The MPSA13 is a widely used NPN Darlington transistor known for its exceptionally high current gain and its ability to amplify very small signals into strong, usable outputs. This article will discuss the MPSA13 device’s specifications, characteristics, operation in circuits, applications, and more.

Catalog

MPSA13 NPN Darlington Transistor Basic

The MPSA13 is a high-gain NPN Darlington transistor designed for applications requiring strong amplification from a very small input signal. With two transistors connected internally in a Darlington configuration, it delivers exceptionally high current gain - often above 5000 - making it ideal for driving relays, LEDs, small motors, and other moderate loads. It operates up to 30 V and supports collector currents up to 0.5 A, making it reliable for a wide range of switching and signal-control circuits. Because of its Darlington design, the MPSA13 requires a higher base-emitter voltage and has a higher saturation voltage compared to standard single transistors.

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MPSA13 Transistor CAD Models

MPSA13 Transistor Pinout

MPSA13 Transistor Alternative

MPSA13 Transistor Specifications

MPSA13 Transistor Working in Circuit

In this circuit, the MPSA13 NPN Darlington transistor (Q3) works as a high-gain booster stage, amplifying very small input signals from IN1. Because the MPSA13 has extremely high current gain, it can take a weak guitar or audio signal and boost it enough to drive the following MOSFET stage. The input signal passes through C3 and R9 before reaching the base of the MPSA13, ensuring the transistor only receives the AC signal while DC biasing stays stable.

When the signal enters Q3’s base, the Darlington configuration provides strong amplification, causing a much larger current to flow from the collector through R5 and R6. This boosted signal then drives the 2N7000 MOSFET (Q1), shaping and strengthening the output tone. The diodes and capacitors around the circuit provide filtering, protection, and additional dynamic shaping.

MPSA13 Characteristic Curves

Noise Voltage  

This curve shows how the transistor’s input-referred noise voltage varies with frequency for different collector currents. At low frequencies, the noise is dominated by 1/f (flicker) noise, resulting in a steep rise as frequency decreases. As frequency increases, the curve flattens, indicating the white-noise region where noise becomes essentially constant. Higher collector currents generally reduce the noise voltage, making the device quieter when biased at higher current levels.

Noise Current  

The noise current curves illustrate how input-referred noise current changes with frequency and collector current. At low frequencies, noise current is relatively small, but it increases with frequency due to rising shot noise and other high-frequency mechanisms. Larger collector currents produce higher noise current because shot noise is proportional to the square root of current. These curves help in choosing an operating current that balances gain and noise performance.

Capacitance vs. Reverse Voltage  

This graph plots the transistor’s junction capacitances as the reverse bias voltage increases. Both collector-base capacitance (Cibo) and collector-emitter capacitance (Cobo) decrease as reverse voltage is raised, a common behavior of semiconductor junctions. The reduction in capacitance with voltage is important for high-frequency performance, as lower capacitance improves bandwidth and reduces signal loading.

High-Frequency Current Gain 

The high-frequency current-gain curve shows how the transistor’s gain behaves with collector current at 100 MHz. Gain increases with current up to a peak value, after which it gradually falls due to high-injection effects and transit-time limitations. This plot helps identify the optimal collector current range for maximum high-frequency performance, especially when the device is used in fast switching or RF applications.

MPSA13 Transistor Applications

Signal amplification for low-level or weak input signals

High-gain audio preamplifiers

Sensor signal conditioning (photodiodes, microphones, small sensors)

Darlington switching stages for driving higher loads

Relay, solenoid, and small motor drivers

LED arrays or high-brightness LED driving

Digital interface level shifting

Low-frequency analog circuits

Control circuits in automation and hobby electronics

General-purpose switching and amplification in low-power systems

MPSA13 Transistor Mechanical Dimension

MPSA13 Advantages & Limitations

Advantages

Very high current gain (Darlington pair structure), typically 5000+

Strong switching capability, suitable for driving relays, LEDs, and small motors

Low input current requirement due to high gain

Useful for amplifying very small signals

Good for low-frequency applications

Easy to interface with microcontrollers thanks to low base drive needs

Widely available and inexpensive

Limitations

Higher saturation voltage (VCE(sat)) because of Darlington configuration

Slower switching speed compared to single transistors due to extra junction

Higher leakage currents

Not suitable for high-frequency applications

Thermal performance can be limited if used at higher collector currents

Requires attention to base-emitter voltage, which is ~1.2–1.4 V (higher than a single BJT)