BJT model and fT simulation with PSpice

MaxMagnificent · 22nd February 2011, 07:27 PM

Hello!!!

I think I am not posting it twice, I am trying to verify fT of 2N3904 (Fairchild Semi) using PSpice, [1] testbench.jpg shows my testbench ckt (analysis directives: .AC LIN 1000 1 500Meg), [2] result.jpg shows my simulation result and [3] datasheet.pdf

Datasheet says fT=300MHz but my simulation result says fT=83.333MHz

As I know fT is a frequency at which small-signal currect gain, |ic/ib|=1

My questions are:

[1] do you see any problem with my testbench?

[2] what does f (=100MHz, 1.0MHz, 10Hz to 15.7KHz) mean in the datasheet's page #2 under small signal characteristics?

your response would be highly appreciated,

sawreyrw · 22nd February 2011, 10:13 PM

Your test circuit looks OK, but I would do it different. I would set the emitter current instead of the base current. Anyway, according to the data sheet the DC collector current will be around 200 uA. fT depends on Ic and it goes down at low values. 83 MHz at .2 mA doesn't surprise me. Try using a higher Ic and see what you get.

The answer to question 2 is these are test frequencies. For example, 100 MHz is used to test fT. At this frequency for an fT of 300 MHZ the current gain would be a minimum of 3.

MaxMagnificent · 23rd February 2011, 05:31 AM

Hi sawreyrw,

Thank you for looking into the testbench, I have tried to improve my Ic (DC biasing current) by increasing Ib to 50uA; simple .tran shows that Ic (DC current) becomes 10.035 mA (Please check the datasheet again, it says Ic=10mA when measuring fT). I guess now all set now to run .ac, result: fT=488MHz (approx) now.

Question #2 from last post,

Quote:

The answer to question 2 is these are test frequencies. For example, 100 MHz is used to test fT. At this frequency for an fT of 300 MHZ the current gain would be a minimum of 3.

I never tested a BJT in realtime so I am not able to understand where to apply 100MHz frequency to test fT? If I look into my testbench there are no scopes to apply 100MHz in any of the sources.

sawreyrw · 23rd February 2011, 05:54 AM

The test frequecny for fT is not too important. When you sweep the frequency in .AC analysis in your test circuit it passes through 100 MHz. This is the point where the vendor measures the current gain. Again, if fT is greater than 300 MHz, the current gain should be greater than 3 at 100 MHz. Perhaps a point you have not understood is that fT is a gain*bandwidth product. In other words, the currrent gain times the 0 db (gain of 1) current gain frequency is equal to fT.

MaxMagnificent · 23rd February 2011, 07:30 AM

Thank you very much sawreyrw! I think I was able to extract fT in PSpice with your help,

your reply on Question#2 is clear to me now.

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22nd February 2011, 10:13 PM	#2
sawreyrw diyAudio Member Join Date: Apr 2006 Location: Minnesota	Your test circuit looks OK, but I would do it different. I would set the emitter current instead of the base current. Anyway, according to the data sheet the DC collector current will be around 200 uA. fT depends on Ic and it goes down at low values. 83 MHz at .2 mA doesn't surprise me. Try using a higher Ic and see what you get. The answer to question 2 is these are test frequencies. For example, 100 MHz is used to test fT. At this frequency for an fT of 300 MHZ the current gain would be a minimum of 3.

23rd February 2011, 05:54 AM	#4
sawreyrw diyAudio Member Join Date: Apr 2006 Location: Minnesota	The test frequecny for fT is not too important. When you sweep the frequency in .AC analysis in your test circuit it passes through 100 MHz. This is the point where the vendor measures the current gain. Again, if fT is greater than 300 MHz, the current gain should be greater than 3 at 100 MHz. Perhaps a point you have not understood is that fT is a gain*bandwidth product. In other words, the currrent gain times the 0 db (gain of 1) current gain frequency is equal to fT.

23rd February 2011, 07:30 AM	#5
MaxMagnificent diyAudio Member Join Date: Feb 2011 Location: New Delhi	Thank you very much sawreyrw! I think I was able to extract fT in PSpice with your help, your reply on Question#2 is clear to me now.