How to Reduce Offset Voltage Caused by Transistor Base Emitter Voltage

Collection of small-signal transistors.

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).

Tips & Warnings

• By gradually turning the variable resistor wiper contact towards the positive end of its track, the transistor can be made to turn on, as the base-emitter offset voltage is overcome and current begins to flow in the transistor base-emitter circuit. As this occurs a current will begin to flow in the transistor collector-emitter circuit, through the 470 ohm resistor and the voltage reading on the multimeter will be reduced as the transistor collector voltage falls. This will continue until the transistor collector voltage falls close to zero, when the transistor is said to be saturated. Reset the variable resistor wiper until the collector voltage of the transistor is equal to 4 volts. This will be close to the center of its linear region of operation. Move the multimeter positive lead to the variable resistor wiper contact and take a note of this voltage reading. Move the multimeter positive lead to the transistor collector terminal again. Move the variable resistor wiper until the transistor collector voltage is equal to 3 volts. Move the multimeter positive lead to the variable resistor wiper contact and again take a note of this voltage reading. By comparing the small change in the variable resistor wiper voltage to the larger change in the transistor collector voltage, an estimation of the voltage gain of this circuit can be obtained. (delta-voltage-collector / delta-voltage-wiper = voltage-gain).

References

• Bipolar NPN Transistor Tutorial
• SourceForge: TinyCAD - tinycad