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The MPSA18 is a well-recognized low-noise, high-gain NPN transistor widely used in audio, sensor, and small-signal amplifier applications. This article explores the MPSA18’s electrical characteristics, pin configuration, CAD models, equivalent substitutes, operating principles, performance comparisons, and more.
The onsemi MPSA18 is a low-noise, high-gain NPN bipolar junction transistor designed for small-signal amplification. With a collector-emitter breakdown voltage of 45 V and a maximum collector current of 200 mA, it performs well in low-power circuits. Its high DC current gain (typically 400–1500) and transition frequency reaching over 100 MHz make it ideal for sensitive audio and general-purpose amplifier applications.
This transistor is commonly used in preamplifiers, sensors, and signal-conditioning stages where clean, low-noise performance is required. Housed in a compact TO-92 package, the MPSA18 remains reliable with a power dissipation rating of around 625 mW, making it suitable for tightly packed circuit boards and low-power design environments.
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| Pin Number | Pin Name | Description |
| 1 | Emitter (E) | Allows current to exit the transistor. Connected to ground or negative side in most NPN applications. |
| 2 | Base (B) | Controls the transistor’s operation. A small current at the base controls a larger current between collector and emitter. |
| 3 | Collector (C) | Main current-carrying terminal. Connected to the load and positive supply in typical NPN configurations. |
| Equivalent Model | Description |
| 2N5088 | High-gain, low-noise NPN transistor commonly used in audio preamps and signal amplification. Very close performance to MPSA18. |
| BC550C | Low-noise, high-gain NPN transistor with similar voltage and current ratings. Popular in audio and precision circuits. |
| NTE47 | NTE-series replacement transistor offering comparable gain and noise characteristics to MPSA18. |
| KSP06 | General-purpose NPN transistor with suitable gain and voltage ratings; can substitute MPSA18 in many small-signal applications. |
| Parameter | Value |
| Lifecycle Status | Obsolete |
| Mounting Type | Through-Hole |
| Package / Case | TO-226-3, TO-92-3 (TO-226AA) |
| Supplier Device Package | TO-92-3 |
| Number of Pins | 3 |
| Weight | 201 mg |
| Number of Elements | 1 |
| Polarity | NPN |
| Part Status | Obsolete |
| Base Part Number | MPSA18 |
| Element Configuration | Single |
| Collector-Emitter Voltage (VCEO) | 45 V |
| Collector-Base Voltage (VCBO) | 45 V |
| Emitter-Base Voltage (VEBO) | 6.5 V |
| Collector-Emitter Saturation Voltage | 80 mV |
| Max Collector Current (IC) | 200 mA |
| Current Rating | 100 mA |
| Collector Cutoff Current (ICBO) | 50 nA |
| Max Power Dissipation | 625 mW |
| Power – Max | 625 mW |
| Frequency (Transition) | 100–160 MHz |
| Gain Bandwidth Product | 100 MHz |
| DC Current Gain (hFE) | 500 (min) @ IC=5 mA, VCE=5 V |
| VCE Saturation (Max) | 300 mV @ 10 mA, 5 mA |
| Operating Temperature | –55°C to +150°C |
| Moisture Sensitivity Level (MSL) | 1 (Unlimited) |
| Dimensions (L × W × H) | 5.2 mm × 4.19 mm × 5.33 mm |
| Lead Free | Lead-Free |
| RoHS Status | Non-RoHS Compliant |
| REACH SVHC | No SVHC |
This circuit is a low-noise, high-gain multi-stage amplifier built around several MPSA18 transistors. The input signal first passes through a coupling capacitor that blocks DC while allowing AC to enter the first transistor stage. Each MPSA18 operates as a common-emitter amplifier, providing strong voltage gain thanks to its very high hFE. The emitter resistors stabilize biasing and help maintain consistent operating points, while the collector resistors set the voltage gain of each stage.
The two main amplification stages run at about 12.85 mA, showing they are biased for linear, low-distortion performance. The final transistor works as an output driver, feeding the signal through another coupling capacitor to isolate the amplifier’s DC bias from the output load. The MPSA18’s low noise and high gain characteristics make it ideal for this precise, low-distortion signal-amplification chain.
Low-noise audio preamplifiers
Microphone preamps and signal conditioners
Instrumentation amplifiers
High-gain small-signal amplifier stages
Sensor signal amplification (temperature, light, vibration sensors)
Low-level signal detection circuits
Active filters and equalizer circuitry
Small-signal switching applications
Buffer and driver stages in low-power systems
RF and high-frequency small-signal amplification (up to ~100–160 MHz)
Analog processing blocks in communication devices
Low-noise stages in radio receivers
Harmonic and tone-shaping sections in guitar effects and pedal circuits
Precision measurement and laboratory equipment
General-purpose NPN transistor applications requiring high gain
| Parameter | MPSA18 | BC550C | MPSA13 |
| Transistor Type | NPN | NPN | NPN (Darlington) |
| Package | TO-92 | TO-92 | TO-92 |
| DC Gain (hFE) | 500–1500 (very high) | 420–800 (high) | 5000 (extremely high, due to Darlington pair) |
| Noise Performance | Very low noise | Low noise | Moderate noise (Darlington) |
| Collector-Emitter Voltage (VCEO) | 45 V | 45 V | 30 V |
| Collector Current (IC max) | 200 mA | 100 mA | 500 mA |
| Power Dissipation | 625 mW | 500 mW | 625 mW |
| Gain Bandwidth (fT) | 100–160 MHz | 100 MHz | ~50 MHz |
| Input Characteristics | High gain, sensitive to small signals | Stable, low-noise, suitable for precision | Very high input impedance |
| Noise Suitability | Excellent for audio | Very good | Not ideal for low-noise audio |
| Speed | Fast | Fast | Slower due to Darlington structure |
Very high DC gain (hFE 500–1500), ideal for amplifying very small input signals.
Excellent low-noise performance, making it suitable for microphone preamps, audio stages, and instrumentation circuits.
High gain-bandwidth (100–160 MHz) supports high-frequency small-signal amplification.
Low collector-emitter saturation voltage, improving efficiency in low-power designs.
Stable biasing characteristics, resulting in low distortion when used in linear amplifier stages.
Through-hole TO-92 package, easy to handle and solder for prototyping and repair work.
Moderate collector current rating (200 mA), allowing use in various low-power applications.
Now obsolete, making it harder to source genuine parts.
Not suitable for high-power switching, due to limited power dissipation (~625 mW).
High gain may lead to stability issues (oscillation) if not properly decoupled in high-frequency circuits.
Noise advantages lost at high currents, where performance degrades.
Limited voltage rating (45 V) restricts usage in higher-voltage designs.
TO-92 package cannot dissipate heat well, requiring careful thermal considerations.
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