Sallen–Key topology


The Sallen–Key topology is an electronic filter topology used to implement second-order active filters that is particularly valued for its simplicity. It is a degenerate form of a voltage-controlled voltage-source filter topology.

Explanation of operation

A VCVS filter uses a voltage amplifier with practically infinite input impedance and zero output impedance to implement a 2-pole low-pass, high-pass, bandpass, bandstop, or allpass response. The VCVS filter allows high Q factor and passband gain without the use of inductors. A VCVS filter also has the advantage of independence: VCVS filters can be cascaded without the stages affecting each others tuning. A Sallen–Key filter is a variation on a VCVS filter that uses a unity-voltage-gain amplifier. It was introduced by R. P. Sallen and E. L. Key of MIT Lincoln Laboratory in 1955.

History and implementation

In 1955, Sallen and Key used vacuum tube cathode follower amplifiers; the cathode follower is a reasonable approximation to an amplifier with unity voltage gain. Modern analog filter implementations may use operational amplifiers. Because of its high input impedance and easily selectable gain, an operational amplifier in a conventional non-inverting configuration is often used in VCVS implementations. Implementations of Sallen–Key filters often use an operational amplifier configured as a voltage follower; however, emitter or source followers are other common choices for the buffer amplifier.

Sensitivity to component tolerances

VCVS filters are relatively resilient to component tolerance, but obtaining high Q factor may require extreme component value spread or high amplifier gain. Higher-order filters can be obtained by cascading two or more stages.

Generic Sallen–Key topology

The generic unity-gain Sallen–Key filter topology implemented with a unity-gain operational amplifier is shown in Figure 1. The following analysis is based on the assumption that the operational amplifier is ideal.
Because the operational amplifier is in a negative-feedback configuration, its v+ and v− inputs must match. However, the inverting input v− is connected directly to
the output vout, and so
By Kirchhoff's current law applied at the vx node,
By combining equations and ,
Applying equation and KCL at the OA's non-inverting input v+ gives
which means that
Combining equations and gives
Rearranging equation gives the transfer function
which typically describes a second-order linear time-invariant system.
If the component were connected to ground, the filter would be a voltage divider composed of the and components cascaded with another voltage divider composed of the and components. The buffer bootstraps the "bottom" of the component to the output of the filter, which will improve upon the simple two-divider case. This interpretation is the reason why Sallen–Key filters are often drawn with the operational amplifier's non-inverting input below the inverting input, thus emphasizing the similarity between the output and ground.

Branch impedances

By choosing different passive components for,,, and, the filter can be made with,, and characteristics. In the examples below, recall that a resistor with resistance has impedance of
and a capacitor with capacitance has impedance of
where is the complex angular frequency, and is the frequency of a pure sine-wave input. That is, a capacitor's impedance is frequency-dependent and a resistor's impedance is not.

Application: low-pass filter

An example of a unity-gain low-pass configuration is shown in Figure 2.
An operational amplifier is used as the buffer here, although an emitter follower is also effective. This circuit is equivalent to the generic case above with
The transfer function for this second-order unity-gain [|low-pass filter] is
where the undamped natural frequency, attenuation, Q factor, and damping ratio, are given by
and
So,
The factor determines the height and width of the peak of the frequency response of the filter. As this parameter increases, the filter will tend to "ring" at a single resonant frequency near .

Poles and zeros

This transfer function has no zeros and two poles located in the complex s-plane:
There are two zeros at infinity.

Design choices

A designer must choose the and appropriate for their application.
The value is critical in determining the eventual shape.
For example, a second-order Butterworth filter, which has maximally flat passband frequency response, has a of.
By comparison, a value of corresponds to the series cascade of two identical simple low-pass filters.
Because there are 2 parameters and 4 unknowns, the design procedure typically fixes the ratio between both resistors as well as that between the capacitors. One possibility is to set the ratio between and as versus and the ratio between and as versus. So,
As a result, the and expressions are reduced to
and
Starting with a more or less arbitrary choice for e.g. C and n, the appropriate values for R and m can be calculated in favor of the desired and. In practice, certain choices of component values will perform better than others due to the non-idealities of real operational amplifiers. As an example, high resistor values will increase the circuit's noise production, whilst contributing to the DC offset voltage on the output of opamps equipped with bipolar input transistors.

Example

For example, the circuit in Figure 3 has and. The transfer function is given by
and, after substitution, this expression is equal to
which shows how every combination comes with some combination to provide the same and for the low-pass filter. A similar design approach is used for the other filters below.

Input impedance

The input impedance of the second-order unity-gain Sallen–Key low-pass filter is also of interest to designers. It is given by Eq. in Cartwright and Kaminsky as
where and.
Furthermore, for, there is a minimal value of the magnitude of the impedance, given by Eq. of Cartwright and Kaminsky, which states that
Fortunately, this equation is well-approximated by
for. For values outside of this range, the 0.34 constant has to be modified for minimal error.
Also, the frequency at which the minimal impedance magnitude occurs is given by Eq. of Cartwright and Kaminsky, i.e.,
This equation can also be well approximated using Eq. of Cartwright and Kaminsky, which states that

Application: high-pass filter

A second-order unity-gain high-pass filter with and is shown in Figure 4.
A second-order unity-gain high-pass filter has the transfer function
where undamped natural frequency and factor are discussed above in the low-pass filter discussion. The circuit above implements this transfer function by the equations
and
So
Follow an approach similar to the one used to design the low-pass filter above.

Application: bandpass filter

An example of a non-unity-gain bandpass filter implemented with a VCVS filter is shown in Figure 5. Although it uses a different topology and an operational amplifier configured to provide non-unity-gain, it can be analyzed using similar methods as with the [|generic Sallen–Key topology]. Its transfer function is given by
The center frequency is given by
The Q factor is given by
The voltage divider in the negative feedback loop controls the "inner gain" of the operational amplifier:
If the inner gain is too high, the filter will oscillate.