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PSpice transistor simulation issue

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mat2ag

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I designed very simple CE amplifier with 2n2222 in PSpice. Ic is about 400mA and transistor is in linear domain but calculated g_m is far from simulated one.
g_m=Ic/Vt => g_m = 400mA/25mV = 16
ic = g_m * vbe => ic = 16 * 0.01 = 0.16 mA but ic is about 44 mA.

Screen Shot 2020-05-08 at 11.29.10 PM.png
Screen Shot 2020-05-08 at 11.36.20 PM.png
Screen Shot 2020-05-08 at 11.37.45 PM.png
 

Hi.

It seems to me that 0.44mA is a peak-to-peak value, while 0.16mA is a peak theoretical value..
So you should compare 0.16 with 0.44/2=0.22
Not exactly equals but sounds good to me.
 

Hi.

It seems to me that 0.44mA is a peak-to-peak value, while 0.16mA is a peak theoretical value..
So you should compare 0.16 with 0.44/2=0.22
Not exactly equals but sounds good to me.

You are not right (plz see the image) Ic is near 90mA peak to peak.
 

The biasing of your amplifier is not correct.BJT works near to saturation mode.., not linear region.
400mA is too high for 2N2222 and therefore model is not valid anymore.. so gm=IC/VT is valid only and only in linear region.
In fact it's derivative ratio of DC Collector Current over B-E Voltage so slope of this curve.But not in saturation mode..
bjt.png
 

The biasing of your amplifier is not correct.BJT works near to saturation mode..

Sorry, to interfer, but I am curious about this statement:
How the Transistor is not in the active region if it has a Vce of 2V and a Vbe of 800mV ?
 

I need to analyze power amplifier so current is rather high, but you said it is near saturation region, plz tell me why? and also I repeat this design and simulation with 2n3904,( other items were intact, therefore Ic = 290mA and vce>4 volt ) and the result is the same.

Plz see this it is far from saturation.

Screen Shot 2020-05-09 at 2.31.27 AM.png
 

You are comparing an idealized against the real transistor characteristic (or more exactly its SPICE model). The idealized characteristic doesn't consider bulk resistance which reduces gm at higher collector currents significantly.
 

But in my case it is converse, pen and paper gm is 16 but the simulation show more than 4 times greater!
 

But in my case it is converse, pen and paper gm is 16 but the simulation show more than 4 times greater!

44/16 is not 4, and as I said above, the amplitude of ic is actually 0.44/2 which is 0.22V.
0.44 V is a peak-to-peak value while 0.16 V is not

So you get 0.22 instead of 0.16 which is not that surprising
pif
 
Last edited:

But in my case it is converse, pen and paper gm is 16 but the simulation show more than 4 times greater!
16*0.01 = 160 mA, not 0.16 mA. According to your calculation method, Iac,pk should be 160 mA but it's 44 mA.
 

44/16 is not 4, and as I said above, the amplitude of ic is actually 0.44/2 which is 0.22V.
0.44 V is a peak-to-peak value while 0.16 V is not

So you get 0.22 instead of 0.16 which is not that surprising
pif

44m is peak current. so peak to peak is about 90mA.
 

16*0.01 = 160 mA, not 0.16 mA. According to your calculation method, Iac,pk should be 160 mA but it's 44 mA.


OK. you are right but 44 is near 1/4 of 160 and I think it is not acceptable result!
 

I need to analyze power amplifier so current is rather high, but you said it is near saturation region, plz tell me why? and also I repeat this design and simulation with 2n3904,( other items were intact, therefore Ic = 290mA and vce>4 volt ) and the result is the same.

But your circuit is NOT a Power Amplifier.It's just forced Small Signal Amplifier.
Do your math again and transform your circuit into Power Amplifier configuration.You play the technical specifications of the transistor at extreme borders..
 

Additional Note : If you use high current, output resistance ( ro) drops much so it might be much lower than load resistance.
Result : The output voltage swing never reaches to computed value.
 

OK. you are right but 44 is near 1/4 of 160 and I think it is not acceptable result!
"Not acceptable" means you didn't yet think about the given explanations for non-ideal transistor behavior. 2N2222 SPICE model has a RE=0.2 emitter bulk resistor parameter which well explains the observed results. You get 1/gm,eff = 1/gm,ideal + RE. A plot of Vbe over Ic visualizes the effect. You get 10% reduction of effective gm at 13 mA.

2n2222.PNG

Other effects discussed in this thread like output resistance or saturation voltage play a role, but are not significant in this case.
 

"Not acceptable" means you didn't yet think about the given explanations for non-ideal transistor behavior. 2N2222 SPICE model has a RE=0.2 emitter bulk resistor parameter which well explains the observed results. You get 1/gm,eff = 1/gm,ideal + RE. A plot of Vbe over Ic visualizes the effect. You get 10% reduction of effective gm at 13 mA.

View attachment 159266

Other effects discussed in this thread like output resistance or saturation voltage play a role, but are not significant in this case.

First. would you explain more detail about how parasitic resistance of emitter affect the current flow in collector (voltages trivial)
Second. As you said above it is about 10% but in my simulation it it about 75%.
Third. As PSpice ref. manual page 209 "Bipolar transistor equations for DC current" said,
Ic did not depend to Re.
( Ic = collector current = area·(Ibe1/Kqb - Ibc1/Kqb - Ibc1/BR - Ibc2) )
 

What do you mean with "voltages trivial"? Apparently you have problems to understand the trivial relations. Collector current is controlled by Vbe, RE is acting as series-series feedback reducing effective gm. Review the formula I posted above, try to understand it. Sketching an equivalent circuit with RE should help.

Second. As you said above it is about 10% but in my simulation it it about 75%.
10% at 13 mA, rising.
 

What do you mean with "voltages trivial"? Apparently you have problems to understand the trivial relations. Collector current is controlled by Vbe, RE is acting as series-series feedback reducing effective gm. Review the formula I posted above, try to understand it. Sketching an equivalent circuit with RE should help.

10% at 13 mA, rising.

In model which I used RE=0.1. I convert it to RE=0 and relaunch the simulation as you said it should be near ideal model but the new Ic swing is about 80mA not 160mA.
 

There are possibly more non-ideal elements in the SPICE model.
 

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