Introduction: Transistor Comparator
This article shows you how to make a simple transistor comparator.
My circuit is an improvement to the design shown in this article:
Components: general-purpose transistors - 3 (1 spare). 10 kohm resistors - 5, 1 kohm resistor - 1, matrix board, insulated wire, 1 mm metal wire.
Optional parts: solder.
Optional tools: soldering iron, wire stripper, multimeter, USB oscilloscope, voltmeter.
Step 1: Design the Circuit
I have drawn this circuit in the old PSpice software.
Calculate the Ve voltage Thevenin equivalent:
Veth = Vs * ReB / (ReA + ReB)
= 3 V * 10,000 ohms / (10,000ohms+ 10,000 ohms) = 1.5 V
Reth = 10,000 ohms || 10,000 ohms = 5,000 ohms
Calculate the comparator input current at a maximum input voltage of 2.5 V:
Ib = (VinMax - Vbe1 - Veth) / (Rb1 + (Beta + 1) * Reth)
= (2.5 V - 0.7 V - 1.5 V) / (10,000 ohms + 101 * 5,000 ohms)
= 0.3 V / 515000 ohms = 0.58252427184 uA = 582.52427184 nA
Ro is used for output short circuit protection.
Step 2: Simulations
Simulations show that the circuit is working well.