Question

What is the difference in the voltage output of a BJT Transistor switch circuit    and a BJT Emitter follower circuit?

Answer 1

The Emitter follower is used to provide current gain and in impedance matching applications.The input to this circuit is applied to the base , while the output is taken from the emitter.The voltage gain is always less than 1 and the output voltage is in phase with input voltage.in the common collector amplifier circuit, the same voltage drop, V_E also represents the output voltage, V_OUT.

If the circuit uses the Bipolar Transistor as a Switch, then the biasing of the transistor, either NPN or PNP is arranged to operate the transistor at both sides of the “ I-V ” characteristics curves.The areas of operation for a transistor switch are known as the Saturation Region and the Cut-off Region

1. Cut-off Region

Here the operating conditions of the transistor are zero input base current ( I_B ), zero output collector current ( I_C ) and maximum collector voltage ( V_CE ) which results in a large depletion layer and no current flowing through the device. Therefore the transistor is switched “Fully-OFF”.

• The input and Base are grounded ( 0v )
• • Base-Emitter voltage V_BE < 0.7v
• • Base-Emitter junction is reverse biased
• • Base-Collector junction is reverse biased
• • Transistor is “fully-OFF” ( Cut-off region )
• • No Collector current flows ( I_C = 0 )
• • V_OUT = V_CE = V_CC = ”1″
• • Transistor operates as an “open switch”
• Then we can define the “cut-off region” or “OFF mode” when using a bipolar transistor as a switch as being, both junctions reverse biased, V_B < 0.7v and I_C = 0. For a PNP transistor, the Emitter potential must be negative with respect to the Base.

2. Saturation Region

Here the transistor will be biased so that the maximum amount of base current is applied, resulting in maximum collector current resulting in the minimum collector emitter voltage drop which results in the depletion layer being as small as possible and maximum current flowing through the transistor. Therefore the transistor is switched “Fully-ON”.

• The input and Base are connected to V_CC
• • Base-Emitter voltage V_BE > 0.7v
• • Base-Emitter junction is forward biased
• • Base-Collector junction is forward biased
• • Transistor is “fully-ON” ( saturation region )
• • Max Collector current flows ( I_C = Vcc/R_L )
• • V_CE = 0 ( ideal saturation )
• • V_OUT = V_CE = ”0″
• • Transistor operates as a “closed switch”

Then we can define the “saturation region” or “ON mode” when using a bipolar transistor as a switch as being, both junctions forward biased, V_B > 0.7v and I_C = Maximum. For a PNP transistor, the Emitter potential must be positive with respect to the Base.