Mumbai University > Information Technology > Sem3 > Analog and Digital Circuits

Marks: 10M

Year: May 14

• The function of an ideal differential amplifier is to amplify the difference between two input signals.

• Following is the circuit diagram for a differential amplifier.

• Many electronic devices use differential amplifiers internally.

• The output of an ideal differential amplifier is given by:

$V_{out}=A_d(V_{in}^+-V_{in}^ -)$

• Where $(V_{in}^+) $ and $(V_{in}^-)$ are the input voltages and $A_d$ is the differential gain.

• In practice, however, the gain is not quite equal for the two inputs.

• This means, for instance, that if $(V_{in}^+) $ and $(V_{in}^-)$ are equal, the output will not be zero, as it would be in the ideal case.

• A more realistic expression for the output of a differential amplifier thus includes a second term.

  $V_{out}=A_d(V_{in}^+-V_{in}^ -) +A_c(\frac{V_{in}^+-V_{in}^ -}{2})$

• is called the common-mode gain of the amplifier.

• $A_c$ As differential amplifiers are often used to null out noise or bias-voltages that appear at both inputs, a low common-mode gain is usually desired.