Obviously with the bias high enough the impedance of the transistor is 0 - it's just a diode. Probably not. The diode model probably includes parasitic resistance in the base and emitter contacts, so the input mbed ota will never go all the way to zero.
Also, your R3 multiplied by the transistor beta will set a lower limit on how low a resistance you measure looking in to the base of the transistor in your circuit. Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered. Asked 4 years, 9 months ago. Active 4 years, 9 months ago. Viewed 4k times. So from this plot it looks like it's about 2. Is this correct? Active Oldest Votes. Yes Obviously with the bias high enough the impedance of the transistor is 0 - it's just a diode.
The Photon The Photon k 3 3 gold badges silver badges bronze badges.This one will be a little meta because I wanted to look at a capability in LTSpice which ironically is very useful, but not often used. This is something you probably already know about, but it is interesting to look at in simulation if you know how to coax LTSpice — no pun intended — into showing you a meaningful graph.
The circuit is super simple. An AC source and a ohm resistor stand-in for a meter ham transmitter. With volts into a ohm load. So far, so good. You can also plot an expression like:. The bad part is the plots that result have a pretty fixed format: some parameter versus time. Sure, you can do math on the Y-axis, but you are always plotting against time.
It is easy to set up Spice to do multiple simulations for comparison. This tells Spice to vary R from 1 to in steps of 10 ohms. Now the plot is a mess, even zoomed in. Besides, this is an easy trend to spot, but it would be nice to have a plot of power versus RL. The answer is the. MEAS or. There are actually two forms of the. MEAS command. The first one computes a single value.
MEAS command is just right for that. Reflect is like a variable name.
LTspice: Using .MEAS and .STEP Commands to Calculate Efficiency
It could have been anything. You can see there is a single value for each step of R. You can put certain operations in to look at all the simulation data from a run and do a calculation on it, again collapsing it to a single number.
Note you can enter these both at one time using the Spice directive command. Or you can enter them one at a time if you prefer. Now what you really want is to get a plot of power out and VSWR vs the resistance.
Then right click in the log. If you have. You can add them like you would any other plot and do the usual things with them. But the X-axis will be the step value, just like we wanted.
Search Forums New Posts. How can I measure input and output impedance? For the DC circuits on this website, I've noticed that 0V DC sources are inserted, and then by knowing the node voltage and the current through a 0V DC source, I could then compute the impedance.
Will this work for BJT amplifiers? Any suggestions would be greatly appreciated! Scroll to continue with content. Ron H Joined Apr 14, 7, Here's a way to measure impedances in LTspice. Plot the resistance vertical axis on a linear scale instead of in decibels. To do this, right-click on the vertical axis and choose the Linear button. I've included the.
ASC file, if you want to run the same sim that I ran. You must log in or register to reply here. Similar threads Measuring weld cable resistance with Ohms Law Measuring Vehicle Sensor Resistance Measuring body resistance Measuring very low resistance of a fuse Measuring moving coil meter resistance for zero-center meter.
Circuit VR: Measuring With LTSpice
You May Also Like. Continue to site. Measuring weld cable resistance with Ohms Law. Jul 18, Measuring Vehicle Sensor Resistance. Jul 26, Measuring body resistance. Apr 5, Measuring very low resistance of a fuse.
May 16, Measuring moving coil meter resistance for zero-center meter. Jan 8, Predicting the efficiency of an application is vital to evaluating design trade-offs of a switching mode power supply. Two useful tools, the. To evaluate efficiency, clearly label your input and output voltage net as IN and OUTrespectively.
Press F4 to place net names. Press F2 and type load2 in the search box to select and place the component. Note the names of the input voltage source V1 and the load current source I1. The variable can be temperature, a model parameter, a global parameter or in our case an independent source. These steps can be defined as linear, logarithmic or as a list of specific values.
Insert a. Here, we step the independent current source, I1from 0. The param directive is used here in the. Please see the help file F1 for more details on. It is important to calculate efficiency when the circuit is operating in a steady state. To ensure this, simulate your circuit, and note when steady state is achieved for all conditions in the. In the. Please note the current direction convention for the input voltage source, V1is into the device, hence the negative sign in the Pin calculation.
The final expression calculates the efficiency using the param directive for clarity. Run your simulation. The Spice Error Log contains data points for the. One neat feature of LTspice is the ability to plot the stepped.Hey everyone this is going to be a simple introduction to generating an AC sweep of a circuit and finding the impedance at any given point, this came up several times in my courses and it was very difficult for me to find any way to do it online so hopefully this will help everyone especially those people like me trying to find answers at 3am.
To do this its very simple the first step is to make your desired circuit I will do another instructable on how to actually do this but leave the voltage source blank. The next step is to right click on the voltage source and select advanced you will see several options and on the right there will be small signal AC analysis, you can set that to anything however I do 1v at 0 degrees.
Then you will want to set up the sweep since this is an ac analysis you will do AC Analysis as the sweep type then select decade and use points per decade you can change this to match your needs but I have yet to experience a problem with this method, and then set the desired frequency range.
Finally you will want to label the input node as you can see it says V1 in the schematic above the voltage source, of course this can be at whatever point you are measuring the impedance at. After you simulate and plot the results you will notice they dont look very conducive to take them and find the impedance, the plots images here are the voltage and current at the battery of course you can select them anywhere in the circuit and you will find a variety of results.
