[sdiy] Transistor Sallen-Key lowpass filter question

Ben Bradley ben.pi.bradley at gmail.com
Mon Nov 19 03:36:53 CET 2018


Yes, it's pure AC, so the transistor has 0V  bias. Easiest thing to do
is add a 6V DC voltage source in series with V1.
On Sun, Nov 18, 2018 at 7:40 PM Tristan <tu at alphalink.com.au> wrote:
>
> Is V1 a pure AC source? V1 should also include a positive DC bias of around 6V for the transistor circuit to
> work. Otherwise you could just be getting half wave rectified pulses coming out on the positive peaks of
> the AC input waveform.
>
> The opamp example probably assumes the opamp is powered by bipolar supply rails so it works with no
> DC bias on the input.
>
> /Tristan
>
>
> On Mon, Nov 19th, 2018 at 10:59 AM, Tom Wiltshire <tom at electricdruid.net> wrote:
>
> > Can anyone tell me why a transistor-based Sallen-Key filter like this one
> > would have such a heavy roll-off at the bass end?
> >
> >       https://electricdruid.net/wp-content/uploads/2018/11/DelayFilterSchematic.png
> >
> > According to the sim (LTSpice), the response looks like this:
> >
> >       https://electricdruid.net/wp-content/uploads/2018/11/DelayFilter.png
> >
> > There’s a significant drop with a -3dB cutoff of about 200Hz. What’s
> > causing that?
> >
> > Using the same passive components, but with a op-amp in place of the tranny,
> > with the tools at..
> >
> >       http://sim.okawa-denshi.jp/en/Sallen3tool.php
> >
> > ..doesn’t give the same effect. Am I seeing a simulation effect or a real
> > phenomena?
> >
> > Thanks,
> > Tom
> >
> >
> >
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>
>
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