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An AC128 transistor is a germanium positive-negative-positive (PNP) small-signal amplifier. It was primarily used in guitar effects equipment and amateur radio sets. The AC128 transistor has become virtually obsolete, as have most other germanium transistors.
A typical PNP transistor, such as the AC128, is composed of a layer of negative material sandwiched between two positive layers. The middle layer serves as the switch and is known as the base. The other layers are called the emitter and the collector, and a supply voltage and a load are connected through the collector and emitter connections. In a PNP transistor circuit, the collector is always positive with respect to the emitter. This portion of the circuit is the output.
Germanium transistors such as the AC128 transistor require less forward bias than silicon transistors. A germanium transistor typically switches on when the base voltage exceeds the emitter voltage by 0.25 volts. A silicon-based transistor requires 0.5-0.6 volts.
The input signal is applied across the emitter-base connections. With no input applied to the emitter-base connections, the transistor is theoretically off. When an input signal applied to the emitter-base connection exceeds the typical 0.25-volt trigger point, the transistor switches on, allowing a much larger current to flow through the collector-emitter output circuit. This configuration is known as a common emitter circuit because the emitter connection is shared by both the input and output circuits. Other configurations are often used for other types of applications.
One issue with the AC128 transistor is leakage current. This leakage current is the amount of current flowing through the transistor with no input signal applied. Excess leakage current can produce unwanted noise in the output stages of the circuit because the transistor never switches off completely.
The leakage current and other properties of the AC128 transistor vary from one transistor to another. In circuits where two transistors work together, they need to be matched as closely as possible. This is so that they will provide the same response in both portions of the circuit.
Matching transistor characteristics requires specialized equipment and a controlled testing environment, so they are often tested by the supplier and sold in pairs that have comparable properties. The specifications for the AC128 can be found on the Internet by searching for the part number. The AC128 transistor has become obsolete and is no longer produced, so most suppliers will provide a list of suitable replacements.