Not understanding BJT basics I guess

Thread Starter

MikeJacobs

Joined Dec 7, 2019
215
So I am either doing something wrong in the sim or I am just not grasping the basics I guess

So I have a simple PNP with a 5 k on the emitter and 20k on the collectors.
I have an idea current source connected to the base of the pnp.
above the 5k there is a 12V voltage source.

I set the current source to 1uA and I get 250uA of current through the 20k. ok cool
then. I made it 10uA on the base and get 450uA of current through the 20k, ok also cool
I make it 100uA and I get 470uA of current through the 20k, ok makes sense starting to approach saturation apparently and im running out of gain
put 1 mA of current on the base and I get 250uA of current......... ummmmm ok now im confused???

If I am on the verge of saturation why the heck is my collector current going down????
Utter confusion instills.

Can anyone explain why this is? Thanks
 

Audioguru again

Joined Oct 21, 2019
4,117
Replace the emitter resistor with a piece of wire so that the emitter resistor does not steal voltage and current.
1mA in the emitter resistor drops the emitter voltage to less than 7V then the collector current cannot be more than 7V/20k= 350uA.
 

Thread Starter

MikeJacobs

Joined Dec 7, 2019
215
Replace the emitter resistor with a piece of wire so that the emitter resistor does not steal voltage and current.
1mA in the emitter resistor drops the emitter voltage to less than 7V then the collector current cannot be more than 7V/20k= 350uA.
ok thanks
But this still does not explain why this circuit is not following the rules of a BJT
 

ronsimpson

Joined Oct 7, 2019
1,750
Tanks for the schematic.
using your example 10ua, 100uA, .... of base current. Results in collector current.
BUT
At some point you have so much current that you can not get it through the resistors. As the
Guru said short out the resistors. I think both resistor. Or make them 1/10 the value or 1/100.
 

KeithWalker

Joined Jul 10, 2017
2,095
The simple explanation is that if you try to make the voltage dropped across R2 greater than 0.7V, it would be greater than the maximum voltage which would appear across the forward biased Base to Emitter junction. As a result, the transistor would become saturated and the voltage from Collector to Emitter would drop to about 200mV. You would also probably exceed the maximum Base Emitter current and damage the transistor.
 

ronsimpson

Joined Oct 7, 2019
1,750
I find that using PNP, is hard to teach.
Here is a NPN version.
R1=1 ohm is only there so it is easy to see current. Emitter current. Some SPICE programs have a hard time showing Emitter current. You can show R1 current.
I find NPN is better for my head. LOL
1637972373751.png
 

dl324

Joined Mar 30, 2015
13,836
I make it 100uA and I get 470uA of current through the 20k, ok makes sense starting to approach saturation apparently and im running out of gain
put 1 mA of current on the base and I get 250uA of current......... ummmmm ok now im confused???
In the 1mA case, the C-B junction is forward biased. That's an indication that it's saturated. The resistors are so large that the results are confusing you.
 

MrChips

Joined Oct 2, 2009
25,036
Think Load Line.
1637987120433.png

R2 = 5kΩ
R3 = 20kΩ
Vcc = 12V

Hence the most current you can have is 12V / 25kΩ = 0.48mA
VR2 = 2.4V
VR3 = 9.6V
VCE = 0V

Now you are trying to pull 1mA through R2.
VR2 becomes 5V. That leaves only 7V across the rest of the circuit.
Ic = 7V / 20kΩ = 0.35mA

As you can see, this is not BJT basics. This is Ohm's Law basics.
 

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MrAl

Joined Jun 17, 2014
8,613
So I am either doing something wrong in the sim or I am just not grasping the basics I guess

So I have a simple PNP with a 5 k on the emitter and 20k on the collectors.
I have an idea current source connected to the base of the pnp.
above the 5k there is a 12V voltage source.

I set the current source to 1uA and I get 250uA of current through the 20k. ok cool
then. I made it 10uA on the base and get 450uA of current through the 20k, ok also cool
I make it 100uA and I get 470uA of current through the 20k, ok makes sense starting to approach saturation apparently and im running out of gain
put 1 mA of current on the base and I get 250uA of current......... ummmmm ok now im confused???

If I am on the verge of saturation why the heck is my collector current going down????
Utter confusion instills.

Can anyone explain why this is? Thanks
Check and see if you are forward biasing the base collector junction and the emitter voltage is rising.

When the transistor goes into saturation the collector emitter voltage goes down, but with a sizable emitter resistor and increasing base current the emitter voltage will increase and hence the collector voltage will increase (NPN) and thus the current through the collector resistor will decrease.
Since at saturation the collector emitter voltage is fairly constant, when the emitter voltage rises so will the collector voltage. With PNP the collector voltage will decrease as the emitter voltage decreases.
See if that is what is happening. Also, try a much lower value emitter resistor with same current levels and see if your results makes more sense to you.

A second idea is if you are using a spice model the base collector junction may forward bias prematurely (note in saturation the base collector junction is slightly forward biased too though).

Also note more typical a circuit like this uses a lower value emitter resistor except for very special circuits.
 

crutschow

Joined Mar 14, 2008
28,538
Below is the simulation of your circuit for a DC Sweep of the I1 base current from 10nA to 100µA (horizontal axis):

As you can see, the PNP collector current (yellow trace) and emitter current (green trace) increase with base current until it is fully turned on at about 2µA base current with a collector current of 470µA, where the collector voltage (blue trace) goes to within about 200mV (saturation voltage) of the emitter voltage (red trace).

At about 10µA base current the emitter current starts to noticeably increase compared to the collector current due to the base current now becoming significant when added to the saturated emitter current.

If you study and understand these graphs, then you will see how the PNP is correctly following the rules of the circuit as configured. :)

1637993803594.png
 
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