Solution:

Given: Differential gain (Ad) = 20,000 and CMRR = 80 dB

To find: Common mode gain (Ac)

Formula: CMRR = 20 log (Ad / Ac)

Calculations:
CMRR = 80 dB
Ad = 20,000

Substituting the values in the formula, we get:

80 = 20 log (20,000 / Ac)

4 = log (20,000 / Ac)

Antilog both sides of the equation, we get:

Ac = 20,000 / antilog (4)

Ac = 2

Therefore, the common mode gain (Ac) is 2.

Explanation:

Differential Amplifier:
A differential amplifier is a type of electronic amplifier that amplifies the voltage difference between two input signals while rejecting any signals that are common to both inputs. The differential gain of an amplifier is the ratio of the change in output voltage to the change in the differential input voltage.

Common Mode Rejection Ratio (CMRR):
CMRR is the measure of an amplifier's ability to reject common-mode voltage signals. It is defined as the ratio of the differential gain to the common-mode gain.

Common Mode Gain (Ac):
Common mode gain is the ratio of the change in output voltage to the change in common-mode input voltage.

In this problem, we are given the differential gain of the amplifier and the CMRR. We are asked to find the common mode gain. Using the formula for CMRR, we can calculate the common mode gain.

By substituting the given values in the formula, we get the result as 2. Therefore, the common mode gain of the differential amplifier is 2.