The feedback circuit is widely used in various electronic circuits. The feedback is to feed back some or all of the output signal (voltage or current) of the amplifier to the input of the amplifier and compare it with the input signal (add or subtract) and use Compare the resulting effective input signal to control the output. This is the feedback process of the amplifier. Any feedback signal that is fed back to the input of the amplifier is used to strengthen the input of the original input signal, so that the input signal is increased by positive feedback. Otherwise, vice versa.
According to its circuit structure, it is divided into: current feedback circuit and voltage feedback circuit. The positive feedback circuit is mostly applied to the electronic oscillating circuit, while the negative feedback circuit is applied to various high and low frequency amplifying circuits. Because of the wide application, we Negative feedback circuits are discussed here. Negative feedback has four effects on amplifier performance:
1. Negative feedback can improve the stability of the amplifier gain.
2. Negative feedback can broaden the passband of the amplifier.
3. Negative feedback can reduce the distortion of the amplifier.
4. Negative feedback can improve the signal-to-noise ratio of the amplifier.
5. Negative feedback has an effect on the output and input resistance of the amplifier.
Figure F1
It is a basic amplifier circuit. This circuit looks very simple, but it contains a DC current negative feedback circuit and an AC voltage negative feedback circuit. R1 and R2 in the figure are BG DC bias resistors, and R3 is an amplifier. The load resistance, R5 is the DC current negative feedback resistor, the branch composed of C2 and R4 is the AC voltage negative feedback branch, and C3 is the AC bypass capacitor, which prevents the negative feedback of AC current.
1. DC current negative feedback circuit.
The base voltage VB of the transistor BG is the divided voltage value of R1 and R2, and the voltage VE of the BG emitter is Ie*R5, then the voltage between B and E of BG=VB-VE=VB-Ie*R5. For some reason (such as temperature change) causes Ie B of BG VE↑, BG base voltage = VB-VE=VB-Ie*R5↓ This makes Ie↓. Stabilize the DC operating point. This negative feedback process is due to Ie It is caused by ↑, so it belongs to the current negative feedback circuit. The emitter capacitor C3 provides the AC path, because if there is no C3, the AC signal will also form a negative feedback due to the presence of R5, which will make the amplification factor of the amplifier large. discount.
2. AC voltage negative feedback circuit
The AC voltage negative feedback branch is composed of R4 and C4, and the output voltage is fed back to the input terminal through this branch. Since the signal at the output of the amplifier and the input signal voltage are mutually opposite in phase, the introduction of the feedback signal is weakened. The role of the original input signal. So it is a voltage negative feedback circuit. R4 is the size of the negative feedback control, C4 acts as a DC-connected AC. When the amplitude of the input AC signal is too large, if there is no R4 and C4 Negative feedback branch, the amplifier will enter the state of saturation or cut-off, causing the output signal to appear clipping distortion. Since the negative feedback is introduced to control the amplitude of the input AC signal, the distortion is avoided.
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