Transistors

Part 2

From Transistors Part 1

• Transistors are used for amplification.

• They have been referred to as a Three-Terminal 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