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Transistors

Part 2

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From Transistors Part 1

Transistors are used for amplification. They have been referred to as a Three-

Terminal Amplifier with one terminal beingcommon to the other two terminals and isusually grounded.

Transistors behave as a resistor whose ohmicvalue is varied by the AC input signal.

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Transistors Part 2

In Transistors Part 2 we will discuss: Transistor configurations Transistor voltage feedback Transistor current feedback Push-Pull & Darlington circuits

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Transistors Part 2

Transistors as an amplifier can be classified by:

Circuit Configuration

Signal LevelsClass of Operation

The Type of Transistor Used This presentation will discuss the circuit

configuration.

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Transistors Part 2

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Transistors Part 2

CE-Common Emitter has the input signalapplied between the base terminal andground, and the output terminal is takenbetween the collector terminal and ground.

CC-Common Collector has the input signalapplied to the base terminal, and the output istaken from the emitter terminal leaving thecollector common to the base and emitter.

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Transistors Part 2

Basic features of CE Amplifiers- Typically the input impedance is about 1-k - The output impedance is approximately 5-k - The circuit provides voltage, current, and

power gain. Power gains range as high as10,000, current gain is usually equal to ac

- There is a 180 phase reversal between theinput and output signals with the voltage gainhaving a negative value.

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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.

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Transistors Part 2

The CB-Common Base Amplifier has the inputsignal applied to the emitter and the outputsignal coming from the collector. This leavesthe base common to both the emitter andcollector.

The base-emitter junction is forward biasedwhile the base-collector junction is reversebiased.

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Transistors Part 2 Basic features of CB Amplifiers- The input has very low impedance

between 1- and 50- .

- The output has medium impedanceabout 1-k .

- Provides good voltage and power gain.

- No phase reversal between the input andoutput voltage.

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Transistors Part 2

Feedback-Stabilizes the DC bias of thetransistor for better amplification of signals.- Current Feedback is the use of a resistor in series

with the emitter.

- Voltage Feedback is used with two resistorsbetween the collector and base and a single

voltage source, V CC, or with the same circuitry butadding an additional resistor and reversed biasedVBB between the base and ground.

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Transistors Part 2

Current Feedback circuit:

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Transistors Part 2 Voltage Feedback Circuit:

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Transistors Part 2

With AC signals we can use feedback tocontrol the oscillations of the amplifier. Whenfeeding energy from the output to the inputmay be either in phase or out of phase withthe input signal.

In-Phase (Positive) Feedback increases thegain of the amplifier, and may, if sufficient,lapse the amplifier into oscillations.

Out-of-Phase (Negative, or inverse) Feedbackreduces gain and prevents oscillations.

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Transistors Parts 2

Current Feedback circuit

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Transistors Part 2Series or CurrentInverse Feedback

Shunt or Voltage Feedback

a.k.a Negative Feedback

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Transistors Part 2

Push-Pull Output set as CE Darlington CC Connection

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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