Transit frequency

The transit frequency denotes the frequency of an amplifying system at which amplification no longer takes place, i.e. is 0  dB , and thus forms the theoretical upper limit for a sensible use. The practical working range of an amplifier circuit is, however, limited much earlier by the so-called cutoff frequency .

electronics

Transit frequency in relation to collector current (operating point)

In electronics , the transit frequency is f _T (engl .: transit frequency ) of an amplifier circuit, the frequency at which the current gain is lowered at short-circuited output to one ( alternating current amplification factor β = 1).

The transit frequency is specified for individual ( discrete ) amplifying components such as transistors , field effect transistors or electron tubes , as well as for amplifier circuits that can be integrated (e.g. operational amplifiers ) or discrete (e.g. transistors in emitter circuits ).

The transit frequency depends on the operating point. Similar to the current gain, the transit frequency also increases with increasing load (higher collector current ), reaches a maximum and then decreases. The direct influencing variables of the transit frequency are the junction capacities and the rail resistance. The transit frequency can be calculated using the h parameters .

${\displaystyle h_{21}(\omega _{T})=\left|{\frac {i_{2}}{i_{1}}}\right|\equiv 1}$

The following applies to transistors in a common emitter circuit: Cutoff frequency f _c  =  f _T  / β. The transit frequency of transistors for high frequency applications is in the upper gigahertz range .

To distinguish it from the transit frequency is the practically relevant cut-off frequency f _c (engl .: cutoff frequency ); it is defined as the threshold at which the power gain has dropped to 50% (≈ −3  dB , voltage and current gain have dropped to ≈ 70.7%).

Control engineering

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

literature

• Wolfgang Bauer, Hans Herbert Wagener: Components and Basic Circuits of Electronics Volume 1 Components. 2nd edition, Carl Hanser Verlag, Munich / Vienna 1981, ISBN 3-446-13305-4 .
• Gregor Häberle, Heinz Häberle, Thomas Kleiber: Expertise in radio, television and radio electronics. 3rd edition, Verlag Europa-Lehrmittel, Haan-Gruiten 1996, ISBN 3-8085-3263-7 .
• Ekbert Hering, Klaus Bressler, Jürgen Gutekunst: Electronics for engineers and natural scientists . Springer Verlag, Berlin / Heidelberg 2014, ISBN 978-3-642-05499-0 .
• Thomas Tille, Doris Schmitt-Landsiedel: Microelectronics. Semiconductor components and their application in electronic circuits, Springer Verlag, Berlin / Heidelberg 2004, ISBN 978-3-540-20422-0 .

Individual evidence

1. ^ Ellinger, Frank, 1972-: Radio frequency integrated circuits and technologies . Springer, Berlin 2007, ISBN 978-3-540-35790-2 .