Editor Version ×
Standard

1.Easy to use and quick to get started

2.The process supports design scales of 300 devices or 1000 pads

3.Supports simple circuit simulation

4.For students, teachers, creators

Profession

1.Brand new interactions and interfaces

2.Smooth support for design sizes of over 5,000 devices or 10,000 pads

3.More rigorous design constraints, more standardized processes

4.For enterprises, more professional users

Ongoing

STD Linear 2 transistor current source and sink

License:

Mode: Editors' pick

  • 5k
  • 0
  • 0
Update time: 2020-03-21 16:24:23
Creation time: 2019-02-05 10:14:38
Description
Simple 2 transistor linear current source using PNP transistors and current sink using NPN transistors. The forward base emitter voltage drop (Vbe) of a bipolar transistor (bjt) is proportional to the log of the base current and rapidly rises to between approximately 0.55V and 0.7V for base currents over a few hundred nA. So for most purposes the Vbe of a bjt when conducting a collector current can be approximated to be a constant 0.6V. Therefore the Vbe of Q1 (Q3) can be considered to clamp the voltage across Riset to 0.6 volts. Therefore the current through Riset is given by: I_Riset = 0.6V/Riset Since the base current of Q1 (Q3) is negligible compared to the current through Riset and the base current of Q2 (Q4) is small compared to the collector current, the collector current of Q2 (Q4), Iconst, can be further approximated to be equal to the current through Riset i.e. Iconst = I_Riset. If the collector current tries to rise then the voltage across Riset would rise and so Q1 (Q3) would conduct more and pull the base of Q2 up (Q4 down) so turning Q2 (Q4) further off and reducing the collector current again. Hence there is negative feedback around the circuit to stabilise Iconst vs. the supply and collector-emitter voltage of Q2 (Q4). In other words the collector current of Q2 (Q4) is constant. In practice, the current does vary somewhat from one pair of devices to another due to the tolerances in current gain and Vbe vs. base current across devices and with the temperature of Q1 (Q3) due to the temperature sensitivity of bjt Vbe of approximately -2mV/degC. For a way to make a precision current source see: https://easyeda.com/andyfierman/High_side_4mA_20mA_current_source-Va86G2oXV In this version the collector current is almost exactly given by the voltage across R2. By replacing the PNP bjt with an NPN and returning R2 to ground instead of the positive supply rail, the 0-20mA current source can be turned into a current sink.
Design Drawing
schematic diagram
1 /
PCB
1 /
The preview image was not generated, please save it again in the editor.
ID Name Designator Footprint Quantity
1 2N3904 Q1 NONE 1
2 1k R1,R2 AXIAL-0.3 2
3 {0.6V/Iconst1} RISET1 AXIAL-0.3 1
4 {0.6V/Iconst2} RISET2 AXIAL-0.3 1
5 SIN(15 12 50) V1 HDR1X2 1
6 Ammeter AICONST1,AICONST2 NONE 2
7 BD139_ON Q2 TO-225 1
8 BC557C Q3 BC557 1
9 BD140_ON Q4 TO-225 1

Unfold

Project Attachments
Empty
Project Members
Related Projects
Change a batch
Loading...
Add to album ×

Loading...

reminder ×

Do you need to add this project to the album?

服务时间

周一至周五 9:00~18:00
  • 0755 - 2382 4495
  • 153 6159 2675

服务时间

周一至周五 9:00~18:00
  • 立创EDA微信号

    easyeda

  • QQ交流群

    664186054

  • 立创EDA公众号

    lceda-cn