Bad Circuit Design 5 - Not Providing a DC Path

Perhaps one of the more common bad designs is not providing a DC path to the input of a circuit.

Consider the op-amp circuit seen below. This looks like a classic linear circuits op-amp problem.

In a real implementation of this circuit the DC voltage on the inverting op-amp input is unknown

and so the circuit will not function properly. A very large resistor can be placed across the feedback

capacitor to provide a DC path to the input (large so that it only affects the DC performance of the

circuit). Why can’t we put a large resistor across the capacitor connected to the input to provide the

DC path needed to get the circuit to work? Answer, the op-amp’s offset voltage will cause the op-amp

output to rail (go to the power supply rails). We need the negative feedback from the output back

to the input to ensure the op-amp biases up to the correct DC operating point.

Note that with the large resistor across the feedback capacitor the DC gain of the circuit is 0 while

without the feedback resistor the DC gain is unknown.

So does the circuit work if we remove the capacitor connected to the input (the resulting circuit is

an integrator)? Answer, no. There still needs to be feedback from the output back to the input to

set a DC operating point that compensates for the op-amp’s offset. Practically, this feedback can

be implemented with (again) a large resistor across the feedback capacitor, a reset switch across the

feedback capacitor (see Figs. 25.25b or 31.50), or if the integrator is part of a feedback loop (see

Fig. 29.47 assuming the capacitor connected below to Vin isn’t present).

This problem can also be a major concern when using floating gate devices in analog design, see

Sec. 16.3.3. Generally, the devices are erased and then slowly programmed with a program-verify

to ensure that they have the desired electrical characteristics. Practically this is a challenge for a

manufacturable analog integrated circuit because the programming has to be precise (< mVs).

Below is one final example of not providing a DC path…this circuit won’t work!

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