Bipolar Junction Transistors (BJT) Current Sources - Current Mirror (084b)

Video Description

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. ------------------------------------- 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. ======================================= LINKS for You: -------------- The Mailbox Indicator Project https://youtu.be/ilPnNqcMxBA Changes in Current Gain (hFE) with changes in Collector-Emitter Voltage (Vce) https://youtu.be/6w0OKF1_INA Kirchhoff's Current Law https://youtu.be/Htrwqby10DA ======================================= Time Markers for Your Convenience ---------------------------- 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