Basic OP-AMP circuits: Inverting, Non-inverting, Summing, Difference amplifier

Basic OP-AMP circuits

This section concentrates on the principles involved with basic OP-AMP circuit viz, (i) inverting and (ii) non-inverting amplifiers.

(i) Inverting amplifier

The basic OP-AMP inverting amplifier is shown in Fig. The input voltage V_in is applied to the inverting input through the input resistor R_in. The non inverting input is grounded. The feedback resistor R_f is connected between the output and the inverting input.

Since the input impedance of an op-amp is considered very high, no current can flow into or out of the input terminals. Therefore I_in must flow through R_f and is indicated by I_f (the feedback current). Since R_in and R_f are in series, then I_in = I_f. The voltage between inverting and non-inverting inputs

is essentially equal to zero volt. Therefore, the inverting input terminal is also at 0 volt. For this reason the inverting input is said to be at virtual ground. The output voltage (Vout) is taken across Rf.

It can be proved that

I_f=V_out/R_f

Since I_in =I_f , then

V_in/R_in = -V_out/R_f

Rearranging the equation, we obtain

-V_out/V_in = R_f/R_in

∴ The voltage gain of an inverting amplifier can be expressed as

A_v = -R_f/R_in

The amplifier gain is the ratio of R_f  to R_in

Finally, the output voltage can be found by

V_out = -R_f/R_in x V_in

The output voltage is out of phase with the input voltage.

(ii) Non-inverting amplifier

The basic OP-AMP non-inverting amplifier is shown in Fig. The input signal V_in is applied to the non-inverting input terminal. The resistor R_in is connected from the inverting input to ground. The feedback resistor R_f is connected between the output and the inverting input.

Resistors R_f and R_in form a resistive ratio network to produce the feedback voltage (V_A) needed at the inverting input. Feedback voltage (V_A) is developed across R_in. Since the potential at the inverting input tends to be the same as the non-inverting input (as pointed out with the description of virtual ground), V_in = V_A.

Since V_A = V_in, the gain of the amplifer can be expressed as

_{Av  =} ^Vou t

V _A

However, V_A is determined by the resistance ratio of R_in and R_f ; thus,

V_A=[R_in/(R_f/R_in)] V_out

A_v=1+(R_f/R_in)

Finally, the output voltage can be found by, Vout = (1+R_f/R_in)V_in

It is seen that the input and output voltages are in phase

(iii) Summing amplifier

The summing amplifier provides an output voltage equal to the algebraic sum of the input voltages.

Fig shows an inverting amplifier, used to sum two input voltages. The input voltages v₁ and v₂ are applied through the resistors R₁ and R₂ to the summing junction (P) and Rf is the feedback resistor. At the point P,

i₁ + i₂ =i_f

Since the voltage at the point P is ideally 0

(V₁/R₁) + (V₂/R₂) =(V_out/R_f)

Hence the output voltage,

V_out=-[(R_fV₁/R₁) +(R_fV₂/R₂) ]

If R₁ = R₂ = R_f  = R, then vout = - (v₁ + v₂)

Hence the output voltage is equal to the sum of the input voltages and the circuit acts as a summing amplifier. The negative sign indicates that OP-AMP is used in the inverting mode.

(iv)Difference amplifier

The difference amplifier is shown in Fig. The output voltage can be obtained by using superposition principle. To find the output voltage v₀₁ due to v₁ alone, assume that v₂ is shorted to ground. Then

V⁺ = R₂V₁ / R₁+R₂

Therefore, with both inputs present, the output is

V₀ = V₀₁ + V₀₂

= (R₃+R_{4 /} R₃)    (R₂/R₁+R₂)  V₁    -    (R₄/R₃)V₂

If R₁ = R₂ = R₃ = R₄ = R

then vo = v₁ - v₂

If all the external resistors are equal, the voltage difference amplifier functions as a voltage subtractor.