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What Can You Do with Two Transistors (BJT)? Part 1

Imagine we have two transistors lying on the work bench on a lazy Sunday afternoon and we'd like to do something exciting out of them.  How much can we do out of them?

One would be a crude Current Source. You may recall that two transistors assumed to be reasonably matched will produce the same current given the same voltage across base and emitter (VBE). Notice that Q1's base and collector are tied to make the base voltage equal. The programmed current is the supply, 5V, minus ~0.6V, divided by the reference resistor (5V-0.6)/RF..  The reference resistor can be made variable, so that the output current varies. The load can be LED, so that makes it dimmable by adjusting the reference resistor. The current source is certainly not precision, but can be useful in many practical applications like the example shown.

Current Source Circuit


Another would be a waveform generator, an Astable Multivibrator. It relies on the capacitor discharging and discharging and the transistor turning ON and OFF to produce the square wave. It is one clever circuit that beginners may take a while to appreciate how it does it exactly. This page does a good job of explaining how it works. It has two outputs complementary to each other. The period of the square wave produced is T = R1C1 + R2C2. The circuit below will have a frequency of 500Hz and duty cycle of 50%. We can play around with the resistors R1 and R2 to modify the duty cycle. Try this circuit, it works.


Astable Multivibrator


This next circuit here, a basic Transistor Voltage Regulator, will need an additional component, a Zener diode, but it's cool nonetheless. The two transistors are configured as Darlington Pair for higher current and better regulation.The circuit in fact can work with just a single transistor for smaller load current. The transistors allow for varying currents load without affecting the regulation of the Zener diode.

Transistor Regulator


We can make this transistor regulator output-variable. We'll show that in the next installment, along  with more two-transistor circuits.




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