EEE 312 Lecture7

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Lecture 7

March 02, 2015

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EEE 312

Electronic Circuits

BJT AC Analysis

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COMPLETE HYBRID EQUIVALENT MODEL

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COMPLETE HYBRID EQUIVALENT MODEL

An operational amplifier, or op-amp, is a very high gain

differential amplifier with high input impedance and low output

impedance. Typical uses of the operational amplifier are to

provide voltage amplitude changes (amplitude and polarity),

oscillators, filter circuits, and many types of instrumentation

circuits. An op-amp contains a number of differential amplifier

stages to achieve a very high voltage gain.

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Operational Amplifiers (Chapter 10)

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Double-Ended Output

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Common-Mode Operation

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DIFFERENTIAL AMPLIFIER CIRCUIT

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If an input signal is applied

to either input with the other

input connected to ground,

the operation is referred to as

single-ended.

If two opposite-polarity

input signals are applied, the

operation is referred to as

double-ended.

If the same input is applied

to both inputs, the operation

is called common-mode.


In single-ended operation, a single input signal is applied. However, due

to the common- emitter connection, the input signal operates both

transistors, resulting in output from both collectors.

In double-ended operation, two input signals are applied, the difference

of the inputs resulting in outputs from both collectors due to the

difference of the signals applied to both inputs.

In common-mode operation, the common input signal results in opposite

signals at each collector, these signals cancelling, so that the resulting

output signal is zero. As a practical matter, the opposite signals do not

completely cancel, and a small signal results.

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The main feature of the differential amplifier is the very large gain when

opposite signals are applied to the inputs as compared to the very small

gain resulting from common inputs. The ratio of this difference gain to

the common gain is called common-mode rejection.

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DIFFERENTIAL AMPLIFIER CIRCUIT (DC BIAS)

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DIFFERENTIAL AMPLIFIER CIRCUIT (AC OPERATION)

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Single Ended AC Voltage Gain

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Single Ended AC Voltage Gain

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Double-Ended AC Voltage Gain

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Common-Mode Operation of Circuit

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Common-Mode Operation of Circuit

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Use of Constant-Current Source

A good differential amplifier has a very large difference gain

Ad, which is much larger than the common-mode gain Ac. The

common-mode rejection ability of the circuit can be

considerably improved by making the common-mode gain as

small as possible (ideally, 0). Larger RE, the smaller is Ac. One

popular method for increasing the ac value of RE is using a

constant-current source circuit.

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Current Mirrors (4.13)

The current mirror is a dc network in which the current

through a load is controlled by a current at another point in the

network. That is, if the controlling current is raised or lowered

the current through the load will change to the same level.

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Current Mirrors

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CURRENT SOURCE CIRCUITS (4.14)

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CURRENT SOURCE CIRCUITS

An ideal current source provides a constant current regardless of the

load connected to it. There are many uses in electronics for a circuit

providing a constant current at a very high impedance. Constant-current

circuits can be built using bipolar devices, FET devices, and a

combination of these components. There are circuits used in discrete

form and others more suitable for operation in integrated circuits.

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Bipolar Transistor Constant-Current Source

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Bipolar Transistor Constant-Current Source

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Basic Op-Amp

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Basic Op-Amp

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Basic Op-Amp

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Inverting Amplifier

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Noninverting Amplifier

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Unity Follower

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Summing Amplifier

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Integrator

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Summing Integrator

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Differentiator

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DIFFERENTIAL AND COMMON-MODE OPERATION

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EEE 312 Lecture7

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Transcript of EEE 312 Lecture7