Section G11: Other Op-Amp Input & Output Considerations We’re going to look at just a few more important aspects of op-amp circuit design in this section: amplifiers with balanced inputs or outputs, and the coupling effect that may occur between multiple inputs. Amplifiers with Balanced inputs or Outputs So far, op-amp circuits we’ve looked at use voltage sources that have one side grounded, or unbalanced. Many times we are not given this option in that neither side of the voltage source can be grounded, so they must be balanced. Depending on the available input and requirements for the output, the op-amp may be used for any required conversions. The illustrations of Figure 9.44, generalized and reproduced below, illustrate the use of op-amps to achieve desired combinations of balanced/unbalanced inputs and/or outputs. (a) Balanced input, unbalanced output (b) Balanced high-impedance input, unbalanced output (c) Unbalanced input, balanced output (e) Balanced high-impedance input, balanced output (d) Unbalanced high-impedance input, balanced output (f) Balanced input, balanced output Coupling Between Multiple Inputs Coupling between inputs can occur when more than one input signal is connected to the inverting or the non-inverting terminal of the op-amp. This phenomenon is exactly what it sounds like – a variation in one signal can couple, or produce, an unwanted input into another signal. Signal coupling may become a significant problem since the resulting crosstalk may change, or distort, the actual signal to be amplified. Figure 9.47a, modified and presented to the right, illustrates a non-inverting weighted summer configuration with two ideal voltage sources, v1 and v2. The resistances indicated in lower case (r1 and r2) represent the internal resistance of the respective voltage source, and the coupled inputs to the op-amp are indicated by v’1 and v’2 (hold on, this will make sense shortly). In this figure, R1 and R2 serve their usual purpose of resistive components chosen to satisfy the bias balance constraint (i.e., RA=R1||R2). Using the principle of superposition, the circuits of Figure 9.47b (below and left, where v2 has been set to zero) and 9.47c (below and right, where v1 has been set to zero) are obtained. Writing the KVL around each of these loops and solving for the coupled inputs (or by using the voltage divider rule), we get: v'2 = r1v2 r2v1 and v '1 = . r1 + r2 + R1 + R2 r1 + r2 + R1 + R2 (Equations 9.98 & 9.99) Notice from the above equations that the coupled inputs v’2 and v’1 depend on the signal applied to the other channel (v1 and v2 respectively). As you may imagine, this effect may become devastating with any significant values of r1 and r2, or if several signals are involved in the summer. This effect may be eliminated by designing a system where the internal resistance of the voltage sources (r1 and r2 in our example above) approach zero. To remove the coupling effect, and the resulting crosstalk, your author suggests that each non-inverting multiple input should be driven with an op-amp that has zero (or very low) output impedance.