Loop reinforcement
In electrical engineering , the loop gain (A g ) indicates the factor by which a signal is amplified which is subject to feedback in an electronic circuit once it passes through the loop. In this case, feedback means that part of an amplified output voltage is fed back to the input. By passing through a loop, it means the passage of the signal from one point back to this point.
To determine this, the loop is separated at any point and - either actually with a signal generator or only as a mental experiment - a weak (alternating voltage) signal is fed in. This is amplified at one point in the circuit, weakened at another and arrives at the point of separation after passing through some components. Then it has either more or less amplitude than the signal fed in. The ratio of the two amplitudes is the loop gain of the circuit.
The phase shift of the fed-in signal is also important for assessing the effects .
Barkhausen's stability criterion applies to an oscillator :
- The phase shift must be 0 ° or 360 °.
- If the loop gain is at least equal to 1, the circuit generates an oscillation.
- If the loop gain is greater than 1, the amplitude of the oscillation increases until - usually due to overdriving the amplifier - the amplitude can no longer grow.
- If the loop gain is less than 1, the amplitude of any disturbance that has arisen in some way becomes smaller and smaller (damped oscillation).
If the phase shift is equal to 180 °, one speaks of negative feedback or negative feedback . In many circuits the phase shift changes as a function of frequency. This must be taken into account , for example, when dimensioning the circuitry of operational amplifiers .
literature
- Ulrich Tietze, Christoph Schenk: Semiconductor circuit technology . 12th edition. Springer, Berlin 2002, ISBN 978-3-540-42849-7 .