Analog buffer without op-amps

One alternative is to build a common collector with a BJT or a common drain with JFET. It works almost as a buffer: high input impedance, low output impedance and almost a unity gain. Being a simple circuit you can implement without problem. JFET produces a better alternative to low input current needed.

Common Collector Common Drain

Common Collector and Common Drain

If you need more inputs/outputs you can use a common collector array chip such as CA3082. It will save you lots of space. Unfortunately I don’t know a chip with common drain array built in.

Common Collector Array Chip

I find it hard to believe that wiring up little SC70 (or smaller) op amps is going to take up too much space on your board. Digikey has 5 pages of SC70 sized buffer amps that would probably do a good sight better job performance-wise than a common-collector BJT, and you could sprinkle them around the board as needed, meaning they’d take up no more space than that BJT.

It seems after some discussion that you are looking for a buffer amplifier.

Digikey has a section for this. if you go to the main area for linear amplifiers and then select for buffer type and in stock you get quite a list. I selected for those with an input current of 2nA typical I got a relatively short list(like 10). This however does not give me an easy way to share the links, so you will have to click it yourself.

These are designed to give you the features you want and in a small package, now they normally come in groups of 2^n, so you will have to get a package of 8, but I am sure you can make it work.

Markrages had a bit of extra input to add in a comment:

Cautions about those buffer amps: Most of them are made for video signals and so they are high bandwidth / high current designs. That’s a consideration if the circuit is battery powered. Also note that (the ones I looked at) are specialized, single vendor parts. More expensive and more availability risk than op-amps or transistors with standardized footprints. Engineers have a duty to only use oddball parts when their special features are necessary and relevant to the design.

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