How to Evaluate Bipolar Transistor Circuits

Identify the base, emitter and collector on the transistor. These are found either on the package, manufacturer's website or data sheet. Some electronics texts also have them in the appendix.

Add the transistor to the breadboard, with each lead being placed in a separate column.

Attach the 270k resistor to the same column that the base lead is in. Next, add the 4 AA battery holder to the circuit, so that 6 volts will be supplied to the base. Do this by placing the red lead of this holder in the same column that the base lead and the 270k resistor are in. Select a column on the breadboard as ground, which means zero voltage, and insert the battery holder's black lead into it.

Select a jumper wire and place one end into the emitter column and the other to ground.

Insert the 1k resistor into the collector column and add the red lead of the 9V battery to this column as well. Connect the battery holder's black lead to the ground column. Insert the batteries into their respective holders.

Measure Vbe, the voltage between the base and the emitter. Do this by turning the multimeter to the voltage setting and then placing its red probe on the base and its black probe on the emitter. Vbe is assumed to be 0.7 volts, but the actual value will be in the range of 0.5 to 0.7 volts.

Calculate Vr, the base voltage across the resistor using Kirchoff's law (which states that the sum of all the currents into a point is the same as the sum of all currents leaving the point). Kirchoff's Law for the left-hand base loop is Vr = Vbb -- Vbe. The base voltage source Vbb is 6 V, which is the battery. The equation is Vr = 6 V -- 0.7 V = 5.3 V. Measure Vr with the digital multimeter by placing one probe on each side of the resistor, and then compare this value with the one previously calculated.

Calculate the theoretical value of Ib, the current through the base resistor, using Ohm's Law (which gives the relationship between the current and the voltage in a circuit). Ohm's Law is V = IR. The equation is Ib = (Vbb -- Vbe) / Rb = (6 V -- 0.7 V) / 270k ohms = 5.3 V / 270k ohms = 19.6 uA, where uA is microamps.

Calculate the collector current Ic. To do this, use the gain hfe or Bbc. The equation is Ic = hfe*Ib. If hfe = 277, then Ic = 277 * 19.6 uA = 5.4 mA, where mA is milliamps.

Calculate Vce, the voltage between the collector and the emitter. From Kirchoff's law, the equation is Vce = Vc -- IcRc = 9 V -- 5.4 mA * 1k-ohm = 3.6 V. Use the digital multimeter and measure the Vce's actual value. Do this by holding the red probe of the digital multimeter on the collector and the black probe on emitter. Compare this measurement with the calculated value for Vce.