Difference Amplifier With No Noninverting Input Attenuator

This circuit is a (operational) difference amplifier, however, there is no resistor from the noninverting input to ground. The voltage applied to the noninverting side does not get attenuated, Rbbc serves only as bias-balance compensating resistor. The noninverting and inverting gains are asymmetric and the output voltage can be expressed as Vo = (K+1) · Viniv - K · Viinv where: K = Rf/Rinv, the ratio between feedback resistor Rf and inverting input resistor Rinv. Viniv = the voltage applied to the noninverting side. Viinv = the voltage applied to the inverting side. For the saved values: K = (20 kΩ / 20 kΩ) = 1 Vo = (1 + 1) · Viniv - (1) · Viinv Vo = 2 · Viniv - Viinv Vo = 2 · (2 V) - (1 V) Vo = (4 - 1) V Vo = 3 V When Sflt is OPEN the circuit is in floating-input mode. Unlike the conventional difference amplifier, this circuit will not work as intended in this configuration. In the absence of a resistor from the noninverting input terminal to ground the current path from the input voltage sources to ground is through the noninverting input terminal of the operational amplifier which is a very high impedance node. The operational amplifier saturates and the voltage at the noninverting input is the algebraic sum of the op amp saturation output voltage and the input voltages.