Figure 1 shows how a Sallen-Key high-pass filter (HPF) can be turned on/off with just one additional transistor and resistor.

Figure 1 Sallen-Key HPF filter topology that can be turned on/off using an additional transistor and resistor.

This solution seems appropriate for the Sallen-Key filter, since the topology itself is quite simple.

The op amp here is a programmable op amp with a current sinking of setting Iset (ie the current through the tuning resistor R4 that sinks into the op amp). The LM146 and LM346 are the operational amplifiers of this type.

An op amp that uses source tuning current (such as the LM4250) can also be used with a simple mirror modification of this circuit. For example, Q1 becomes a PNP transistor and must be connected to the +E terminal instead.

Resistor R3 is bypassed when the op amp is turned off. When transistor Q1 is off, tuning current flows in Iset terminal (pin 9). So the op amp is turned on and has a very low output impedance which consists of resistor R3 in a voltage divider which effectively eliminates the bypass (more than 80dB) leaving the filtered output.

If the transistor Q1 is on, the current Iset flows into Q1 and cannot activate Iset terminal. This is due to the threshold voltage (0.8 V to 0.9 V) at the I terminalset. So, the op amp is turned off and its output is effectively disconnected from the circuit.

The output is now determined by a voltage divider made up of resistor R3 and the load impedance. To emphasize this, the next cascade is shown as a buffer, which is optional. Note: the filter components (R1 and C1) are connected in parallel to the resistor R3, this cannot be ignored.

A version of the LPF is shown in Figure 2. It uses a unipolar power supply and transistor Q1 (SRC1204) with built-in bias resistors, making the circuit even simpler.

Figure 2 An LPF topology that can be turned on/off using a single-pole power supply and transistor Q1 (SRC1204) with built-in bias resistors.

Peter Demchenko studied mathematics at Vilnius University and worked in software development.

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