Hello everyone and thanks

Please
I want to use two transistors to switch between two references Vref01 and Vref02 by biasing the transistors Base (V_base) ,high for Vref01 and low for Vref02 ,this is simple in the case of the of the Vref02 transistor(low); but when the voltage on the base (V_base) get high sometimes I have Vref is more positive than V_base and this will cause the biasing of the tow transistors on the same time


*** Please help to get a best solution



*** I tried this way and it seems work but I don’t know why



***Please if this solution is really good tell me why it works

NOTE: I find this solution by trying a lot of things

I don’t need to keep the other V_ref when the second is on V_Out

Thanks a gain

 

What kind of accuracy and stability do you want to maintain?

 

hello again and thanks
-+7mv is good accuracy
for the stability really i don't know how to answer you ; because i don't know how this can affect the vOut
thanks again

 

I would use sn74lvc1g3157 or similar analog switch.

 

thanks kubeek
please i want to do it with a simple transistors if it is possible (four maximum)

 

If you want 7mV accuracy, you will need to use MOSFETs not BJTs.

What is the voltage and polarity of the two Ref voltages?

 

The BJT solution has losses that are difficult to account for. The Vce drop could be a couple hundred millivolts to larger values at some low base currents. With a MOSFET you get a small rds(on) that you can adjust for.

 

If you want to use BJTs this three transistor circuit will work with a possible loss of around 10mv. It all depends on the load on Vout and the values of the biasing resistors.
What is the load on Vout?

 

Thanks DickCappels, kubeek, crutschow, Papabravo and sghioto

So I understood that I have to use a MOSFET , I think that I cannot use a MOSFET on this application for another considerations (reparations, price, simplicity…)

Please

The really reference is amplified for 4 times, so I thought for example, that 1v is the result of 0.25v

What is the voltage and polarity of the two Ref voltages?

Is positive voltage between 0.8x and 3.xx v so with 4 times is 2.4 to 9.xx

However I understood now that what I need really is to stabiles this V_Out

Because in final this V_Out is compared with another voltage amplified for 4 times and this last voltage is the changed voltage without any intermediaries’ components

So it does not really matter if the reference drop some milivolts while is still stable

What is the load on Vout?

I think there will be no load because those references will be compared to another

thanks sghioto , I will try your suggestion and I will tell you later

 

Is positive voltage between 0.8x and 3.xx v so with 4 times is 2.4 to 9.xx

Are you saying the reference voltages are amplified 4 times after they come out of Vout or before?

 

Thanks sghioto

Are you saying the reference voltages are amplified 4 times after they come out of Vout or before?

the reference voltages are amplified 4 times before they come out of Vout
i amplified 4 times another positive voltage to get Vref01 and Vref02

 

the reference voltages are amplified 4 times before they come out of Vout

OK, this same version actually works down to .8 volt reference.

 

OK, this same version actually works down to .8 volt reference.

please what means this !?

 

It means the reference voltage can be as low as .8 volt before amplification

 

Thanks sghioto
Ok, I will try your two suggestions and I will tell you as soon as possible.

 

Just the one in post #12 should suffice.

 

The post #12 is a very good try and just might be what you need.

If you choose to go the transmission gate route, I see that my distributor has the HEF4066BT which is 4 channels of analog switches which should be fine if your load is light, is just over U.S. 21¢ each in single quantities.

 

Hello everyone and thanks

Please sghioto ; can you help me to adapter your schematic on my real application?
The logic circuit in my precedent schemas was the output of Omp LM324n (high and low)

This is part of my real circuit

If you choose to go the transmission gate route, I see that my distributor has the HEF4066BT which is 4 channels of analog switches which should be fine if your load is light, is just over U.S. 21¢ each in single quantities.

thanks DickCappels , maybe , because my application has many separated loads, please can you let me take look on it

 

Datasheet attached.

Inside the package are four SPST normall-open switches. You new two enable signals, one for each switch. It is possible to use one switch as an inverter to make an enable signal that does the opposite of the original enable signal. If your load(s) are not high impedance you can use and opamp voltage follower as a buffer.

Take note of the on resistances listed in table 7.

 

Maybe I missed it but is there any reason you can't use small relays with silver or gold plated contacts? This would reduce any loss through semiconductor devices. Relays with very low coil currents.

Ron