Differential Amplifier: Principles and Applications in Electronics, Lecture notes of Electronics

Download Differential Amplifier: Principles and Applications in Electronics and more Lecture notes Electronics in PDF only on Docsity!

DIFFERENTIAL AMPLIFIER

DIFFERENTIAL AMPLIFIER

 Differential Amplifier  two CE stages in parallel with a common emitter resistor  two input voltages: v 1 (non-inverting) and v 2 (inverting)  two collector voltages  no lower cut-off frequency  differential output: v out = v c

• v c  differential input: v out

= A

V (v 1

• v2)

DIFFERENTIAL AMPLIFIER

 Non-inverting Input and a Differential Output  v out

= A

V

(V

1

 Non-inverting Input and a Single-ended Output  v ou t = A V

(V

1 ) but A V will be half as much

DIFFERENTIAL AMPLIFIER

 Inverting Input and Differential Output  v out

= - A

V (v 2

 Inverting Input and Single-ended Output  v out

= - A

V (v 2 ) but voltage gain will be half as much

DIFFERENTIAL AMPLIFIER

 DC Analysis Second Approximation

 I

T

= (V

EE

– V

BE

) / R

E

 Effect of Base Resistors: IT = (VEE-VBE)/ RE + RB/2dc)

DIFFERENTIAL AMPLIFIER

Example: Calculate the currents and voltages

using ideal and second approximations.

AC ANALYSIS OF A DIFFERENTIAL AMPLIFIER

 Single-Ended Output Gain  each transistor has an r e  the biasing resistor R E is in parallel with the r e of the right transistor  R E is much greater than r e

AC ANALYSIS OF A DIFFERENTIAL AMPLIFIER

 Simplified Equivalent Circuit  input voltage v i across the first r e is in series with the second r e  ac voltage across the tail resistor is half of the input voltage

 Single-ended output: A

V

=R

C

/2r

e

AC ANALYSIS OF A DIFFERENTIAL AMPLIFIER

 Inverting-Input Configurations

 the inverting input v

2

produces an amplified and

inverted ac voltage at the final output

AC ANALYSIS OF A DIFFERENTIAL AMPLIFIER

 Differential-Input Configurations

 both inputs are active at the same time

 Differential Input: A

V

(v

1

• v 2

AC ANALYSIS OF A DIFFERENTIAL AMPLIFIER

 Input Impedance

 In CE stage: z

in

= r

e

 In a differential amplifier: z

in

= 2r

e

AC ANALYSIS OF A DIFFERENTIAL AMPLIFIER

 Example: What is the ac output voltage? If

=300, what is the input impedance of the

differential amplifier?

AC ANALYSIS OF A DIFFERENTIAL AMPLIFIER

 Common-Mode Gain  same input voltage, v in(cm) is being applied to each base  common mode signal  differential amplifier does not amplify common-mode signals

AC ANALYSIS OF A DIFFERENTIAL AMPLIFIER

 Equivalent Circuit

 since equal voltages v

in(cm)

drive both inputs

simultaneously, there is almost no current through

the wire between the emitters