two transistors to switch between two references Vref01 and Vref02

Thread Starter

AMIRAAM

Joined Dec 31, 2019
37
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

ESCH01.png

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

ESCH02.png

***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
 

Thread Starter

AMIRAAM

Joined Dec 31, 2019
37
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
 

crutschow

Joined Mar 14, 2008
27,761
If you want 7mV accuracy, you will need to use MOSFETs not BJTs.

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

Papabravo

Joined Feb 24, 2006
16,836
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.
 

sghioto

Joined Dec 31, 2017
2,395
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?
1619018751805.png
 
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Thread Starter

AMIRAAM

Joined Dec 31, 2019
37
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
 

DickCappels

Joined Aug 21, 2008
7,746
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.
 

Thread Starter

AMIRAAM

Joined Dec 31, 2019
37
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

ESCH03.png
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
 

DickCappels

Joined Aug 21, 2008
7,746
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.
 

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Reloadron

Joined Jan 15, 2015
6,067
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
 
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