or contact resistance.

Remote sensing capability extends Kelvin connection benefits to the load location. Sense leads return to the regulator's feedback input, allowing compensation for voltage drops in output cables. Remote sensing requires attention to stability, as the added loop phase shift from cable capacitance and inductance may cause oscillation without appropriate compensation.

Noise Sources and Minimization

Thermal noise in resistive elements sets fundamental limits determined by temperature, bandwidth, and resistance value. Lower resistance values reduce thermal noise but may increase current and power dissipation. Optimal design balances these factors within system constraints.

Shot noise in semiconductor junctions contributes current-dependent noise that increases with operating current. Reference and regulator designs must balance bias current for stability against shot noise contributions.

Flicker noise, also called 1/f noise, dominates at low frequencies and varies widely between components and technologies. Discrete JFET input stages often achieve lower 1/f noise than integrated alternatives, explaining their continued use in demanding applications.

Popcorn noise and burst noise create random step changes in output that, while statistically infrequent, may corrupt sensitive measurements. Screening for burst noise requires statistical testing over extended periods, often impractical except for the most critical applications.

Layout and Shielding Techniques

Physical layout profoundly impacts achievable noise performance. Separating high-current paths from sensitive nodes prevents magnetic coupling, while ground plane breaks or slots can reduce circulating currents that create ground voltage drops.

Electrostatic shielding prevents capacitive coupling from noise sources. Driven shields surrounding sensitive nodes, held at signal potential through low-impedance buffers, eliminate capacitive current flow that would otherwise corrupt measurements.

Magnetic shielding using high-permeability materials attenuates external magnetic fields that would induce noise currents in circuit loops. Mu-metal and similar alloys provide excellent shielding but require proper termination to avoid creating additional coupling paths.

Vibration sensitivity, while not strictly electronic noise, affects ultra-low-noise systems through microphonic effects in components. Ceramic capacitors with certain dielectrics and mechanical resonances in circuit board mounting can convert vibration into electrical signals indistinguishable from true noise.

PSRR Measurement Methods

Power supply rejection ratio measurement requires injecting known ripple onto the regulator input while measuring resulting output variation. Direct injection using summing amplifiers or coupling transformers places controlled AC signals on the DC input supply.

Network analyzers provide the most complete PSRR characterization, sweeping frequency while measuring input-to-output transfer function. This approach reveals resonances, filter breakpoints, and phase information essential for stability analysis when cascading multiple regulation stages.

Oscilloscope-based measurement suits spot-frequency testing with signal generators providing input ripple. This method, while less comprehensive than swept measurements, allows real-time observation of waveform distortion and nonlinear effects that might not appear in magnitude-only measurements.

Practical Testing Considerations

Output noise floor limits meaningful PSRR measurement. A regulator with 10 microvolt output noise cannot demonstrate 100 dB rejection of 1 millivolt input ripple, as the noise floor equals the expected output signal. Bandwidth limiting and signal averaging improve measurement resolution at the expense of testing time.

Load current affects PSRR performance in most regulators, often degrading rejection at higher currents as the pass element operates in different regions. Testing across the full load range reveals worst-case performance that specifications might not capture.

Common-mode rejection in differential measurement systems may mask true PSRR if input ripple couples equally to both differential inputs. Proper guarding and attention to measurement system ground connections prevent this masking effect.

Multi-Stage Filtering Evaluation

Complex power supply systems often cascade multiple filtering stages for maximum ripple rejection. Evaluating such systems requires understanding interaction between stages, including possible resonances when filter characteristics align unfavorably.

Post-regulation using second linear regulators after primary regulation provides both increased rejection and isolation from load-induced disturbances. This approach trades efficiency for performance, acceptable in noise-critical applications with moderate power requirements.

Active ripple cancellation circuits sense residual ripple and inject antiphase corrections, potentially achieving rejection beyond passive filter capabilities. These circuits require careful stability analysis and may introduce their own noise contributions that offset rejection improvements.