Closed Loop OPAMP Configuration

Closed Loop Amplifier:

 The gain of the OPAMP can be controlled if fedback is introduced in the circuit. That is, an output signal is fedback to the input either directly or via another network. If the signal fedback is of opposite or out phase by 180° with respect to the input signal, the feedback is called negative fedback.

An amplifier with negative fedback has a self-correcting ability of change in output voltage caused by changes in environmental conditions. It is also known as degenerative fedback because it reduces the output voltage and,in tern,reduces the voltage gain.

If the signal is fedback in phase with the input signal, the feedback is called positive feedback. In positive feedback the feedback signal aids the input signal. It is also known as regenerative feedback. Positive feedback is necessary in oscillator circuits.

The negative fedback stabilizes the gain, increases the bandwidth and changes, the input and output resistances. Other benefits are reduced distortion and reduced offset output voltage. It also reduces the effect of temperature and supply voltage variation on the output of an op-amp.

A closed loop amplifier can be represented by two blocks one for an OPAMP and other for a feedback circuits. There are four following ways to connect these blocks. These connections are shown in fig. 1.

These connections are classified according to whether the voltage or current is feedback to the input in series or in parallel:

• Voltage – series feedback
• Voltage – shunt feedback
• Current – series feedback
• Current – shunt feedback

Fig. 1

In all these circuits of fig. 1, the signal direction is from input to output for OPAMP and output to input for feedback circuit. Only first two, feedback in circuits are important.

 Voltage series feedback:

It is also called non-inverting voltage feedback circuit. With this type of feedback, the input signal drives the non-inverting input of an amplifier; a fraction of the output voltage is then fed back to the inverting input. The op-amp is represented by its symbol including its large signal voltage gain A_d or A, and the feedback circuit is composed of two resistors R₁ and R_f. as shown in fig. 2

Fig. 2

The feedback voltage always opposes the input voltage, (or is out of phase by 180° with respect to input voltage), hence the feedback is said to be negative.

The closed loop voltage gain is given by

The product A and B is called loop gain. The gain loop gain is very large such that AB >> 1

This shows that overall voltage gain of the circuit equals the reciprocal of B, the feedback gain. It means that closed loop gain is no longer dependent on the gain of the op-amp, but depends on the feedback of the voltage divider. The feedback gain B can be precisely controlled and it is independent of the amplifier.

Physically, what is happening in the circuit? The gain is approximately constant, even though differential voltage gain may change. Suppose A increases for some reasons (temperature change). Then the output voltage will try to increase. This means that more voltage is fedback to the inverting input, causing v_d voltage to decrease. This almost completely offset the attempted increases in output voltage.

Similarly, if A decreases, The output voltage decreases. It reduces the feedback voltage v_f and hence, v_d voltage increases. Thus the output voltage increases almost to same level.

Different Input voltage is ideally zero.

Again considering the voltage equation,

           v_O = A_d v_d

 or       v_d = v_O / A_d

Since A_d is very large (ideally infinite)

         \ v_d » 0.

and    v₁ = v₂ (ideal).

This says, that the voltage at non-inverting input terminal of an op-amp is approximately equal to that at the inverting input terminal provided that A_d is very large. This concept is useful in the analysis of closed loop OPAMP circuits. For example, ideal closed loop voltage again can be obtained using the results