Electrical – OTA LTSpice model help

ltspiceota

(This is my first post go easy on me)

I'm trying to use an OTA to simulate a floating voltage controlled resister like explained here, but I cannot for the life of me figure out how the mota model functions.

I'm aware of the general idea of an OTA. Differential input, biasing current multiplier, etc. But I cannot get the model in LTspice to function properly. I thought at first that I was supposed to connect a voltage to the bottom left I/O pin, but then I figured out it was a ground reference…

I'm having trouble getting any output from the OTA, and the current doesn't seem to be entering the biasing input.

Any and all help would be appreciated!

Green V(n010): Differential voltage measured from negative input to positive input

Blue V(n015): Output node of OTA

Red I(R10): Current through R10 (supposedly biasing output current, but not working)

Best Answer

Since you are using the ota, then maybe you are not looking for a real-life case, instead, you want a behavioural model. If so, the simplest voltage dependent resistor is the behavioural resistor:

R1 nodeA nodeB R=f( V(control) )

where V(control) is the controlling voltage and f(x) is an optional function of your choice. Being a behavioural element, it has advantages and disadvantages.

Another approach, a bit more involved, but without the behavioural resistor, is using this:

The circuit on the left is the "engine": Iref generates a unity current over S1 (a VCSW), generating a voltage. This is then compared with a reference voltage, Vref, who represents the desired time varying resistance. The difference goes through an error amplifier and a loop filter (Gfb and Cfb), generating a controlling voltage for the switch. This voltage can control any other switch in the schematic (like S2) that needs a varying resistance as supplied by Vref.

It's not perfect, either: the bandwidth is limited by the time constant of Cfb and its series resistance (1 \$\Omega\$, thus only Cfb's value matters -- Rpar is there just for a DC path to ground), the on/off values of the switche's resistances are the lower/upper limits of its variance, and, not lastly, the number of elements/nodes. OTOH, it's blazingly fast, despite this last shortcoming.

As for the OTA, here are three ways to use it, based on various parameter settings:

A1 uses iout, which means symmetric, tanh() limited output (black trace), A2 uses different isrc and isink for asymmetric, tanh() limited output (blue trace), and A3 uses the linear flag, which means that any voltage going beyond the limits will have the gain halved (red trace). All are compared to the normal V(a)*V(b) (gren trace).

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From the LTSPice IV manual:

X. Subcircuit 
Syntax: Xxxx n1 n2 n3... <subckt name> 
[<parameter>=<expression>] 
Subcircuits allow circuitry to be defined and stored in a 
library for later retrieval by name. Below is an example of 
defining and calling a voltage divider and invoking it in a 
circuit. 
* calling a subcircuit 
* 
* This is the circuit 
X1 in out 0 divider top=9K bot=1K 
V1 in 0 pulse(0 1 0 .5m .5m 0 1m) 
* This is the subcircuit 
.subckt divider A B C 
R1 A B {top} 
R2 B C {bot} 
.ends divider 
.tran 3m 
.end

Notice that params: never appears in the LTSpice syntax for a subcircuit call.

I am guessing that including this token in your X card has confused LTSpice about how the subcircuit should be called.

Electronic – Has LTspice been extended to support the modified-Chan, non-linear hysteretic model for coupled inductors (transformers)

LTspice supports the Chan core since long ago. Unfortunately, it doesn't support direct coupling with other than linear inductances (i.e. the default inductance). However, as you found out in your searches, there are ways to circumvent this. If it were me, I'd use this link for my needs, but there are other links, as well (and also some examples in the default installation of LTspice).

A few remarks: there is no "specialized" symbol denoted L, that is the general SPICE notation for inductance, ever since 40+ years ago. The parameters are specialized, and this can be checked in the LTspice manual. Even if it is pretty spartan, I highly recommend reading it, at least once, it may save you tons of searches on the net. Also, in the LTspice Yahoo Groups archive and message list there should be more than enough examples to get you started, if the LTwiki link doesn't do it for you.

This is a simpler version to that from the LTwiki, it only accounts for primary/secondary resistances:

Note that I used G sources for their superior convergence over E sources. If you need more than two windings, extending this should be fairly easy, as the Chan core is only used as a "prototype", with one turn; the sources take care of the windings (and anything else that may be added).

The example on the LTwiki, though, should be pretty self-explanatory, I'm afraid that, if you are looking for a simple, "place L, add coupling" sort of transformer, there is no such thing and you're likely be asking for external libraries which will, most probably, have the same arrangement under the hood.

Best Answer

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