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How to Reduce Offset Voltage Caused by Transistor Base Emitter Voltage

A transistor is a current-operated, solid state, electronic device. It has three connections, base, emitter and collector. A small current flowing between the base and emitter will produce a larger current flowing between the collector and emitter. These currents have a maximum level above which the device is likely to fail. To prevent this, the base-emitter junction must be fed though a suitable resistive source and the collector-emitter junction must be terminated by a suitable resistive load. A small change of voltage applied to the base resistor will result in a larger voltage change in the load resistor. This is called the voltage amplification of the circuit. There is an offset voltage of approximately 0.6 volt in the base-emitter junction which must be exceeded before current will flow. A fixed voltage is applied to the base-emitter connection to overcome this offset and bias the transistor into a linear mode of operation.

Things You'll Need

Resistor 10 K ohm 0.25 watt
Resistor 470 ohm 0.25 watt
Pack of leads with clips
9-volt battery
Variable resistor 5 K ohm
Resistor 1 K ohm 0.25 watt
Small NPN transistor
Multimeter
TinyCAD (open source schematic capture program for MS Windows)

Instructions

- 1
  Connect the 10 K ohm resistor, the 470 ohm resistor and one end of a connecting lead together. The other end of the connecting lead will go to eventually to the battery positive terminal. Connect the other end of the 10 K ohm resistor to an outer terminal of the variable resistor. Connect the wiper terminal of the variable resistor to the base connection of the transistor via the 1 K ohm resistor. Connect the other outer terminal of the variable resistor to the emitter connection of the transistor and to the negative terminal of the battery. Connect the other end of the 470 ohm resistor to the collector connection of the transistor. Draw the circuit using TinyCAD and check that it is correct.
- 2
  Turn the wiper contact of the variable resistor to its most negative position. Set the multimeter to range to 0 to 20 Volts DC. Connect the multimeter positive lead to the transistor collector. Connect the multimeter negative lead to the battery negative terminal.
- 3
  Check your circuit connections again to ensure there are no errors. Connect the remaining connecting lead clip to the battery positive terminal. The meter reading should be equal to the battery voltage. (9 volts approximately).

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