Negative feedback in an amplifier 

Concept:

Negative feedback circuit:

The feedback amplification factor is given by:

 Af = \(\frac{A_o}{1 + A_o β }\)

Where,

Ao is open-loop gain

Aoβ is the loop gain.

Explanation:

The negative feedback in amplifiers causes:

• Reduced the gain and increases the stability in G.
• Increases the bandwidth to maintain constant gain-bandwidth product
• Reduces the distortion and noise in the amplifier
• The signal-to-noise ratio is not affected.
• The voltage gain (Av) of an amplifier is defined as the ratio of output voltage to the input voltage.

Av = Vo/Vin

Here, Vo is the output voltage of an amplifier and Vin is the input voltage of an amplifier.

• In a negative feedback amplifier, closed-loop voltage gain is given 

            Av = Vo / Vin = 1/(1+Aoβ)

Here, β = feedback factor,

Ao = open-loop gain of the amplifier.

• This expression clearly shows that closed-loop voltage gain has reduced by introducing negative feedback. 
• We know that a product of gain and bandwidth is inversely proportional so here bandwidth of amplifier will increase ;

            (gain × bandwidth = 0.35)

• A negative feedback amplifier decreases the current gain.