The Virtual Contact
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, or if you live in a Dilbertian world of lastminute spec changes.
With the switch S open, source A goes through voltage-follower A and R feed 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 good rejection of A, the output impedance of follower B must be much, much lower than the value of R feed ; so an opamp output must be used directly.
At first this technique 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.
There is also the rejection of B when the switch is open to consider. The impedance of R feed mean there is the potential for capacitative crosstalk across the open switch contacts. The amount depends on the value of R feed and on switch construction.
If the offness of B is more important than the offness of A, then R feed should be a lower value, to minimise the effects of the capacitance. Do not make R feed too low as A drives through it into effectively a short circuit when B is selected.
If the offness of A is more important, R feed 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.
The rejection of A shown above worsens at high frequencies, as the dominant-pole of opamp B reduces its open-loop gain and the output impedance rises. The slopes are 6 dB/ octave as usual.
This technique is particularly useful for switching between three sources with a centre-off toggle switch.
Excerpt from Small Signal Audio Design by Douglas Self © 2014 Taylor & Francis Group. All Rights Reserved.
About the Author
Douglas Self studied engineering at Cambridge University, then psychoacoustics at Sussex University. He has spent many years working at the top level of design in both the professional audio and hifi industries, and has taken out a number of patents in the field of audio technology. He currently acts as a consultant engineer in the field of audio design.