Current Source Analysis

[Duave]

Thank you for any help that you can offer.

Did I answer all of the questions correctly and thoroughly? Can you please find any errors, and point them out to me so that I can fix them?

1. The problem statement, all variables and given/known data

Show that :

VB = 1.6V
VE = 1.0V
IE = 1.0 mA
IL = 1.0 mA
What is the voltage compliance range of the current source?
For what range of load resistors RL will the circuit function as a current source?

2. Relevant equations

RB1/RB2 = {10V - VB}/VB
....................................................................... ............................
IL = IE = VB - 0.6/RE
....................................................................... .....................
VLimit < Vcomp < {VRE/RE}RL = 1.03mA[/B]
....................................................................... .............................................
iout = Vin/RE
..................................................................

3. The attempt at a solution

2(a)1

RB1/RB2 = {10V - VB}/VB
....................................................................... ...........................
8.2k/1.6k = {10V - VB}/VB
....................................................................
5.125VB = {10V - VB}
..............................................................
5.125VB + VB = 10V
...........................................................
6.125VB = 10V
.....................................
VB = 10V/6.125
....................................
VB = 1.63V
................................

2(a)2
.................................
IL = IE = VB - 0.6/RE
....................................................................... .....................
IL = IE = 1.63 - 0.6/1000k
....................................................................
IL = IE = 1.03/1000k
............................................................
IL = IE = 1.03mA
.......................................................
VB - VBE = VE
...................................................................
VE = 1.03V
...............................
VE/RE = 1.03V/1000k
.............................................................
IE = VE/RE = 1.03mA
....................................................................... ......


What is the voltage compliance range of the current source?

The device is in comlpliance when:

VLimit < Vcomp < {VRE/RE}RL = 1.03mA[/B]
....................................................................... ......
VCC - ILRE < Vcomp < {VRE/RE}{VCC/IE}
....................................................................... ..................
10V - (0.001A)(1000[ohms]) < Vcomp < {1V/1000[ohms]}{10V/0.001A}
....................................................................... ..........................................
10V - 1V < Vcomp < 10V
..................................................
9V < Vcomp < 10V
..................................................


For what range of load resistors RL will the circuit function as a current source?

The range of load resistors can be found by:

iout = Vin/RE
.................................................................
9V/1000[ohms] < Vcomp/RL[ohms] < 10V/1000[ohms]
....................................................................... ...................................
Your assistance will be greatly appreciated

 

[Ratchit]

Duave,

I don't know what RB1 and RB2 are, but it looks like everything you calculated is somewhat correct. the base resistor current is 10v/(1k6+8k2) which approximately 1 ma. The transistor base current will be negligible and can be ignored when compared with 1 ma. Vbe is approx 0.6 volts, so that makes Ie and Ic 1 ma. The variable load resistor can range from 0 to 8k4 ohms. If it is higher than that, the collector and base will not be reversed biased, and the transistor will not be in the active region anymore.

Ratch

 

[Duave]

Duave,

I don't know what RB1 and RB2 are, but it looks like everything you calculated is somewhat correct. the base resistor current is 10v/(1k6+8k2) which approximately 1 ma. The transistor base current will be negligible and can be ignored when compared with 1 ma. Vbe is approx 0.6 volts, so that makes Ie and Ic 1 ma. The variable load resistor can range from 0 to 8k4 ohms. If it is higher than that, the collector and base will not be reversed biased, and the transistor will not be in the active region anymore.

Ratch

Ratch,

Thank you for your reply. Can you tell me what I need to consider in order to make the last answer correct? How do you know that the resistor can range from 0 to 8k4 ohms? I am confused about, Beta, saturation, active region, and voltage compliance range. Can you expound on the math connection of these things?

Thanks again for your response.

 

[Ratchit]

Ratch,

Thank you for your reply. Can you tell me what I need to consider in order to make the last answer correct? How do you know that the resistor can range from 0 to 8k4 ohms? I am confused about, Beta, saturation, active region, and voltage compliance range. Can you expound on the math connection of these things?

Thanks again for your response.

I did not see beta mentioned in the problem. Usually it is around 100. You can easily look up beta, saturation, and active region in a plethora of textbooks and web references. I have not heard of voltage compliance with respect to transistors. The collector has to have at least 1.6 volts in order to reverse bias the collector-base. If the variable resistor in the collector is too high, the voltage at the collector will be below 1.6 volts, and the transistor will go from the active region into the saturation region. (10 volts -1.6 volts)/1 ma = 8k4 ohms.

Ratch

 

[MrAl]

Hi there,

Just to note, the gain of the transistor is 40.