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What Can You Do With Two Transistors (BJT)? Part 2 - Transistor Regulator

In the first installment of this series, we talked about what exciting things we can do with just two transistors, in particular BJT transistors. Some of us might probably think that we don't talk a lot about BJT transistors nowadays, not with the thrill, excitement, and trend brought about by high advancements in the technology in the Internet of Things, Machine Learning, Machine Intelligence, Advanced Algorithm, etc.. But we argue that these little circuits and components are constantly in the background of the advancements we see around us and we should not ignore them, as much we will not ignore what we currently enjoy and what the future holds for us.

Let's continue to look at the Two-Transistor Series Regulator we talked about in Part 1. The circuit below is a different version, now using the transistor to provide feedback from the output. This makes the output voltage higher. The output is still derived from the zener voltage.
The output voltage is now a scaled -up version of D1's Zener Voltage (VZ) plus the VBE drop on Q2. The 1N4735 has 6.2V zener voltage. This voltage, VZ + VBE (Q2), appears across R3 (VR3). The output voltage therefore is :

VOUT = VR3(1 + R2/R3)

We see from the equation that the amplification is provided by R2 and R3. We can make this circuit variable by replacing R2 and R3 with a potentiometer, which is R2 in the diagram below.


A variation of the circuit below shows an additional resistor, R4, that provides the majority of the bias to D1, so that Q1 doesn't have to do it alone that anymore allowing for better and improved regulation. The zener diode typically needs a substantial amount of current to bias it in regulation. With the improved circuit, that bias is now provided by R4.


So there you go, nice voltage  regulators that can be handy when we need to derive a smaller supply voltage, like 5V to supply our ICs from a higher source like battery. I didn't go into the small design details of the above circuit, but they can be used as they are for general purposes. In the coming installments we'll be showing some more useful and practical two-transistor circuits.

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