The transistor as a switch (and inverter)

In the common-emitter circuit with supply V_CC, collector resistor R_C:

V_CC = V_CE + I_C R_C (8-7)

(where V_CE is the voltage between collector and emitter.)

• Large positive base voltage (input high): large base current, so a large collector current I_C. The voltage drop I_C R_C becomes large and V_CE falls to a very small value (about 0.2 V). The output is close to zero. The transistor is like a closed switch (saturation).
• Small or negative base voltage (input low): no base current, I_C is almost zero, so I_C R_C ≈ 0 and V_CE ≈ V_CC. The output is high. The transistor acts as an open switch (cut-off).

• Thus the transistor can be switched on and off by the input voltage.
• Notice that a high input gives a low output and a low input gives a high output; the circuit is an inverter (NOT gate).