Transistor as an Amplifier
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• How do we use the transistor as an amplifier?
• First, we must connect it appropriately to the supply voltages,
input signal, and load, so it can be used
• A useful mode of operation is the common-emitter configuration
Common Emitter Configuration
• To make a practical circuit, we have to add bias and
load resistors to ensure the transistor is at the
desired operating point (operating in the right current
How it works
• A signal, such as music from a CD player, is applied to the input
• Let’s examine what happens when such a signal increases the
base voltage by DVin.
• The emitter voltage is always 0.7V below Vb, so if Vb changes by
DVin , so does Ve.
• Thus the emitter current increases by DVin /Re.
• But Ic=-aIe@-Ie, so it also increases by DVin /Re.
• Thus the voltage at the collector will increase by
-DVin RL/Re (that is, it will decrease)
• In this case RL/Re is 10, so the circuit amplifies
the input voltage signal by a factor of -10.
• In general, the gain is -RL/Re. The negative sign
indicates that a increase in input voltage leads to
a decrease in output voltage.
• This is an example of an inverting amplifier
Improved Voltage Regulator
• Buffer regulated zener diode output with transistor in
emitter follower (common collector) configuration.
• Current output from zener boosted by b (50-200)
• Less current drawn in standby mode
• Need to boost zener voltage by 0.7V.
• Here the transistor is used as a switch to close relay
contacts by driving the coil.
• Note the need for the flyback diode to prevent
damage to the transistor from the high voltages
created by the coil when the current is switched off.