LT Spice For Radio Amateurs: PART 4

So lets bring everything we have done so far together into 1 project. So we use a new component in this schematic a transistor. To add the transistor select components->npn. The we can right click the component and select a transistor type. For this circuit we will use some rather standard 2n2222 and the 2n2219a.

So in the above circuit we have a voltage source V2 where you will notice the proper use of Label Nets. Anywhere a Label Net has been placed with the value of 12v, the 12 volts from  V2 will be applied. Run a transient analysis with the following values to see for yourself how the Label Net works “.tran 0 0.000001 0.0000001”.


Ok, so what is the above circuit. It is a rather poorly designed buffer and amplifier. What is does however is demonstrate everything we have done so far. We have voltage and signal sources, we have the voltage divider R1 and R2, we have DC Blocking caps C2, 3 and 5, we have L1 blocking AC from entering the power rail. Something else you might notice is we are using 200 ohms for the load and source impedance.

So, once you have the circuit built, run a simulation and follow the signal through the circuit. There is so much that can be learned from probing about the circuit. Voltages can be found on the base, emitter and collector of the transistors, the current flowing through the transistors can be found and we can see the amount of total gain of the amplifier.

So for those who have been paying attention, you will have noticed that the input signal is 0.2v p-p, the green trace in the above circuit. And the output at the top of R6 is close to 5v p-p, which means that the gain is close to 27db.

And there you have it, you now know enough to be able to make complex circuits in LT Spice and simulate them and make usable measurements to define some parameters of amplifiers.


LT Spice For Radio Amateurs: PART 3

Flow control is something we need to do in radio circuits, we want to keep things out, let only some things pass and all that. We certainly do not want the output of our amplifiers being sent out the power jack into the house wiring and turning the house into a giant antenna for our oscillator circuit. Nor do we want DC being passed into our amplifier circuits either, because that adds a DC offset to the signal that is often not desirable. So, what we do is use DC and AC blocking. So lets knock up a circuits that demonstrates these principles. No gimmies this time, you are on your own.

In the above schematic we have a 7mhz signal source consisting of a 1v p-p sine wave with a 5v dc offset. Signal impedance is set to 50 ohms and a 50 ohm load is used.

So when this circuit is run, out-a should show the 1v signal with 5v offset and when probing the other side of the DC blocking cap, all we have left is the 1v p-p signal as the cap blocks the 5dc from passing on in the circuit.

An inductor does the opposite of the dc blocking cap, it blocks the ac signal and allows dc to pass through. The green trace is out-a and the blue trace is out-b. So from the above 2 simulations we have a good demonstration of the DC blocking and AC blocking action of capacitors and inductors. Add in a voltage divider and a transistor to both of these and we have a simple amplifier.

And with that, you now know a bit more about electronics fundamentals and a good grounding in some of the fundamental aspects of using LT Spice in using signal sources, voltage sources, dc offsets and making complete circuits. Have a play with values in the above circuits and see what the outcomes might be. As this will demonstrate why certain values are often chosen and used in different designs.


LT Spice For Radio Amateurs: PART 2

Ok meat and potatoes time. Lets look at some simple circuits that are electronics fundamentals to illustrate the basics of using LT Spice.

FILE: Voltage-Divider

So you built the voltage divider circuit above, or if you were lazy you downloaded the one I have linked above. Either way, you now have a complete circuit that can be simulated. We have a voltage source providing 12 volts to the circuit, we have R1 and R2 forming a voltage divider, we have a ground and we have 2 Net Labels, Out-A and B.

In this instance the net labels are not being used for their proper purpose, but rather to provide a convenient place to probe the circuit under simulation. So, lets run this thing and see if it does what we expect, in proving Ohms Law is true and working HEHE.

So if you assembled the circuit yourself, right click the schematic and select RUN. This will bring up the simulation command window. We will be performing a Transient Analysis, plug in some values as I have above, and click ok. The window will now split and show you the simulation window.

