10. Amplifiers with balanced Inputs or Outputs

Amplifiers with balanced Inputs or Outputs

The op-amp systems presented so far in this text are driven by voltages where one side of the source is grounded. Numerous op-amp applications require that we deal with voltages that are balanced; that is, neither side of the voltage source can be grounded. Op-amps are useful for converting a balanced input to a grounded (or unbalanced) output.

Figure 44 illustrates various configurations of balanced inputs and outputs. These configurations use the second method of balancing, that of cascading two op-amps.

As an example, in the configuration shown in Figure 44(b) if v_in is supplied from a high-impedance source, the 5kΩ resistors are no longer useful. Bias current may not be balanced in each amplifier. However, the effects of equal offsets due to unbalanced bias currents in the two input amplifiers should cancel one another. The alternative arrangement shown in the figure avoids this problem.