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Blog The Filter Wizard Remastered

Active filters that still work even when their amplifiers don’t

This is an addendum to an earlier article in which the Filter Wizard forgot to do a very important simulation. The apparently undesirable behavior predicted by that simulation turns out to not be a problem after all, and further demonstrates the robustness of the design approach.

In my haste to get “A Fast-Settling Bias Voltage Filter with High Ripple Rejection” out there, I forgot to cover an important aspect, that on the face of it might look to be a show-stopper. It turns out that it’s not so big a deal, though it does demonstrate something quite interesting about that class of filter design.

To recap, in that article I presented a ‘DC-free’ filter that not only effectively suppressed ripple on a very high voltage bias line, but also settled very quickly following changes in that voltage. Do check the article out for the full story if you’ve not already done so.

In the previous article, figure 10 proudly showed the settling behaviour of a couple of variants of that filter topology to a step change in the input voltage. As you can see from the units, the particular step I was studying was a unit step, i.e. a 1 V change in input voltage. I implied, however, a rapid settling time also to the much larger input step that results when the actual 180 V bias voltage we were filtering was turned fully on and off.

Let’s look at the predicted outputs of the op-amps in the circuit (figure 7 in the previous piece) I when we apply a full-sized 180 V step to the input. First of all, let’s use a simple non-powered op-amp macromodel. Whichever simulator you use, it’ll have one of these – you can even use the simplest of all amplifier approximations, a voltage-controlled voltage source or SPICE ‘E’ component, to make an ideal op-amp for this purpose. It’s a great way of abstracting the ideal behaviour of a circuit with only the simplest of amplifier characteristics.

Figure 1 shows the result. Unfortunately, our simulations predict a negative spike of 600 V at the output of each op-amp when the input voltage steps up from zero to 180 V. A same-size positive-going spike follows when the input voltage steps back down to zero again. Now, fellow engineers, it doesn’t take class-leading smarts to work out that this kind of voltage excursion isn’t available from real amplifiers that are being powered from a 5 V supply. In practice, the outputs can only go close to ground or 5 V, and they will then saturate. As a result, the inverting inputs of the amplifiers, previously held at ‘virtual earth’ as a consequence of the feedback resistor, will be pulled away from that state of electronic grace by current flowing through the capacitors connected to them. Eventually the inverting input voltage will hit the clamp voltage set by your input protection scheme.

step response
Figure 1. What the amplifier outputs of the filter circuit would do with no supply voltage limitations.

Speaking of input protection, in the earlier article I did mention that the op-amp terminals, which are exposed to large voltage changes through the capacitors, need to be protected with tough devices, such as chunky Zener diodes and surge suppressors – just good basic practice when handling voltage excursions outside your supply rails. These need to withstand quite significant currents during that step, if the source can supply it.

Surely this input and output saturation is fatal for the filter’s performance, right? You usually expect that an op-amp-based circuit becomes pretty non-functional if the amplifier outputs hit the rails. Well, once this has happened with our circuit here, the circuit clearly can’t work as intended because the amplifiers have an incremental voltage gain of zero and aren’t doing anything (except getting warm). The circuit just looks like a ladder of resistors and capacitors, and it won’t have either the complex pole pairs or the stop-band zeroes. It will take longer to settle and won’t have the necessary stopband rejection.

To check the behaviour when we put some clipping diodes in to protect things, I put a 4.7 V Zener diode across each op-amp output and each inverting input. My simulator only had a model for a rather weedy 500 mW-rated diode, but it soon became clear that you might need quite a high current rating here. Also, to ensure that all the error current does go through the diodes, I put a 40 Ω resistor in series with each op-amp output and gave it a 25 mA current limit. You could also usefully put a resistor between each summing node and the actual amplifier inverting input.

Figure 2. This is a ’real’ test schematic, now with built-in voltage and current limiting.

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