Test: Feedback Amplifier

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 (A_v) of an amplifier is defined as the ratio of output voltage to the input voltage.

Av = V_o/V_in

Here, V_o is the output voltage of an amplifier and V_in is the input voltage of an amplifier.

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

            A_v = V_o / V_in = 1/(1+A_oβ)

Here, β = feedback factor,

A_o = 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.

Concept:

A negative feedback network is shown below.

Where A is the gain of the amplifier without feedback and feedback gain = β

Calculation:

Case 1:

Gain without feedback, A = 200

Gain with feedback A_CL = 100

Case 2:

Gain with feedback, A_CL = 150

Feedback, 

Concept:

The gain of a feedback system is given by:

A = Open Loop gain

A_f = Closed Loop Gain

β = Feedback/Transmission factor

Calculation:

Given A_f = -100 and A = -500

1 + Aβ = 5

Aβ = 4

β = 4 / -500 = -0.008

In a current series feedback, current is sampled from the output and voltage is feedback to the source.

In the given amplifier circuit, the feedback signal becomes zero by opening the output feedback.

Hence, it is a current series feedback.

Given that β = 0.1and we know that 

So we can write 

∴ A₁ + A₂ =78.75

In a series shunt feedback network, feedback is connected in series with signal source but in shunt with the load. Error voltage from feedback network is in series with the input. Voltage fed back from output is proportional to output voltage, hence parallel or shunt connected. The current in and voltage out connection refers to a shunt-shunt connection.

R_I = V_I/I_I = 4/2m = 2kΩ
R_IF = R_I(1+A.β) = 2k(1+500)
= 1002kΩ.

In series-series feedback, the output is current sampled, that is it is in series with the load. Also, input is a voltage mixer, which is in series with signal source. So feedback factor
Β = V_F/I_L in Ohms.

The resistance R4 is the feedback network resistance. There is no bypass capacitor being used. The resistance is not directly connected to either the input node or output node. Hence it’s a current series feedback.

Given that input is 14V, feedback is 6V and source is 20 V, we can see
V_I = V_S – V_F, which is voltage mixing. Also, β is in ohms that is voltage/current.
Since output of feedback is voltage and input is current, the output has current sampling.
Thus, configuration is a series-series feedback/current – series feedback.

_F = A/(1+Aβ)
Aβ = 24
A = 8
Relative change in gain = dA_F/A_F = dA/A(1+Aβ)
dA_F/A_F = 2/8*25 = 0.01.

In feedback systems, the feedback signal is in proportion with the output signal.
X_F ∝ X_O
X_F = βX_O, where β is the feedback factor or reverse transmission factor.

In setting the gain of the voltage series feedback amplifier, the ratio of two resistors is important and not the absolute value of these resistors. For example: If a gain of 11 is desired, we choose R₁ = 1kΩ and R₁ = 10kΩ or R₁ = 100Ω and R_F = 1kΩ.

A voltage shunt feedback amplifier forms a negative feedback because, any increase in the output signal results in a feedback signal into the inverting input causing a decrease in the output signal.

A voltage follower is a special case of non-inverting amplifier ( or voltage series feedback amplifier) and it has a gain of unity.