NOTICE: This is a treatment of what is the CLASSIC/BASIC, textbook current sources. These, in their current form, have some significant shortcomings. I will be presenting the new and improved version of this current source in the next video.
TERMINOLOGY: Yes, while these are sinking current, they are still referred to as a current source.
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We all understand what a voltage source is, like batteries and power supplies. But current sources are somewhat of an enigma to a lot of people.
Current sources are one of the core circuits in a LOT of application circuits including, but not limited to, op amps, instrumentation amplifiers, difference amplifiers, Gilbert Cells and LED drivers.
In this video I am going to show you what a voltage source is and explain how a current source is both the same and different than that.
I will show you how to design your own current source using a single Bipolar Junction Transistor (BJT) and as a Current Mirror, which uses two BJTs.
I will be using an example application of driving a consistent current through an LED given widely varying supply voltages.
We will also discover how the current gain of a BJT will change depending on the Collector-Emitter Voltage (Vce) and how this affects the performance of our current source when the output is subjected to wide variations in voltage.
Finally, I will compare the two approaches for this specific application: our LED driver with widely varying supply voltages.
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LINKS for You:
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The Mailbox Indicator Project
https://youtu.be/ilPnNqcMxBAChanges in Current Gain (hFE) with changes in Collector-Emitter Voltage (Vce)
https://youtu.be/6w0OKF1_INAKirchhoff's Current Law
https://youtu.be/Htrwqby10DA=======================================
Time Markers for Your Convenience
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00:05 Introductory Comments
00:54 What is a Voltage Source?
02:03 What is a Current Source?
03:16 Our Application for this Video: LED Driver
04:24 The Bipolar Junction Transistor as a Current Source (Sink)
06:33 Single Transistor Current Source (sink)
Design
06:33 The Zener
09:22 The Transistor
12:01 Simulation Results
14:28 Seeing the change in hFE with changes in Vce
15:37 The Current Mirror
15:37 The Introduction
Design
16:45 The Zener
17:00 Determining our hFE
Kirchhoff's Current Law (KCL)
19:51 Write It for this Circuit
21:03 I(R2)
22:25 I(B1) & I(B2)
23:44 I(C1)
24:51 Solving for R2
26:24 Simulation Results
28:00 Comparisons
29:53 Final Comments and Toodle-Oots