I need to plot the frequency response of the transconductance of this circuit in the picture with LTSpice but I don't know how. Can you please help me urgently how should I do it?
Thanks
Since you want frequency response, do a .AC simulation; not a DC sweep.
LTSpice lets you plot anything vs anything else (waveform arithmetic), e.g. branch currents vs node voltages, etc. It even knows that a plot of I(...)/V(...) has the units of Mhos...
I am confused about where the real "output" is on that circuit. If the "output" is the difference between the collector currents, then use that for the numerator of the plotting expression?.
By the same token, wouldn't the "input" be (Vi+ - Vi-), which would be the denominator of the plotting expression?
The above picture is not the frequency response. The text at the bottom of the schematic is ".dc v1....." it needs to start out ".ac........".
Attached is a file "videoamp.asc". Run sim on it and click on the "output" to see a plot of the frequency response. This is the frequency response of the voltage gain.
I would change the scale on the Y-axis to numeric instead of decibels. Click on the Y-axis in the plot window. You can change the method of how the ratio is plotted...
I plot in db because often amplifiers are rated in db.
Example; -3db at 100mhz or -6db at 150mhz. When a amplifier is rated at -3db at 100mhz they are really saying the gain has dropped by 3db. So the amplifier might have 20db at 1mhz but the gain has dropped back by 3db at 100mhz so the gain is really 17db.
In your plot the gain starts out at -67db. I think something is wrong. The over all look of the plot looks good and what I would expect.
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Now I see: you are plotting transconductance while I plot voltage gain. So -67db might be just fine. Plot linear or log does not matter as long as you know where 1/2 is.
Thanks MikeMl and ronsimpson
Now, how can I determine these:
1- Specify the region where Gm is linear
2- Determine the bandwidth of Gm
3- where the frequency is half?
...
In your plot the gain starts out at -67db. I think something is wrong. The over all look of the plot looks good and what I would expect.
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Now I see: you are plotting transconductance while I plot voltage gain. So -67db might be just fine. Plot linear or log does not matter as long as you know where 1/2 is.[/QUOTE]
1mA of output per 1volt of input would be -60db... That is why I suggested plotting it as a ratio (still logarithmic), rather than dB
Thanks MikeMl and ronsimpson
Now, how can I determine these:
1- Specify the region where Gm is linear
2- Determine the bandwidth of Gm
3- where the frequency is half?
But I don't know how to measure the bandwidth... Which part of my frequency response is considered as bandwidth. Sorry I know my questions are stupid, but I'm a beginner and need to learn...