Frequency characteristic equivalent circuit of operational amplifier
Time:2022-10-04
Views:1717
Today we will learn the frequency characteristic equivalent circuit of the operational amplifier.
Amplitude frequency characteristic and phase frequency characteristic
1、 Frequency characteristic equivalent circuit
Learning about operational amplifier, we know that the internal structure of operational amplifier is three-level: input stage, intermediate stage and output stage.
Each stage can be equivalent to a cascade of an amplifier with infinite bandwidth and an RC low-pass network. See the figure below.
The equivalent RC time constant is different due to different circuit forms and gain of circuits at all levels.
The output stage is in the form of a voltage follower. Its gain is the lowest, but its bandwidth is the widest (that is, the RC low-pass cutoff frequency is the highest). That is, the RC time constant is the minimum.
The open-loop low frequency gain of the operational amplifier is quite high (above 100dB). This is mainly provided by the input stage (differential amplifier) and the intermediate stage (common radiation amplifier).
Therefore, the RC time constant of input stage and intermediate stage is the largest, that is, the cut-off frequency is the lowest.
2、 RC low-pass characteristics
1. Circuit form
2. Transfer function
3. RC low-pass characteristics
3、 Features of RC Qualcomm
1. Circuit form
2. Transfer function
3. Features of RC Qualcomm
Amplitude frequency characteristic and phase frequency characteristic
4、 Concept of stability
The three stages of the internal circuit of the operational amplifier will produce phase shift at a certain frequency. The more the frequency is close to or more than the characteristic frequency (fo) of the equivalent RC low-pass network of each stage of the circuit, the greater the phase shift will be generated (at the same time, the smaller the transmission coefficient of this stage).
The maximum phase shift of each stage of circuit shall not exceed 90 °
When the cumulative phase shift of the three-level circuit reaches 180 ° at a certain frequency, the output sine wave of the op amp at that frequency will be in inverse phase with the input sine wave.
Because the op amp must use negative feedback in linear applications, and the op amp is inverted at a certain frequency, at this time, the negative feedback will evolve into positive feedback at that frequency.
If AF>1 is also met at the frequency meeting the phase condition, self-excited oscillation will occur.
At this time, the linear circuit formed by the operational amplifier will not work properly—— It is called unstable.
Therefore, the stability of the op amp refers to the possibility that the op amp will not generate self-excited oscillation.
In various linear circuits composed of operational amplifiers (amplifiers, voltage followers, active filters and various operational processing circuits, etc.), whether they can work stably must be considered.
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