Gain–bandwidth product

The transition frequency is the frequency of a reinforcing system in which there is no more gain, that is 0 dB, and thus forms the theoretical upper limit for a sensible use. The practical working range of an amplifier circuit is, however, much earlier, limited by the so-called cut-off frequency.

Electronics

In electronics, the transit frequency fT (English: transit frequency) of an amplifier circuit, the frequency at which the gain is decreased with short-circuited output to one ( alternating current amplification factor β = 1).

The transit frequency is specified for individual (discrete ) reinforcing components such as transistors, field effect transistors or electronic tubes, as well as amplifier circuits, the integrated (for example, operational amplifier ) or discretely (for example, transistor -emitter circuit ) can be constructed.

The transition frequency depends on the operating point. Similar to the current gain, the gain crossover frequency increases as the load increases (higher collector current ) first, reaches a maximum and then decreases. Direct determinants of transit frequency are the junction capacitances and the bulk resistance.

To be distinguished from the transit frequency is practically the relevant cut-off frequency fc (English: cutoff frequency); It is defined as the threshold, wherein the power amplification is decreased to 50% ( ≈ -3 dB, the voltage or current gain are lowered to ≈ 70.7 %).

For resistors in the emitter circuit applies: cutoff frequency fc = fT / β. The transit frequency of the transistors for high frequency applications is in the upper gigahertz range.

Control Engineering

In control engineering, the transition frequency is used to optimize a control circuit. At the transition frequency the gain of the open loop exactly is one. The higher the transition frequency of the open loop, the shorter is the reaction time in the closed loop.