Gas station without pumps

2017 July 1

Multiple-feedback bandpass active filter

Filed under: Uncategorized — gasstationwithoutpumps @ 12:14
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I wanted to improve my understanding of active filters, beyond the very simple ones that I included in my textbook, so I decided to try implementing a bandpass filter using the multiple-feedback topology.  To make the design more concrete, I decided to make one that was centered at 2kHz, but firmly rejected 1.5kHz and 2.5kHz.

Here is the design I came up with. The top part is the filter, while the bottom is just a reference voltage buffer. I’ll derive the values below.

I_1= (V_{in} -V_{m})/R_1
I_2= (V_{ref}-V_{m})C_{2} s = (V_{o}-V_{ref})/R_2
I_3= (V_{ref}-V_{m})/R_3
I_4= (V_{o}-V_{m})C_{1}s
I_1 + I_2 + I_3 +I_4 = 0

We can simplify a bit, by replacing V_{m}-V_{ref} with -(V_{o}-V_{ref})/(\tau_2 s), where \tau_2 = R_2 C_2. Note that this corresponds to replacing the amplifier, R2, and C2 with an inverting high-pass filter.

It is conventional to recenter the voltages so that V_{ref} is zero, which simplifies the algebra.  Doing that gives us

(V_{in}+V_{o}/(\tau_2 s))/R_1 + V_{o}/R_2 + V_{o}/(\tau_2 s R_3) + V_{o}(1+1/(\tau_2 s))C_{1}s=0

Multiplying both sides by \tau_2 s and rearranging gives us

-V_{in}\tau_2 s/R_1  = V_{o} \left(1/R_1 + \tau_2 s /R_2 + 1/R_3 + (\tau_2 s +1)(C_1 s)\right)

\frac{V_{o}}{V_{in}} = -\tau_2 s / \left({1+ \tau_2 s R_1/R_2 + R_1/R_3 + (\tau_2 s +1)(R_1 C_1 s)}\right)

More correctly, that should be \frac{V_{o}-V_{ref}}{V_{in}-V_{ref}} = \ldots, but I’ll just talk about the gain G.

Defining \tau_1 = R_1 C_1 we get

G = -\tau_2 s / \left(1+ R_1/R_3+  (\tau_2 R_1/R_2 + \tau_1) s + \tau_1\tau_2 s ^2)\right)

G = -(\tau_2 s/R_1 )/ \left(1/R_1+1/R_3+  (\tau_2 /R_2 + \tau_1/R_1) s + \tau_1\tau_2 s ^2/R_1)\right)

If we have C_1=C_2=C, which is a common design constraint, we can simplify to

G = -(R_2 C s/R_1 )/ \left(1/R_1+1/R_3+  2 C s +C\tau_2 s ^2)\right)

G = -(R_2 /R_1 )/ \left((1/R_1+1/R_3)(Cs)^{-1}+  2  +\tau_2 s)\right)

We can define R_p = R_1 || R_3 = R_1R_3/(R_1+R_3) and \tau_p = R_p C_1.  Then we have

G = -(R_2 /R_1 )/ \left((\tau_p s)^{-1}+  2  +\tau_2 s)\right)

If we let s = j \omega, we get

G = -(R_2 /R_1 )/ \left(2  +j (\tau_2 \omega-(\tau_p \omega)^{-1})\right),

whose magnitude is maximized when \tau_2 \omega-(\tau_p \omega)^{-1}=0 or \omega = 1/\sqrt{\tau_2\tau_p}, and at that maximum the gain is - R_2/ (2 R_1).

The corner frequencies, where the gain drops by the square-root of 2 occur when the real and imaginary parts of the denominator have the same magnitude, or \tau_2 \omega-(\tau_p \omega)^{-1}= \pm 2. If we take only the positive values of \omega, we get \omega = \left(\pm 1 + \sqrt{(\tau_p+\tau_2)/\tau_p}\right) /\tau_2, for a bandwidth of 2/\tau_2, or 1/(\pi R_2 C_2) Hz.

For C=10nF and R2=220kΩ, we get a bandwidth of 144.7Hz.  With R1=6.8kΩ and R3=300Ω, Rp=287.32Ω, with time constant 2.8732 µs, giving a center frequency of 2001.8kHz.  I built this filter and tested it with the Analog Discovery 2:

The overall match of the filter to the theory is pretty good.

But when we look in detail at the peak, we can see that the center frequency is off a little and the gain isn’t nearly as big as it should be. I suspect that the problem is poor matching of the very cheap 10nF capacitors.

I also tried exciting the filter with wideband noise from the Analog Discovery 2, and doing an FFT of the input and output to get a different view of the response:

The filter response looks a little larger taking the difference of FFTs (after averaging many FFTs, since each individual one has a lot of noise). Note the 60 Hz spike.


1 Comment »

  1. […] Multiple-feedback bandpass active filter I built a simple active filter to be part of a tone-detection circuit (looking for 2kHz). […]

    Pingback by Digital tone detection with Goertzel’s algorithm | Gas station without pumps — 2017 July 9 @ 00:28 | Reply

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