If you move the mouse around in the schematic window you will notice that the cursor will change from cross hairs to a probe when you hover over OUT-A and B and will look like a current meter when you hover over the voltage source. So we can measure the voltages at points A and B and the current flowing in the circuit. And if you look in the above image, you will see 2 traces, the green one is 12v from out-a and the blue one is 6v at out-b, just what we expected for a voltage divider with 2 10K resistors.

Change the value of the resistors and then run the simulation again, notice what effect that has on the voltage at out-b, its going to change, by how much will depend on the value you change it to.

FILE: Pi-Attenuator

Another simple resistor only circuit is the pi attenuator, I think off the top of my head that this attenuator is -3db. Load it up and run it and see if i am right.

This time we are using a voltage source as a signal. Select a sine wave, give it an amplitude of your choice, its value is Peak to Peak, set the frequency of the signal, I gave it 7 megahertz. Now, this is important, all signal sources need to have their impedance set. The Series Resistance box will set the signals impedance, and being that we like to have 50 ohms impedance everywhere, lets set the series resistance to 50R. Also, all circuits need a load impedance also, if you look at the schematic you will note there is a 50R resistor to ground after the circuit under test, this sets our load impedance to 50R also. 50R in, 50R out, with known impedances we should get accurate results.  Click ok, then run the simulation using the transient analysis numbers shown in the above schematic. Probe points A and B and you should have something like the image below.

The green trace is our 1v p-p 7mhz input signal, the blue trace is our attenuated now 700mv p-p signal. And now you know enough about how to use LT spice to test simple circuits, using both DC voltage sources and AC signal sources. Change the values about and see what happens, the good thing is none of this costs any components or solder. Just your time and some self learning.


LT Spice For Radio Amateurs: PART 1

Ok, so sometime ago I started writing a series of articles for the WIA AR Magazine from a beginner to homebrew persective. Now for various reasons of no real importance to this story part way though i changed my mind and pulled the pin on the whole thing, but I still have my original files and so now seems as good a time as any to edit them up, add some more information and present them here for those who might be looking for a very basic intro into LT Spice.

I am going to assume you know how to download and install the software and get it up and running. And I am only going to gloss over using the interface and get straight into building basic circuits in the program and testing them. For some circuits I will provide the LT Spice files, for others I will not. The whole point about this series is to learn HOW TO DO IT, not just play with the crap I have already done.

So, lets get into it.

The Interface:

Everything you need to know is pretty much in this one picture. So run the program, click FILE->NEW SCHEMATIC and start clicking things.

  1. The Scissors: are used to remove components and wires when you stuff things up.
  2. The Pencil: is used to connect parts together with wires.
  3. Ground: every circuit has to have a ground point. It wont work without one.
  4. Label Net: Meh kind of ignore this for now.
  5. Resistor: used to place resistors in the circuit.
  6. Capacitor: used to place caps in the circuit.
  7. Inductor: used to place inductors in the circuit.
  8. Diode: used to place diodes in the circuit.
  9. Components: components are things like IC’s, transistors, voltage and signal sources.
  10. The Hands: used to move things or drag things around in the schematic.
  11. Text: to add notes to the schematic.
  12. Spice Directive: High end feature used for making complex inductors like bifilar and trifilar windings.


When you place a passive component in the schematic it has no value. To set its value you right click on it and this window pops up. You can enter the values in Ohms, Farads or Henry’s and thats ok if you know exponential notation, or you can shorthand things for resistors 10R, 10K, 10M will be 10 ohms, 10,000 ohms and 10 million ohms, capacitors 10pf, 10nf, 10uf for pico, nano and micro farads, and inductors 10nh, 10uh for nano and micro henry’s.


Finally, components, the main things we will be placing are NPN transistors, N type MOSFETS and Voltage Sources. Voltage sources can be DC or AV volts like for powering things and they can also be AC Signal Sources.

And with that, you now know how to place components into a schematic and find all the parts for might need to build, test and simulate many different circuit types in LT Spice.