This will change the graph but the units are still going to be in decibels so you will want to right click on the Y axis and change it to linear then hit ok and the units will now be in Ohms.
After you have converted to impedance it still may be hard to read the graph and a simple fix is to right click on the label of the graph and select a cursor under Attached Cursor I used 1 and 2 so I could measure at multiple points as you can see in the image with a results window showing up. Thanks for reading feel free to ask any questions and if this goes well I will try to create more like it. Thanks a lot for this! It helped me a lot. If I did everything right I guess the attached image should be showing the correct result for this buffer's input impedance around k.
Is it correct? Thanks again! Reply 1 year ago. Where is the I v2 taken at? If you have your voltage and current at two different places your result will change, depending on how the voltage and current change. I V2 is taken from AC 1, its reference name is V2. At least that's my understanding. I tried I Vin but is says it has no current. Thanks for you reply! By transistor2 Follow. More by the author:.
Did you make this project? Share it with us! I Made It! Half Square Triangles Kinetic Art by andrei. FrediM 1 year ago on Step 4. Reply Upvote. FrediM transistor2 Reply 1 year ago.In previous articles, we introduced modeling noise with LTspice and simulating noise sources in LTspice.
This article shows how to use LTspice to measure the noise of an op-amp and of an op-amp combined with a dual-JFET input stage. How accurate are the measurements? Are they useful? To find out, the results are compared to noise specifications in datasheets. An excellent summary of op-amp noise and how noise sources combine can be found in this article and app note from Analog Devices.
An excellent explanation of converting an RMS value to peak-to-peak is in this video from Analog Devices. The first component to test is a low-noise op-amp. The circuit is a standard non-inverting amplifier with a resistance in series with the non-inverting input to measure bias current noise.
The values for the power supplies and input resistor have been parameterized to make them easy to change and use in. STEP directives. More on this later. Adding this undocumented attribute to a resistor tells LTspice to ignore the resistor as a noise source. This feature is very useful because the extra noise from resistors does not have to be subtracted from the measurement.
Measuring Impedance Using LTspice
The noiseless attribute can be added using the Component Attribute Editor. Start this editor by holding down the control key and right-clicking on the resistor body.
With this feature, the ONLY noise source is the op-amp. Not a lot of work went into choosing this part. The ADA caught my attention from a quick scan of the low-noise op-amp selection table from Analog Devices. Then, a check of the datasheet showed the noise is well specified.
It was not chosen because the datasheet specs did or did not match LTspice testing. Here is the noise section of the ADA datasheet. The specifications are used in the comparison table below. Measurement conditions are set with statements on the left-hand side. PARAM statements provide values to the variables in the schematic. STEP statements run multiple simulations with different variable values. For example, two runs are done with different values of the input resistor. There is a small value for the input voltage noise measurement Step 1 and a large value for the input current noise measurement Step 2.
Measurements are specified with.Please Note: All the information presented here is my own work and not from Linear Technology. Please also read my sites general disclaimer. After selecting a component, you have the option to place another of the same type press Escape to cancel this.
Components can be moved around by pressing F7. Wires are drawn by pressing F3 or from the edit menu. After re-arranging components, press F7 and move the component, F5 deletes a wire or component and F3 draws a wire. To edit the value of a component right click its symbol and enter a new resistance value. The designation can also be changed with a right click, e. The ground terminal is always node 0 in any circuit.
The next line contains the simulation command, followed by some options, the final line is an. More about spice netlists can be found in the Spice Primer article. To display the voltage across resistor R2, it is simply the difference between the node voltages. Hovering the mouse cursor near a wire the node will be displayed on the bottom left of the main LTspice window.
As the topmost terminal is displayed as n, and the lower connection is Vout then entering the expression: V n - V vout as shown above will compute the voltage across R2.
Replace 1 and 2 with the appropriate node number in your circuit. Change the value of the multiplier for other values of decimal places, e. The example above is the same circuit with all of the above rounding techniques.
Generally you would fix all values with the same number of places but the voltage across VR4 and current through R4 are set at 2dp decimal placesthe voltage across VR2 is rounded to dp and Vout has no rounding at all. To enter the simulation you can use the menu or type "s" and enter the simulation command.
Unless you have experience with spice based simulators, you will find it easier to go to the simulation menu and choose option, "edit simulation command" as shown below:. The options for the simulation command appear. Click on the DC sweep tab. The fields below are the name of the source current or voltage to be swept and from our schematic this is V1. The next field is used to set a linear or logarithmic sweep, and then there is the start value, stop value and increment.
If increment is left out, then the default value of 1 is used. The full simulation command is. After running the circuit and clicking on Vout, the following result is displayed. As current through a resistor is proportional to voltage across it, then there's no surprise that the graph is a straight line. The horizonal x-axis displays the voltage source V1, while the vertical y-axis is a plot of Voltage at the resistor R1 and R2 junction, more conveniently labelled as Vout.
If my graph looks a little different to yours its because I "tweaked" the graphs axis. Right click at the left hand side y-axis you will see a dialog box like below, where you can change start, stop and increment values. Its the same for the x-axis as well. Using Measure Commands If you want to know the current or voltage at a particular node when some condition is reached you can use a measure command.
A measure command starts with a period followed by "measure" or "meas" and uses the following syntax:. The syntax looks quite daunting, but let me offer some further explanation. Any statement or expression in square brackets "[ ]" is optional, and may be omitted.