The tolerance of a small-signal digital transistor's internal resistors directly affects its current control accuracy, gain stability, and temperature characteristics, ultimately determining the overall reliability of the circuit.
In a voltage-divider network, the tolerance of the internal resistors directly changes the set base bias voltage. A small-signal digital transistor uses a base-emitter resistor and a base-collector resistor to form a voltage-divider circuit, providing a stable bias current for the transistor. If the tolerances of the two resistors differ—for example, one is ±20% and the other is ±30%—the actual deviation in the voltage-divider ratio may exceed 50% of the designed value. This deviation causes the base current to deviate from the expected value and the collector current to vary nonlinearly. In amplifier circuits, gain drift can cause signal distortion; in switching circuits, it can lead to logic level errors and reduce noise immunity.
The stability of the current gain factor is highly dependent on the accuracy of resistor matching. The DC current gain (GI) of a small-signal digital transistor is determined by the internal resistors and the transistor's hFE. The formula is GI = hFE × [R2/(R1+R2)]. If the tolerance of R1 is ±30%, the tolerance of R2 is ±20%, and the ratio of the two tolerances reaches ±20%, the GI fluctuation range may increase by more than 30%. This fluctuation manifests as output signal amplitude jitter in low-frequency amplifier circuits, while in high-frequency circuits it may cause phase noise, degrading signal transmission quality.
The effect of temperature on resistor values further exacerbates the instability caused by tolerances. The resistance of semiconductor materials increases with temperature, and the temperature coefficients of different materials vary significantly. If the internal resistors use materials with high temperature coefficients, the actual resistance change may far exceed the tolerance range when the ambient temperature fluctuates. For example, in an industrial environment with temperatures ranging from -20°C to 85°C, the temperature drift of the resistor value may add up to ±10%, which, combined with the initial tolerance, will result in a total deviation exceeding the design tolerance. This cumulative effect is particularly prominent in precision measurement circuits or long-term steady-state operation scenarios, potentially causing gradual degradation of circuit performance or even failure.
In high-frequency applications, resistor tolerances can also affect the resonant characteristics of transistors. The built-in resistors and parasitic capacitors of small signal digital transistors form an RC network, whose resonant frequency is inversely proportional to the resistor value. If resistor tolerance causes the resonant frequency to deviate from the designed value, the transmission efficiency of high-frequency signals will be significantly reduced. For example, in RF switching circuits, a shift in the resonant frequency can cause signal reflections, increase the standing wave ratio (SWR), and even damage downstream equipment. Furthermore, impedance mismatch caused by tolerances can reduce the circuit's power transmission efficiency and increase energy consumption.
To mitigate the impact of tolerances, coordinated optimization is required in resistor material selection, process control, and circuit design. Using metal film resistors with low temperature coefficients can reduce temperature drift, while laser trimming technology can achieve resistance accuracy within ±1%. In circuit design, the reliance on individual resistor values can be reduced by introducing negative feedback networks or employing resistor ratio matching strategies. For example, in differential amplifier circuits, selecting tolerance-matched resistor pairs can improve the common-mode rejection ratio and significantly enhance the circuit's anti-interference capabilities.
The packaging and layout of small signal digital transistors also influence tolerance effects. Surface mount technology (SMT) packages are suitable for miniaturized circuits due to their small size and high integration density, but thermal management requires careful attention. Through-hole packages, however, are suitable for higher-power circuits due to their excellent heat dissipation and mechanical strength. In PCB layout, avoid placing high-precision resistors near heat sources and minimize the length of traces connecting them to transistors to minimize the effects of parasitic parameters.
The tolerance of the internal resistors of small signal digital transistors can affect circuit stability through mechanisms such as voltage division deviation, gain fluctuation, temperature drift, and degradation of high-frequency characteristics. By optimizing resistor materials, process control, circuit design, and layout, these tolerance effects can be effectively minimized, improving circuit reliability.
