Need help creating a subcircuit for an inverting amplifier in LTSpice for Mac

Thread Starter

SwtNemesis

Joined Oct 14, 2016
17
Hello,
I am having difficulty creating an inverting amplifier sub-circuit using LTSpice for Mac. I have attached the a screenshot of the necessary specs but have not been able to correctly simulate it. A pdf of my newest attempt is also attached along with the .asc file from LTSpice. Any help would be much appreciated as I am new to LTSpice but am eager to learn to use it as a replacement for PSpice.
 

Attachments

JoeJester

Joined Apr 26, 2005
4,326
I don't see where you have the controlling voltages connected to the VCVS and your output is shorted in your PDF
 
Last edited:

Thread Starter

SwtNemesis

Joined Oct 14, 2016
17
I don't see where you have the controlling voltages connected to the VCVS and your output is shorted in your PDF
Ok...so I guess I'm pretty lost then. How do connect the VCVS and fix the output? I tried connecting it using a net label but I guess that won't do. I have run and setup other circuits (ie low/high band pass) but have never setup something like this in PSpice or LTSpice.
 

Alec_t

Joined Sep 17, 2013
11,549
Instead of the E source (read the Help regarding convergence issues with E sources) you could use a resistor-loaded Behavioural Current source, or a Behavioural Voltage source with the property v={gain}*v(in-,in+). Nodes you wish to use as inputs/outputs should be labelled if you want to use the schematic in a hierarchy or for creating a model.
 

Thread Starter

SwtNemesis

Joined Oct 14, 2016
17
Ok but I'm told that this circuit is straight forward in nature. Is my set up correct or does it need to be tweaked? When I run it, I get nothing but current. Did I choose the proper run? Are my components correctly set up? I guess my issue is translating the written schematic into LTSpice schematic.
 

eetech00

Joined Jun 8, 2013
1,967
Hi

If you need to create the equivalent of that shown in the screen shot pdf, you would use an arbitrary behavioral voltage source ( "Bv" Device) in LTspice. The equation would be added to its value field. For example, if avol=100k, then the value would be set to "V=100k*(V(V1)-V(V2))" without quotes. You would need to add node labels to label the opamp input wires (the two wires connected to the resistor) "V1" and "V2". However, the recommended LTspice device for opamps is a VCCS because is converges better.
 

Thread Starter

SwtNemesis

Joined Oct 14, 2016
17
Yes. The assignment has constraints but I just want to get it working first. I'll take any advice as long as it works. If I set up this circuit exactly like the screenshot, it will have floating nodes. So I guess I really need instruction on how to set this circuit up from start to finish. I also need to do a second run using the LM741 opamp. Since it's not on the list of components, I assume that I will need to import its specs somehow? My professors always recommend using LTSpice as opposed to PSpice for simulations but don't know how to use it at all really.
 

Alec_t

Joined Sep 17, 2013
11,549
Here is an outline of the Spice amp. You were on the right track originally, except that you had V2 grounded directly, and no control inputs to the E source. Note the addition of arbitrary, but high value, resistors R1 and R2 to keep Spice happy (it needs a ground as reference) without significantly affecting the amp behaviour.
AmpModel.JPG
 

Thread Starter

SwtNemesis

Joined Oct 14, 2016
17
Here is an outline of the Spice amp. You were on the right track originally, except that you had V2 grounded directly, and no control inputs to the E source. Note the addition of arbitrary, but high value, resistors R1 and R2 to keep Spice happy (it needs a ground as reference) without significantly affecting the amp behaviour.
View attachment 115892
Ok. I have set it up according to your pic but what type of run do I need to perform? Also, where do the values for v1 and v2 come into play? Do I need to set up a separate voltage source and use the net name to reference it? I have done something similar when I did a band pass filter in LTSpice.
 

Alec_t

Joined Sep 17, 2013
11,549
The two output nodes should be labelled differently. Vout is the voltage difference between the two output terminals of the E device. You are supposed to model the arrangement in the Screen shot pic in post #1.
 

JoeJester

Joined Apr 26, 2005
4,326
Can @SwtNemesis create a x10 amplifier using the normal opamp symbols? Once they do that, it's a matter of replacing the opamp with the model the instructor wants them to use.

@Alec_t Is there a need for R1 in your example? The ground would be provided via the voltage sources returns or a ground connection. Is there a list of things LTSpice have done to "keep spice happy"? I know I add a path to ground in some circuits to satisfy spice in TINA, and I routinely start my simulations at zero volts to show the reactions as if I'm turning on the power switch.
 

Alec_t

Joined Sep 17, 2013
11,549
Is there a need for R1 in your example? The ground would be provided via the voltage sources returns or a ground connection.
Agreed, that would be the normal situation. I was trying to keep things general, allowing for a floating voltage source possibility.
 

JoeJester

Joined Apr 26, 2005
4,326
@SwtNemesis

You just have the basic information your teacher has given you. You have not constructed the -10 gain.

The second part of your assignment is to make a -10 gain amplifier using the uA741 model. Can you do that?
 

Thread Starter

SwtNemesis

Joined Oct 14, 2016
17
Yes. Where do I attach the Rf of 10ohms and R1 of 1ohms? How can I simulate uA741? It is not on the list of components so Im assuming that I have to import its specs.
 

Alec_t

Joined Sep 17, 2013
11,549
You have set V2 as a 10V DC source. If you want to do an AC analysis you need to set V2's "AC amplitude" property.
I think you need to re-read the 'Help' regarding the different types of analysis possible with Spice.
 
Top