# Current Source Analysis

1. Mar 16, 2014

### 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?

https://scontent-a.xx.fbcdn.net/hphotos-prn1/t1.0-9/1012267_10151937081750919_819156677_n.jpg

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

2. Mar 16, 2014

### rude man

"Compliance range" needs to be better defined.

The circuit is a current source for R_L only if the 8.2K resistor stays at +10V.
Then you can determine how low Vcc to R_L can be before the transistor saturates.

There is in addition a high voltage limit due to the breakdown voltages of the transistor, but that can't be determined with the data you're given.

The last question's answer depends on the voltage to R_L as well as the value of R_L, so I guess assume Vcc = 10V for R_L.

3. Mar 16, 2014

### Duave

@rude man

Thank you. I will make corrections.

4. Mar 17, 2014

### Duave

@rude man,

If the compliance voltage is the range of output voltage in a constant current (dc) power supply over which the load regulation is within certain limits, how can I use this knowledge to verify that my last answer is correct. Can the variable load resistor range from 0 to 8k4 ohms. If it is higher than that, what will happen?

Thank you

5. Mar 17, 2014

### rude man

If your load is 8000 ohms, what is the collector voltage going to be?
If it's 9000 ohms, what then?

Hint: if the emitter to collector voltage is < about 0.2V the transistor is in saturation.

BTW this is not voltage compliance. It assumes the voltage to RL is a constant +10V.

Voltage compliance is a function of R_L. Try to figure out how low the voltage at the top of R_L can be for a given R_L.