ELEX3 Differential Amplifiers

DIFFERENTIAL AMPLIFIER CIRCUITS

ENGR. JOMERV. [email protected]

0921 4111851

By the end of the discussion, the student must be able to:

1. Define a differential amplifier.

2. Draw the differential amplifier circuit.

3. Enumerate the different input combinations used in differential amplifiers.

4. Perform Dc bias analysis of differential amplifiers

5. Perform AC analysis on differential amplifiers

GENERAL AMPLIFIER

CONCEPTS REVIEW

GENERAL AMPLIFIER CONCEPTS REVIEW:

DIFFERENTIAL AMPLIFIER CIRCUITS

DIFFERENTIAL AMPLIFIER CIRCUITS

Basic Differential Amplifier Circuit

The differential amplifier circuit is an extremely popular connection used in IC circuits.

The circuit has two separate inputs, two separate outputs and that the emitters are connected together.

While most differential amplifier circuits use two separate voltage supplies, the circuit can also operate using a single supply.

A number of input signal combinations are possible:

1.SINGLE – ENDED2.DOUBLE - ENDED3.COMMON - MODE

1. SINGLE ENDED-INPUT

- An input signal is applied to either input with the other input connected to the ground. - Due to the common-emitter connection, the input signal operates both transistors, resulting in an output from both collectors.

2. DOUBLE – ENDED (DIFFERENTIAL) INPUT

Double-ended differential 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.

3. COMMON-MODE OPERATION

Double-ended output with single-ended input

Double-ended output

- 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

DC BIAS ANALYSIS OF A DIFFERENTIAL AMPLIFIER

- With ac inputs obtained from voltage sources, the dc voltage at each input is essentially connected to 0V.

With each base voltage at 0V, the common emitter dc bias voltage is:

The emitter dc bias current is then:

Assuming that the transistors are well matched ( as would occur on an IC unit):

Resulting in a collector voltage of:

EXAMPLE 1

Calculate the dc voltages and currents in the circuit below.

ANSWERS:

IE = 2.5 mA

IC = 1.25 mA

VC = 4.1 V

AC OPERATION OF A DIFFERENTIAL AMPLIFIER

AC Connection of a differential amplifier

AC equivalent of a differential amplifier circuit

SINGLE – ENDED AC VOLTAGE GAIN

- To calculate the single-ended ac voltage gain, Vo/Vi, signal is applied to one of the inputs with the other connected to the ground.

Connection to calculate Av1 = Vo/ Vi1

AC equivalent circuit

The ac base current can be calculated using the base 1 input KVL equation. And assuming that the transistors are well matched:

With RE very large, the circuit for obtaining the KVL equation simplifies to the circuit below:

From which we can write:

If we also assume that:

Then,

And the output voltage magnitude at either collector is:

The single-ended voltage gain at either collector is:

EXAMPLE 2

Calculate the single-ended output voltage Vo1 for the circuit below.

ANSWER:

IE =193 μA

IC = 96.5 μA

VC = 4.5 V

re = 269 ohms

Av = 87.4

Vo = 0.175 V

DOUBLE - ENDED AC VOLTAGE GAIN

Where Vd = Vi1 – Vi2

COMMON-MODE OPERATION CIRCUIT

Common- mode connection

While a differential amplifier provides large amplification of the difference signal applied to both inputs, it should also provide as small an amplification of the signal common to both inputs.

AC circuit in common-mode connection

Where:

Which can be rewritten as:

The output voltage magnitude is then:

Providing a voltage gain magnitude of:

EXAMPLE 3

Calculate the common-mode gain for the amplifier circuit below:

ANSWER:

A = 0.54

BiFET, BIMOS, AND CMOS DIFFERENTIAL AMPLIFIER CIRCUITS

1. BiFET DIFFERENTIAL AMPLIFIER

2. BiMOS DIFFERENTIAL AMPLIFIER

3. CMOS DIFFERENTIAL AMPLIFIER

SEATWORK NO. 2

ANALYSIS:

1. Calculate the dc bias values of Ic and Vc for the matched transistors of:

2. Calculate the dc bias values of Ic for the matched transistors of the circuit below.

3. Calculate Vo in the circuit below:

ASSIGNMENT NO. 2I. RESEARCH: (submit a hardcopy)1.Use of Constant-current source in differential amplifiers.II. ANALYSIS:2. Calculate Vo in the circuit. note: β = 75

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