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Difference between Voltage Saturated and Current limited

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mona123

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Difference between Voltage Limited and Current limited

I do not understand when someone asks me if my amplifier or oscillator is voltage or current limited? Can someone explain this with some example? Thanks.
 

lamoun

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Hello,

Let's say we have an OPAMP with a large enough input to sent it to saturation (either pos or neg)
[OPAMP specs: sat_voltage = 10V , max_current = 1mA]

If the load is a cap or a big resistor (or a combination such as Zload > 10K) the output will reach the saturation voltage as long as Zload > Vsat/Imax => Zload > 10/1mA => Zload > 10k.

If the Zload < 10k - lets say 1k - then the maximum current won't be enough to reach the saturation voltage. The output voltage will then be Vo = Imax * Zload = 1mA * 1k => Vo = 1V.

So if the max output voltage is limited by the saturation voltage (read supply voltage) of the circuit, then it is voltage limited.
If the max output voltage is limited by the maximum output current (small Zload) then the circuit is current limited.

Hope it helps!
 

Miguel Gaspar

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Saturation refers to an unusuall response. If you have an amplifier with gain = 1 an you increase the input the usuall response at the output is an increase of 1V, thats the normal response. When you reach teh saturation region, an increase of 1V at input doesn't gives you a 1V, perhaps 0.3V of variation.
Limiting is whe you have not reached the saturation region, but you put a device to control the gain.
Voltage limiting for example use two diodes in the output of the opamp


http://images.elektroda.net/18_1303919058.jpg This one activates dte diodes when the voltage reaches about 0.7v and limit tha gaiof the circuit.

http://images.elektroda.net/100_1303919058.jpg In this one qhen Ie of Q1 increasesso the Vbe of Q2 does, when it reaches about 0.7V the Q2 activates and IC2 stole current limiting the Ib1 which limits Lc1.

I hope that it helps you.

greetings
 

mona123

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Thanks for the explaination. Can you please explain it in terms of oscillator please?
 

Miguel Gaspar

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They are explained in terms of oscillator.
You kmust refer to the BArkhaussen criterias.
Please tell me wich king of oscillator are you using: sinusoidal or relaxation
 

mona123

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sinsuoidal colpitts oscillator...
 

Miguel Gaspar

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For a sinusoidal oscillator you must avoid saturation . . voltage or current.
For example if your Vcc = 12V and if Vee = -12 your maxium swing doesn't reach the 12 V.. You must limit the amplitud to avoid it.
Please first investigate the barkhausen creteria.
 

mona123

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For a sinusoidal oscillator you must avoid saturation . . voltage or current.
For example if your Vcc = 12V and if Vee = -12 your maxium swing doesn't reach the 12 V.. You must limit the amplitud to avoid it.
Please first investigate the barkhausen creteria.

Thanks for helping me out but I am not understanding what does barhausen criteria has to do with voltage or current saturation? Isnt barkhausen criteria to do with loop gain or phase and voltage or current limit tradeoff to do with output r dissipated power in oscillator?
 

LvW

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Thanks for helping me out but I am not understanding what does barhausen criteria has to do with voltage or current saturation? Isnt barkhausen criteria to do with loop gain or phase and voltage or current limit tradeoff to do with output r dissipated power in oscillator?

Yes, the Barkhausen criterion - more correctly: the modified start-up criterion from Barkhausen - has something to do with the saturation effect.
In order to start safely, the oscillator must be designed for a loop gain "slightly" larger than unity. As a result, we observe the following effect: The more the loop gain exceeds unity, the more saturation takes place (equivalent to large THD values).
That means: A loop gain of 1.001 (theoretically !) would lead to a rather small saturation/distortion - however, such a value of only 0.1% above unity is hard to achieve and, secondly, would lead to a rather long start-up phase.
Therefore, a trade-off has to be found between the degree of non-linearity (caused by an effective amplitude limiting mechanism) and the oscillator start-up behaviour. Insofar, there is a certain relation between the fulfillment of the oscillation condition and the degree of saturation (if there is no other amplitude control).
 

mona123

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Thanks. do you have any paper or reference that explains in detail about it? You are also talking in terms of amplitude separation. But I am more interested in knowing how to understand current and voltage amplitude saturation and its cause and how can i design it for either and what are its advantages/disadvantages?
 

LvW

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Thanks. do you have any paper or reference that explains in detail about it? You are also talking in terms of amplitude separation. But I am more interested in knowing how to understand current and voltage amplitude saturation and its cause and how can i design it for either and what are its advantages/disadvantages?

* The start-up procedure for a harmonic oscillator is not easy to describe here; you should read some appropriate chapters in electronic textbooks (waveform generation, oscillators).
* As far as voltage saturation is concerned: Each active circuit (with trtansistors or opamps) is powered externally.
I think it is evident that any signal output cannot exceed the limits set by these external dc power supply voltages.
* I cannot see any relevant advantages connected with this limitation. However, I am afraid it cannot be avoided.
 

mona123

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OK let me ask in a little simpler terms the answer I am trying to calrify. When will I design current limited oscillator and not voltage limited oscillator? Thanks.
 

LvW

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I am afraid there is no general answer because it depends on the design - in particular on the impedance level of the used components.
* case 1: Opamp oscillator with voltage output and relatively large impedances that are connected to the output. In this case, the max. amplitude will be voltage limited (due to the supply voltages).
* case 2: If the impedances connected to the output are to small (that means: a relatively large output current) the output amplitudes are limited by the current capabilities of the output stage. Remember: The opamp internal output impedance is never zero!
 
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