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Dependent sources
A dependent source is a voltage or current source whose value depends upon sensing current or voltage in another component, modified by a polynomial equation. This allows any relation between the sense and value, such as linear, squared, cubed etc - all controlled by a simple set of coefficients. It helps if you are a mathematician too.
Dependent sources follow the four basic SPICE types. Any component or a sensor can be used for the sensors, although usually current senses must be voltage sources only. Use a current METER for this. The polynomial form is used, with clear indication of coefficients meaning and effect, from the DEPENDENT SOURCES menu.
To get to this menu, firstly you will need to have drawn any of the following dependent sources:
These are, in order,
- VV, Voltage controlled voltage source
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VC, Voltage controlled current source
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CC, Current controlled current source
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CV, Current controlled voltage source
Now, select the source that you have just drawn, and either double-click on it, or click-on PARAMS in the left hand menu. In the PARAMS menu, there will be a field called 'Dependencies..'. Select this field and either double-click on it or click-on SET .
The DEPEND menu should now be open...
Using this menu it is possible to specify a SPICE dependent source. The list on the left shows the components used to sense from, be it voltage across them or current through them.
Dependent sources are set up for Polynomial operation, which means that a limitless number of coefficients can be used. The polynomial form allows quick and easy construction of voltage and current summers, multipliers, squarers etc.
The dimension of the polynomial is equivalent to the number of dependencies. The general form is:
P0 + P1.V1 + P2.V2 .....Pn.Vn
+ Pn+1.V1.V1 + Pn+2.V1.V2 + ...... P2n.Vn.Vn
+ P2n+1.V1^2.V1 + ....
For practical purposes, this means that when using 'dim' dependencies or sensors, P0 is always an offset voltage, P1 to Pdim multiply the amount of each dimension (set to 1.0 for a summer), and Pdim+1 will be the square of the first dimension. e.g.:
| Type | Sensors | Polynomial Coefficients
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| Two input voltage multiplier | Sensors: R1, R2. | Coefficients: 0.0 0.0 0.0 1.0
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| Two input voltage summer | Sensors: R1, R2. | Coefficients: 0.0 1.0 1.0
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| One input voltage squarer | Sensor: R1 | Coefficients: 0.0 0.0 0.0 1.0
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- P0 is a parameter within the PARAM menu for the component
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P1, P2 etc. are listed in the right-hand list of the DEPEND menu, in expression form.
A typical application for dependent sources is in modelling op-amps, such as the basic ideal op-amp supplied with SUMO in the form of a sub-circuit:
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