I need to switch negative and positive voltage with npn transistors, but I am not sure if npn transistors work in both ways.
How to do something like this with transistors?
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\$\begingroup\$ Look up a triac, you might be able to use it. \$\endgroup\$– keffeCommented Jun 1, 2018 at 17:26
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2\$\begingroup\$ @Etwus: What are you really trying to do? Your circuit indicates that you want to load a 400 V peak-to-peak squarewave signal source with a switchable voltage divider. You have omitted details on current and load resistor values which, of course, have a very high significance on the solution. \$\endgroup\$– TransistorCommented Jun 1, 2018 at 17:32
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\$\begingroup\$ Sorry for not being clear, what I want to do. I am trying to make switchable voltage divider, which should be driven by 3V signal. It will convert 200V, 100V, 20V and 5V voltage to 1.25V. Very low current, under 200uA. \$\endgroup\$– EtwusCommented Jun 1, 2018 at 17:50
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\$\begingroup\$ Right, well that information belongs in the question rather than in the comments. (1) Presumably you want to tap off at the 'dot' on the schematic. If so then draw that in. (2) Is there a load attached to this point? If so then what is its input impedance? (3) What will happen if it's on the 5 V range and the input rises to 200 V? (4) What is the switching frequency of the input signal? (5) What is the context to all of this? What are you making? \$\endgroup\$– TransistorCommented Jun 1, 2018 at 21:42
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\$\begingroup\$ bipolar transistors don't work well with AC voltages above max(V_eb) \$\endgroup\$– Jasen Слава УкраїніCommented Jun 2, 2018 at 4:26
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1 Answer
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Use an optically isolated AC MOS relay or a mechanical relay.
Eg. 
If you are really insistent on using NPN transistors you could put the transistor inside a bridge rectifier and power the base using a DC-DC converter. It would have 2 diodes drop, plus a bit, however.
Thyristors are another possibility but they don't turn off until the current drops below a certain level, due to external reasons (such as the source being AC).
Based on your additional information you could consider a divider from +/-200V to 1.5V and a programmable gain amplifier to step the 100/20/5 up by a factor of 2/10/40. That takes an op-amp, a low voltage analog multiplexer and a few resistors.
answered Jun 1, 2018 at 17:26