Transistors
Part 2
From Transistors Part 1
• Transistors are used for amplification.
• They have been referred to as a ThreeTerminal Amplifier with one terminal being
common to the other two terminals and is
usually grounded.
• Transistors behave as a resistor whose ohmic
value is varied by the AC input signal.
Transistors Part 2
• In Transistors Part 2 we will discuss:
– Transistor configurations
– Transistor voltage feedback
– Transistor current feedback
– Push-Pull & Darlington circuits
Transistors Part 2
• Transistors as an amplifier can be classified by:
Circuit Configuration
Signal Levels
Class of Operation
The Type of Transistor Used
• This presentation will discuss the circuit
configuration.
Transistors Part 2
• Configurations are:
CE-Common Emitter
CC-Common Collector (Emitter Follower)
CB-Common Base
• The most commonly used configuration the
transistor is in when used as an amplifier is
the common emitter.
Transistors Part 2
Transistors Part 2
• CE-Common Emitter has the input signal
applied between the base terminal and
ground, and the output terminal is taken
between the collector terminal and ground.
• CC-Common Collector has the input signal
applied to the base terminal, and the output is
taken from the emitter terminal leaving the
collector common to the base and emitter.
Transistors Part 2
•
-
Basic features of CE Amplifiers
Typically the input impedance is about 1-kΩ
The output impedance is approximately 5-kΩ
The circuit provides voltage, current, and
power gain. Power gains range as high as
10,000, current gain is usually equal to βac
- There is a 180⁰ phase reversal between the
input and output signals with the voltage gain
having a negative value.
Transistors Part 2
• Basic features of CC Amplifiers
- The input circuit can have very high
impedance.
- The output impedance is relatively low.
- The voltage gain is approximately equal to 1.
- Provides good current gain, but less voltage
gain than that of the CE amplifier.
- No phase reversal between the input and
output voltage.
Transistors Part 2
• The CB-Common Base Amplifier has the input
signal applied to the emitter and the output
signal coming from the collector. This leaves
the base common to both the emitter and
collector.
• The base-emitter junction is forward biased
while the base-collector junction is reverse
biased.
Transistors Part 2
• Basic features of CB Amplifiers
- The input has very low impedance
between 1-Ω and 50-Ω.
- The output has medium impedance
about 1-kΩ.
- Provides good voltage and power gain.
- No phase reversal between the input and
output voltage.
Transistors Part 2
• Feedback-Stabilizes the DC bias of the
transistor for better amplification of signals.
- Current Feedback is the use of a resistor in series
with the emitter.
- Voltage Feedback is used with two resistors
between the collector and base and a single
voltage source, VCC, or with the same circuitry but
adding an additional resistor and reversed biased
VBB between the base and ground.
Transistors Part 2
• Current Feedback circuit:
Transistors Part 2
• Voltage Feedback Circuit:
Transistors Part 2
• With AC signals we can use feedback to
control the oscillations of the amplifier. When
feeding energy from the output to the input
may be either in phase or out of phase with
the input signal.
• In-Phase (Positive) Feedback increases the
gain of the amplifier, and may, if sufficient,
lapse the amplifier into oscillations.
• Out-of-Phase (Negative, or inverse) Feedback
reduces gain and prevents oscillations.
Transistors Parts 2
• Current Feedback circuit
Transistors Part 2
Series or Current
Inverse Feedback
Shunt or Voltage Feedback
a.k.a Negative Feedback
Transistors Part 2
Push-Pull Output set as CE
Darlington CC Connection
Transistors Part 2
• Transistors Part 3
- Field Effect Transistors (FETs)
• Transistors Part 4
- Silicon Controlled Rectifiers (SCRs)
- Class of Operation (Load-Line Analysis)
- Class A
- Class B
- Class AB
- Class C