Voltage Shunt Feedback Amplifier

Figure 1, shows the voltage shunt feedback amplifier using OP-AMP.

Figure 1

The input voltage drives the inverting terminal, and the amplified as well as inverted output signal is also applied to the inverting input via the feedback resistor R_f. This arrangement forms a negative feedback because any increase in the output signal results in a feedback signal into the inverting input signal causing a decrease in the output signal. The non-inverting terminal is grounded. Resistor R₁ is connected in series with the source.

The closed loop voltage gain can be obtained by, writing Kirchoff's current equation at the input node V₂.

The negative sign in equation indicates that the input and output signals are out of phase by 180. Therefore it is called inverting amplifier. The gain can be selected by selecting R_f and R₁ (even < 1).

Inverting Input at Virtual Ground

In the Figure 1, shown earlier, the non-inverting terminal is grounded and the- input signal is applied to the inverting terminal via resistor R₁. The difference input voltage V_d is ideally zero, (V_d = V_o/ A) is the voltage at the inverting terminals (V₂) is approximately equal to that of the non-inverting terminal (V₁). In other words, the inverting terminal voltage (V₁) is approximately at ground potential. Therefore, it is said to be at virtual ground.

Input Resistance with Feedback

To find the input resistance Miller equivalent of the feedback resistor R_f, is obtained, i.e. R_f is splitted into its two Miller components as shown in Figure 2.

Figure 2

Therefore, input resistance with feedback R_if is then

Output Resistance with Feedback

The output resistance with feedback R_of is the resistance measured at the output terminal of the feedback amplifier. The output resistance can be obtained using Thevenin's equivalent circuit,shown in Figure 3.

i_o = i_a + i_b

Since R_o is very small as compared to R_f + (R₁ || R₂)

Therefore, i.e. i_o = i_a

V_o = R_o i_o + AV_d

V_d = V₁ – V₂ = 0 - BV_o

Similarly, the bandwidth increases by (1+AB) and total output offset voltage reduces by (1+AB).