Need help to plot frequency response of transconductance of BJT with LTSpice

[ATMega]

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

 

[MikeMl]

First read the definition

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...

 

[ATMega]

Based on the picture of the circuit that I added now, transconductance can be calculated by
Gm=io1/Vi
To achieve
io1
I used
Ic1−Ic2
. is it correct?

 

[MikeMl]

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?

 

[ATMega]

then, this would be the Gm?

 

[ronsimpson]

I need to plot the frequency response of the transconductance of this circuit in the picture with LTSpice but I don't know how.[/ATTACH]

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.

 

[ATMega]

thanks! this would be the Freq. Response of Gm? Am I right?

 

[ATMega]

And if it's correct, how can I determine the bandwidth of Gm????

 

[MikeMl]

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...

 

[MikeMl]

Are you looking for gain-bandwidth product, or the frequency at which the transconductance falls to half?

 

[ATMega]

MikeMl Questions are:
1- Specify the region where Gm is linear
2- Determine the bandwidth of Gm

the scales are dec- logarithmic- linear. Which one is more proper?

So far, the plot is correct?

 

[ronsimpson]

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.
------------------------------------------------------------------------------------------
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.

 

[ATMega]

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?

 

[MikeMl]

...
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.
------------------------------------------------------------------------------------------
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

 

[MikeMl]

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?

All by inspection, just by looking at the plot.

 

[ATMega]

inspection in AC sweep (frequency response)?
How about bandwidth of Gm? also in frequency response?

 

[MikeMl]

inspection in AC sweep (frequency response)?
How about bandwidth of Gm? also in frequency response?

Yes

Only the linearity would be done using the .DC sweep

 

[ATMega]

Thank you so much MikeMl

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...

 

[MikeMl]

First, read a definition of "Bandwidth"

Your textbook should also have a definition...

 

[ATMega]

so, can I consider the start frequency, until -3dB?
My text book has info about analog circuit designs, no explanation for bw specially for this case.