08-11-2012, 05:06 PM
Bipolar Transistors
Bipolar.ppt (Size: 389.5 KB / Downloads: 176)
Introduction
Bipolar transistors are one of the main ‘building-blocks’ in electronic systems
They are used in both analogue and digital circuits
They incorporate two pn junctions and are sometimes known as bipolar junction transistors or BJTs
Here will refer to them simply as bipolar transistors
An Overview of Bipolar Transistors
While control in a FET is due to an electric field, control in a bipolar transistor is generally considered to be due to an electric current
current into one terminaldetermines the currentbetween two others
as with a FET, abipolar transistorcan be used as a‘control device’
Bipolar Transistor Operation
We will consider npn transistors
pnp devices are similar but with different polarities of voltage and currents
when using npn transistors
collector is normally more positive than the emitter
VCE might be a few volts
device resembles two back-to-back diodes – but has very different characteristics
with the base open-circuit negligible current flows from the collector to the emitter
Summary of Bipolar Transistor Characteristics
Bipolar transistors have three terminals: collector, base and emitter
The base is the control input
Two polarities of device: npn and pnp
The collector current is controlled by the base voltage/current IC = hFEIB
Behaviour is characterised by the current gain or the transconductance
Bipolar Transistor Amplifiers
A simple transistor amplifier
RB is used to ‘bias’ thetransistor by injecting anappropriate base current
C is a coupling capacitorand is used to couple theAC signal while preventingexternal circuits fromaffecting the bias
this is an AC-coupled amplifier
Bipolar.ppt (Size: 389.5 KB / Downloads: 176)
Introduction
Bipolar transistors are one of the main ‘building-blocks’ in electronic systems
They are used in both analogue and digital circuits
They incorporate two pn junctions and are sometimes known as bipolar junction transistors or BJTs
Here will refer to them simply as bipolar transistors
An Overview of Bipolar Transistors
While control in a FET is due to an electric field, control in a bipolar transistor is generally considered to be due to an electric current
current into one terminaldetermines the currentbetween two others
as with a FET, abipolar transistorcan be used as a‘control device’
Bipolar Transistor Operation
We will consider npn transistors
pnp devices are similar but with different polarities of voltage and currents
when using npn transistors
collector is normally more positive than the emitter
VCE might be a few volts
device resembles two back-to-back diodes – but has very different characteristics
with the base open-circuit negligible current flows from the collector to the emitter
Summary of Bipolar Transistor Characteristics
Bipolar transistors have three terminals: collector, base and emitter
The base is the control input
Two polarities of device: npn and pnp
The collector current is controlled by the base voltage/current IC = hFEIB
Behaviour is characterised by the current gain or the transconductance
Bipolar Transistor Amplifiers
A simple transistor amplifier
RB is used to ‘bias’ thetransistor by injecting anappropriate base current
C is a coupling capacitorand is used to couple theAC signal while preventingexternal circuits fromaffecting the bias
this is an AC-coupled amplifier