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Old 17th June 2007, 05:01 PM   #1
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Join Date: Jul 2006
Default Please explain a servo circuit

I have been reading the forums, and the word "servo" keeps

poping up, I must have fell asleep when that topic was covered,

or I learned it by another name. I have been reading the "op amp

book", I have attached a small quote from chapter 1. Op amps

are very important, and I want to eliminate any confusion I may

have, please explain the servo circuit.

Thank You......
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File Type: pdf servo.pdf (11.3 KB, 709 views)
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Old 17th June 2007, 05:11 PM   #2
BWRX is offline BWRX  United States
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As it is being used around here, a servo is used to null DC output offset.
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Old 17th June 2007, 05:24 PM   #3
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Default Please explain

What is DC output offset please,

Please elaborate, where is it used? I know that DC should

never reach the speakers, how is the "servo" used in a pre amp?

is it an op amp? can you show a circuit?
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Old 17th June 2007, 05:34 PM   #4
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DC is the average of a signal. it is a constant output. a DC offset is a constant (unchanging) signal present of the output of an amplifier (opamp, power amp, ect).

DC offset is affected by a few things. if you hook the opamp up with input shorted, you'll get some baseline offset. using feedback you can reduce this.

a servo uses an opamp in the feedback path of another amplifier to increase the DC gain of the feedback loop to force this error closer to a zero.
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Old 17th June 2007, 05:37 PM   #5
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Default I found this

I searched, and found this explanition,
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Old 18th June 2007, 08:09 AM   #6
gootee is offline gootee  United States
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Default Re: Please explain a servo circuit

Quote:
Originally posted by ppcblaster
I have been reading the forums, and the word "servo" keeps

poping up, I must have fell asleep when that topic was covered,

or I learned it by another name. I have been reading the "op amp

book", I have attached a small quote from chapter 1. Op amps

are very important, and I want to eliminate any confusion I may

have, please explain the servo circuit.

Thank You......
A "servo" system is an "automatic control system", using negative feedback.

In EE (electrical engineering), at least back when I was an EE student at Purdue in the second half of the 1970s, it would have been an entire third-year (out of of four years) undergrad course, called something like "Control Systems", or "Automatic Control Theory", covering analysis and synthesis of "classical" (as opposed to "modern") negative-feedback-based automatic control systems.

If you remember anything about Laplace Transforms, Differential Equations, Bode Plots, et al, you shouldn't have too much trouble learning or re-learning all of the essentials of automatic control theory.

Basically, what was covered in the undergrad "control systems" course enabled one to start with the transfer function of an open-loop "plant" and then, after determining a desired overall transfer function, derive the transfer function needed for the feedback path, such that the closed-loop transfer function would become the desired overall transfer function. Of course, many kinds of system-analysis methods, performance criteria, design methods, and lots of other related stuff, were also covered.

The basic block diagram of the simplest form of a feedback control system has an input, then a "summing junction" that subracts the feedback from the input, then the "plant" block with its associated transfer function in the forward path, then the output, from which a feedback path branches back to the "minus" input of the summing junction near the input. In the more-general case, the feedback path also contains a block with its own transfer function.

"Classical" feedback control systems basically take a desired output and use feedback to generate what is essentially an error signal (i.e. the output of the summing junction; the difference between the desired output and the actual output of the plant (but, more generally, the output as processed by the feedback block's transfer function)), which is applied back into the plant's input. In a properly-designed and well-behaved control system, this has the effect of forcing the output to change, in a well-behaved way, to become equal to the desired output.

When using opamps to implement such a feedback control system, a "difference amplifier" opamp circuit could, for example, function as the "summing junction", to subtract the feedback of the output from the desired output. (See the well-known classic application notes AN-31 and AN-20, at http://www.national.com , for straight-to-the-point examples of the basic opamp amplifier topologies.)

But if the feedback path contains anything other than a constant (i.e. pure gain, i.e. "proportional" feedback), such as integration and/or differentiation for example, then often, in opamp-based circuits, the "summing junction" functionality is combined with one of the other operations, if possible, to lower the number of opamps needed for implementation of the system as a real circuit. An example of that is shown in the "DC SERVO" circuit I posted in another thread, very recently (the Chipamps thread named "PA100 with DC SERVO?"), which has now been posted at

http://www.fullnet.com/~tomg/gooteesp.htm

In that DC SERVO example circuit, the integrator opamp used in the feedback path is ALSO performing part of the subtraction of the output from the desired output, and then integrating the resulting "error" signal. Note that the "desired output", in that case, is "ground", i.e. zero volts, since that control system's purpose is to try to force the DC component of the main amplifier's output to always be zero. (And also note that integrating or low-pass filtering an AC signal results in the mean (average) of the signal, which is just its DC component, or "offset".) i.e. If you were viewing the main amp's output on an oscilloscope: The servo loop keeps the average value of the amplifier's audio output waveform centered, vertically, with respect to the horizontal axis (zero volts).

Real circuits can sometimes look quite a bit different than the nice, neat block diagrams of feedback control systems. You might notice, in the DC SERVO example circuit, that the sign of the subtraction of the output from the desired output seems to be reversed, with the desired output being subtracted from the output, instead. It is. But that's because the feedback loop eventually goes to the main amplifier's inverting input (the main/final "summing junction"), reversing the sign again.

---------

Fully understanding opamp amplifier circuits, in general, also necessitates understanding classical feedback control system theory, since virtually all opamps used as amplifiers are used with a negative feedback loop (i.e. from output to inverting input). Using classical feedback control system theory, virtually all aspects of opamp amplifier performance and stability, etc, can be fully and rigorously analyzed, predicted, designed, etc.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html
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Old 18th June 2007, 12:02 PM   #7
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Default Thank You Tom

Now I know where to study.
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