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

[ronsimpson]

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.

Start at left side of your graph. (-67fb) Move to the right until the line starts down. (-70db)=-3db

 

[ATMega]

Thanks!
How about the linear region of Gm?

 

[MikeMl]

What linear region?

 

[ATMega]

either I'm wrong on making this plot or there's no linear region here! And I don't know which one is correct...

 

[MikeMl]

I'm guessing that you could create a more linear transconductance plot by playing with the original circuit design: changing bias, operating point, resistor ratios, or something...

 

[ATMega]

ok, thanks a lot!

 

[ronsimpson]

Note V1 and V2 are not the same. One has "+" on top and the other has "-" on top.
I think both bases need to be about 1 volt up from ground. Then the input is the difference between the two bases. I think you have one at +.3V and the other -.3 volts.
View attachment 89490[/QUOTE]

 

[ATMega]

you mean this way?

 

[ronsimpson]

Please attach your file.

 

[ATMega]

 

[ronsimpson]

Please attach your file.

If you look at post #6 there is a .asc file. All you have to do is click on it and it will open up in spice.
Can you attach your (.asc) file in the same way. UPLOAD A FILE The files are usually under crogram files/ltc/LTspiceIV/

 

[ATMega]

Got it
And thanks a lot for your continuous support

 

[ronsimpson]

I design amplifiers but I don't know what this amp should be doing.
It has crossover distortion. If you passed a sign wave through it you would get the top 1/3 and bottom 1/3 but the middle 1/3 would not be there.
I changed V2 from 1V to 12V. The amp needs 3 or more volts to work at all.
I do not have the right transistors so I changed to 2N2222 transistors.
I biased up the bases to 800mV so V1=V2=0.8 volts.
Now it voltage amplifiers.
I also changed the sweep function. 0.75V to 0.85V and because the gain is high I change the stepping to 0.0001 volts.
When B1 does not equal B2 the amplifier works well. 775mV to 795mV and 805mV to 825mV. But from 795mV to 805mV the amplifier does not work.
I am using 2N2222 transistors and you might be using (text book transistors).

 

[ronsimpson]

Here is the circuit.

 

[ronsimpson]

OK, I fixed the crossover distortion.
It bothered my head that all the current sources were the same value. The emitter current should be slightly higher than the collector current.
I changed I3,I4 to 0.683mA. In my case that gets the right base current for the 2N2222 to function.

 

[ATMega]

the function of this circuit is known as fixed transconductance. All current sources should be the same to remain the transconductance at same level for both transistors. It's kinda painful circuit. If it helps, I can send you the pages of the text book.
Btw, thanks

 

[ronsimpson]

OK, but if the collector current and the emitter current is the same then the base current is zero. Not a happy amplifier. I looked at some amplifiers and I3+I4 are one current source. I have all wise used three current sources.

I could see your amp working if the transistors have a infinite current gain.

 

[ATMega]

here are the pages of the text book.
Then, any idea??

 

[ronsimpson]

Your book said to ignore the base current. But you can't in a real amplifier.
You have a resistor across the top two current sources. This causes them to output less current. So the top/bottom current are not equal.
With R1,R2 on the collectors you really reduce the gain of the amplifiers.
I moved R1, R2 to the emitters and this fixed the base current problem and the cross over distortion problem.

 

[ronsimpson]

sorry sent wrong file.
Try this.