Loop gain

The loop gain (Ag ) indicates in electrical engineering, the factor by which a signal is amplified, which is exposed in an electronic circuit of a feedback once it goes through the loop. Feedback means in this case that a part of an amplified output voltage is fed back to the input. With the passage of a loop is here meant to this point back to the passage of the signal by a period.

To determine separating the loop at any point and feeds - either really with a signal generator or just as a thought experiment - a weak ( AC ) signal. This is amplified at a point of the circuit, and is attenuated in the other after passing through some components back to the point of separation. Then it has either more or less amplitude than the incoming signal. The ratio of the two amplitudes is the loop gain of the circuit.

To assess the impact, it also depends on the phase shift of the input signal.

When an oscillator the stability criterion of Barkhausen applies:

• The phase shift must be 0 ° or 360 °.
• Is the loop gain of at least 1, the circuit generates a vibration.
• Is the loop gain is greater than 1, the amplitude of oscillation increases, until - usually by overdriving the amplifier - the amplitude can not grow.
• If the loop gain is less than 1, the amplitude of a kind incurred disorder always smaller ( damped oscillation ).

If the phase shift is 180 °, it is called a negative feedback or negative feedback. In many circuits, the phase shift changes as a function of frequency. This is to take into account, for example, in the design of the circuit of operational amplifiers.