What feedback does

116

01:16:10

This circuit demonstrates the power of feedback. 

Feedback (closed loop) has a lot of advantages over open loop. The left amplifier is open loop, while the second amplifier is closed loop. Feedback is only done with a single resistor, and the feedback gain can be adjusted by adjusting the resistor. Feedback gives you control of the output impedance too, among others.

Notice how the closed loop system removes the crossover distortion from the output. The open loop system has a gain of about 221, while the closed loop only has a gain of about 2. 

published 9 years ago

hurz

9 years ago

You mixed a little the meaning of closed and open.... Cause in open the gain of the opamp will dominate with equal factor for both circuits. And closed they have also the same gain. The only thing is the point of feedback. Check this http://everycircuit.com/circuit/5668386847064064

uberula

9 years ago

Hi. Yes I agree with you that feedback sole purpose shouldn't be to remove the crossover distortion, and its best to use something like a Vbe multiplier. I only used the op amp as a pre amp, and I refer to the class b amplifier being open loop or close loop.

thebugger

9 years ago

Yes a negative feedback has many advantages not only crossover distortion cancelling. It also keeps the output stable with load variations (a very important principle in voltage regulators) it flattens the frequency response (important in some types of amps), and also it can be made nonlinear which is an important part of stabilising oscillators output voltage. It's also often used for DC thermal drift compensation in amps, and to limit the upper boundary of the bandwidth. I used to make my amps with a low open loop gain and without feedbacks in the beginning, but I now have learned to appreciate the importance of negative feedbacks and include them in every design, and a lot of them too. DC, AC, Frequency dependant, local, complete and such....

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