Differential Amplifier: Operation, Modes, and Common-Mode Rejection, Assignments of Computer science

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DIFFERENTIAL AMPLIFIER

DIFFERENTIAL AMPLIFIER

• A differential amplifier is an amplifier that produces outputs that are a

function of the difference between two input voltages.

• A differential amplifier has two inputs and two outputs.
• The differential amplifier has two basic modes of operation: ✓Differential mode (two inputs are different) ✓Common mode (two inputs are the same)

BASIC OPERATION – BIAS VOLTAGE ON INPUT 1, WITH INPUT 2 GROUNDED

The positive voltage on the base of Q1 increases 𝐼𝐶 and raises the emitter voltage to

This action reduces the forward bias (𝑉𝐵𝐸) of Q2 because its base is held at 0V (ground), thus causing 𝐼𝐶2 to decrease. The net result is that the increase in 𝐼𝐶 1 causes a decrease in 𝑉𝐶 1 , and the decrease in 𝐼𝐶 2 causes an increase in 𝑉𝐶 2.

BASIC OPERATION – BIAS VOLTAGE ON INPUT 2, WITH INPUT 1 GROUNDED

The positive voltage causes Q2 to conduct more, thus increasing 𝐼𝐶2 and raises the emitter voltage to

This action reduces the forward bias (𝑉𝐵𝐸) of Q1 because its base is held to ground, thus causing 𝐼𝐶1 to decrease. The net result is that the increase in 𝐼𝐶 2 causes a decrease in 𝑉𝐶 2 , and the decrease in 𝐼𝐶 1 causes an increase in 𝑉𝐶 1.

MODES OF SIGNAL OPERATION

SINGLE ENDED DIFFERENTIAL INPUT

a) Signal Voltage is applied to input1(input2 grounded) ▪ Inverted amplified signal voltage appears at output 1 ▪ A signal voltage appears in phase at the emitter of Q1. ▪ Since the emitters of Q1 and Q2 are common, the emitter signal

becomes an input to Q2.

▪ Q2 amplifies the signal and appears noninverted at its output2. b) Signal Voltage is applied to input2 (input1 grounded) ▪ Inverted amplified signal voltage appears at output 2 ▪ A signal voltage appears in phase at the emitter of Q ▪ Since the emitters of Q1 and Q2 are common, the emitter signal becomes the input to Q1. ▪ Q1 amplifies the signal and appears noninverted at output1.

MODES OF SIGNAL OPERATION

DOUBLE ENDED DIFFERENTIAL

INPUT

(a) In this configuration, two opposite polarity

signals are applied to the inputs.

(b) Shows the output signals due to the signal

on input 1 acting alone as a single-ended input.

(c) Shows the output signals due to the signal

on input 2 acting alone as a single-ended input. Notice that in (b) and (c) the signal on output 1 are of the same polarity. The same is also true for output 2.

(d) By superimposing both output 1 signals

and both output 2 signals from (b) and (c), we get the total output signals.

COMMON-MODE REJECTION

• Common-mode signals are usually noise signals such as the 60Hz power line,

or other sources.

• The importance of the common-mode rejection lies in the situation where an

unwanted signal appears commonly on both diff-amp inputs.

• Common-mode rejection means that this unwanted signal will not appear on

the outputs and distort the desired signal because of the cancellation that was

mentioned earlier.

COMMON-MODE REJECTION RATIO

• The measure of an amplifier’s ability to reject common-mode signals is a parameter called common-mode rejection ratio.
• It is the ratio of the differential voltage gain, 𝐴𝑉(𝑑) , to the common-mode gain, 𝐴𝑐𝑚. 𝐶𝑀𝑅𝑅 = 𝐴𝑉(𝑑) 𝐴𝑐𝑚
• The higher the CMRR, the better. A very high value of CMRR means that the differential gain is high and the common-mode gain is low.
• The CMRR is often expressed in decibels (dB) as 𝐶𝑀𝑅𝑅 = 20 𝑙𝑜𝑔 𝐴𝑉(𝑑) 𝐴𝑐𝑚

REFERENCE:

Floyd, T. L. (2012). Electronic devices – conventional current version (9th ed.). New

Jersey: Pearson Education, Inc.