Page updated: 18 Feb 2002

What do you do if you need a changeover switch- to select one of two signal sources- but only have a make contact? Here is a technique that can be a lifesaver when you have screwed up on ordering a switch with a long leadtime.

Fig 1 demonstrates the principle. With the switch S open, source A goes through voltage-follower A and Rfeed to the output voltage-follower. With the switch closed, the much lower impedance output of voltage-follower B takes over and the contribution from A is now negligible. To give a good "offness" of A, the output impedance of follower B must be much, much lower than the value of Rfeed; this is not hard to arrange.
Fig 2 shows how good the offness can be using 5532 opamps as the voltage followers. At first this looks a bit opamp-intensive. However, there is often no need to use dedicated voltage-followers, if a similar low-impedance feed is available from a previous stage that uses an opamp with a large amount of negative feedback. Likewise, the output voltage-follower may often be dispensed with if the following load is reasonably high. It is shown here to emphasise that in one position of the switch, the output impedance is definitely not zero.

There is also the offness of B to consider, when the switch is open. The impedance is medium, and there is the potential for capacitative crosstalk across the open switch contacts. The amount depends very much on switch construction.

If the offness of B is more important than the offness of A, then Rfeed should be a lower value, to minimise the effects of the capacitance. Do not make Rfeed too low as A drives through it into effectively a short circuit.

If the offness of A is more important, Rfeed should be higher to increase its ratio to the output impedance of B; be aware that making it too high may introduce excessive Johnson noise.

Above: Fig 2. Offness of A for 2K2, 4K7 and 10K Rfeed resistors, using 5532 opamps.

The offness shown in Fig 2 rises at high frequencies, as the opamp dominant-pole begins to reduce the open-loop gain. The slopes are 6 dB/octave as usual.

This technique is particularly useful for switching between three sources with a centre-off toggle switch.

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