what is dependent sources.

[sohail qureshi]

m
slama experts.....
my question is about dependent sources...that

what is dependent sources....?? plz gives some examples........tnx in advance

 

[atripathi]

A dependent source is whose o/p is function of something else, i.e. dependent upon something else.
Any Opamp circuit or transistor opearating in linear region is an example of dependent sources. In each of these cases output is dependent upon input.

 

[WimRFP]

In simulation, dependent sources are sources (current or voltage) that are controlled by another variable. For example the Voltage Controlled Voltage Source is a dependent source.

 

[sohail qureshi]

would you give the example of voltage control voltage source........///???

 

[WimRFP]

In real world an inverting or non-inverting opamp circuit where

Vout = c1(Vin + c2), alternative form: Vout = c1(Vin) + c3

c1 is the gain (as determined by the resistors, can be negative or positive) and c2, c3 is just a voltage offset (mostly determined by resistors also).

You may know that due to non-zero output impedance, the output voltage is slightly dependent on output current (for the opamp circuits).

 

[LvW]

In simulation, dependent sources are sources (current or voltage) that are controlled by another variable. For example the Voltage Controlled Voltage Source is a dependent source.

I am sorry, but - up to now - no answer is correct.
A voltage-controlled voltage source is - as indicated by the name - a controlled source that is controlled by an independent signal caused by any input. But it is not a dependent source.
So what means "dependent"?
A dependent voltage or current source is a source that depends on some internal state variables (another voltage or current within the circuit) that is NOT caused by the input signal only. Such a device is used very often for modelling active devices for the purpose of circuit simulation.

---------- Post added at 12:00 ---------- Previous post was at 11:38 ----------

Perhaps a simple example is helpful:

If you connect a dependent current source I in series with an ohmic resistor Rb you have a very simple diode model.
Rb acts as a linearizing path resistance and the current source depends on the voltage Uj that is developped across the two current source nodes according to the following formula:
I=Io*(exp(Uj/Ut) - 1) with Io=selected constant and Ut=thermal voltage.
The model can be improved with a junction capacitance Cj in parallel with the dependent source.

 

[WimRFP]

LvW: It depends on the area of application. First hit on google related to simulation: "A dependent source is either a voltage or current source whose value is proportional to some other voltage or current in the circuit." So the output "depends" on the input.

Maybe Sohail can give us the context.

One can extend this to non-linear relations also (as in the Arbitrary Current/Voltage Source where you can enter non-linear equations).

 

[LvW]

LvW: It depends on the area of application. First hit on google related to simulation: "A dependent source is either a voltage or current source whose value is proportional to some other voltage or current in the circuit." So the output "depends" on the input.

Maybe Sohail can give us the context.

I think a definition must not depend on the area of application.
As your reference says: proportional to some other voltage or current in the circuit.
This does NOT mean the input signal, which is considere to be independent on the connected circuit.
More than that: Do not blindly rely on google or wikipedia. As my above example shows, a dependent source is NOT necessarily proportional to some state varaibles. In general, it is a function of an internal voltage(current) - and this function can be specified by the user (linear, quadratic, log, exp,...).

 

[WimRFP]

LvW: Regarding definitions and applications, I agree with your view, but real life is different, as you may know. I can follow your views, but both Beige Bag and Orcad spice simulators consider the E, F, H and G blocks as "dependent sources". I am sorry, I can't change the manuals.

 

[LvW]

Hi WimRFP,

I know what you mean - but I think the misunderstanding originates from a characterization that was somewhat incomplete (in the discussion up to now).
Of course, a "dependent source" at the same time is also a controlled source because its output is controlled by a signal. This signal can be an input (then the output is simply "controlled" by the input signal) or any other variable within the circuit.
In the latter case it is a "voltage or current controlled dependent voltage or current source".
The term "dependent" is an indication for a sort of feedback within the circuit, which means that the source output influences all other voltages/currents - and, thus, also oneself.
In the following, I list some references which support this view:

***broken link removed**
*https://inst.eecs.berkeley.edu/~ee100/sp05/lecture_notes/PSPICEDependentSources.pdf
***broken link removed**
(Sorry I changed the last ref., it was wrong)

Here is a quote from the latter reference:
SPICE2 allows circuits to contain dependent sources characterized by any of the four equations
i=f(v) v=f(v) i=f(i) v=f(i)


I think this excerpt clarifies that the source signal depends on another variable within the circuit.
By the way: The letters E and G, respectivlely, characterize the output signal only (F and H do not exist anymore in PSpice). If they are used simply as "controlled" or "dependent controlled " sources depends on the way the control signal is derived.

---------- Post added at 17:16 ---------- Previous post was at 16:20 ----------

Only now the best and shortest explanation just has come into my mind:

In a dependent source the control function itself is a function of a circuit variable.
In a controlled source the control function is constant (gain or transconductance value, but can be, for instance, frequency dependent).

 

[sohail qureshi]

tnx you all............i get it

 

[FvM]

I am sorry, but - up to now - no answer is correct.
A voltage-controlled voltage source is - as indicated by the name - a controlled source that is controlled by an independent signal caused by any input. But it is not a dependent source.

I must confess, that I wasn't yet aware of a principle difference between dependent and controlled sources in circuit simulation. And I still doubt, that it exists.

The classical SPICE terminology distinguishes between independent and controlled sources. Based on this determination, "dependent" can't be but a synonymous of "controlled". So what could be a purpose of discriminating between the latter?

Technically, both functions are provided by the same class of elements, generally they are creating a link between node voltages and/or mesh currents. As a special case, the controlling quantity may be independent of internal states. But it doesn't change the nature and behaviour of these elements. A voltage source only linked to an external voltage is still a dependent source. And a current source with it's output tied to the input to simulate a resistor can be still considered a controlled source.

At present, I won't abandon using "controlled source" as the generic term for the E,G,F and H elements, keeping in mind that they are dependent sources as well.