Open loop OP-AMP Configurations

In the case of amplifiers the term open loop indicates that no connection, exists between input and output terminals of any type. That is, the output signal is not fedback in any form as part of the input signal.

In open loop configuration, The OP-AMP functions as a high gain amplifier. There are three open loop OP-AMP configurations.

The Differential Amplifier

Figure 1, shows the open loop differential amplifier in which input signals V_in1 and V_in2 are applied to the positive and negative input terminals.

Figure 1

Since the OP-AMP amplifies the difference the between the two input signals, this configuration is called the differential amplifier. The OP-AMP amplifies both ac and dc input signals. The source resistance R_in1 and R_in2 are normally negligible compared to the input resistance R_i. Therefore voltage drop across these resistances can be assumed to be zero.

Therefore;

V₁ = V_in1 and V₂ = V_in2

V_o = A_d (V_in1 – V_in2)

where, A_d is the open loop gain.

The Inverting Amplifier

If the input is applied to only inverting terminal and non-inverting terminal is grounded then it is called inverting amplifier.This configuration is shown in Figure 2.

V₁= 0, V₂ = V_in

V_o = - A_dV_in

Figure 2

The negative sign indicates that the output voltage is out of phase with respect to input 180 ° or is of opposite polarity. Thus the input signal is amplified and inverted also.

The non-inverting amplifier

In this configuration, the input voltage is applied to non-inverting terminals and inverting terminal is ground as shown in Figure 3.

V₁ = + V_in and V₂ = 0

V_o = + A_dV_in

This means that the input voltage is amplified by A_d and there is no phase reversal at the output.

Figure 3

In all there configurations any input signal slightly greater than zero drive the output to saturation level. This is because of very high gain. Thus when operated in open-loop, the output of the OP-AMP is either negative or positive saturation or switches between positive and negative saturation levels. Therefore open loop op-amp is not used in linear applications.