3730 views|0 replies

1782

Posts

0

Resources
The OP
 

Feedback circuits in electronic circuits [Copy link]

Feedback circuits in electronic circuits

This article comes from: http://www.51dz.com/dp.asp?dl=&xl=02&so=&fw=&gjz=&pg=2&sn=2832#&i=wangfei Feedback circuits are widely used in various electronic circuits. Feedback is to feed back part or all of the amplifier output signal (voltage or current) to the amplifier input end for comparison with the input signal (addition or subtraction), and use the effective input signal obtained by comparison to control the output. This is the feedback process of the amplifier. Any feedback signal fed back to the amplifier input end that strengthens the original input signal and increases the input signal is called positive feedback. The opposite is true. According to its circuit structure, it is divided into: current feedback circuit and voltage feedback circuit. Positive feedback circuits are mostly used in electronic oscillation circuits, while negative feedback circuits are mostly used in various high and low frequency amplifier circuits. Because of its wide application, we will discuss negative feedback circuits here. Negative feedback has four effects on amplifier performance: 1. Negative feedback can improve the stability of amplifier gain. 2. Negative feedback can widen 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 impact on the output and input resistance of the amplifier.    Figure F1 is a most basic amplifier circuit. This circuit looks very simple, but it actually contains a DC current negative feedback circuit and an AC voltage negative feedback circuit. R1 and R2 in the figure are the DC bias resistors of BG, R3 is the load resistor of the amplifier, 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 generation of AC current negative feedback. 1. DC current negative feedback circuit. The base voltage VB of the transistor BG is the voltage divided by R1 and R2, and the voltage VE of the emitter of BG is Ie*R5. Then the voltage between B and E of BG = VB-VE = VB-Ie*R5. When some reason (such as temperature change) causes BG's Ie ↑ then VE↑, the voltage of BG base emitter = VB-VE = VB-Ie*R5↓ so that Ie↓. The DC operating point is stabilized. This negative feedback process is caused by Ie↑, so it belongs to the current negative feedback circuit. The emitter capacitor C3 provides an AC path, because if there is no C3, the AC signal will also form a negative feedback effect due to the existence of R5 when the amplifier is working, which greatly reduces the amplifier's amplification factor. 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 through this branch. Back to the input end. Since the signal at the output end of the amplifier is in anti-phase with the input signal voltage, the introduction of the feedback signal weakens the effect of the original input signal. Therefore, it is a voltage negative feedback circuit. R4 controls the amount of negative feedback, and C4 plays the role of blocking DC and passing AC. When the amplitude of the input AC signal is too large, if there is no negative feedback branch of R4 and C4, the amplifier will enter a saturation or cut-off state, causing the output signal to be clipped. Since the input AC signal amplitude is controlled by the introduction of negative feedback, distortion is avoided.












This post is from Analog electronics

Guess Your Favourite
Find a datasheet?

EEWorld Datasheet Technical Support

Copyright © 2005-2024 EEWORLD.com.cn, Inc. All rights reserved 京B2-20211791 京ICP备10001474号-1 电信业务审批[2006]字第258号函 京公网安备 11010802033920号
快速回复 返回顶部 Return list