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.