Electronic DC Voltmeter circuit

[MSAKARIM]

This circuit is a DC voltmeter, I want to know why the two transistors are arranged in this manner?
Is Q1 is common emitter and Q2 is common collector?
why suppely E connected like this?

 

[KlausST]

Hi,

The transistors try to amplify the measurement current (input voltage / resistor).

why suppely E connected like this?

Whats wrong with it?
How do you you expect it to be wired?

******

But it´s a really bad circuit. It may be from 1960 to 1970. So really outdated.
It´s not accurate at all and it drifts a lot with temperature.
--> The displayed value is not reliable.

Klaus

 

[FvM]

Another possible explanation is that the schematic has errors and the real circuit is different, if it's at all the schematic of a real instrument.

Range resistors seem to fit a standard 50 uA moving coil instrument (20 kOhm/V), it won't use amplifier transistors.

I rather guess that the schematic is a mixed-up combination of resistor measurement circuit, regular voltage input. Transistors may either provide overvoltage clamp or a different function.

 

[D.A.(Tony)Stewart]

Anecdotal:
I recall drawing power from a DMM battery and noticed as in this circuit that Vbat+ and Vin+ were common.
But this schematic looks flawed with hFE dependancy. I wonder if PNP is installed in reverse to reduce the hFE to near 1.

2009 was the earliest similar match of this schematic, on www.Circuitstoday.comhttp://www.circuitstoday.com/dc-voltmeters
by Author jojo

I agree with FVM Frank's description and the design is flawed.
Always check who created the design and look for missing information as a clue to an unfinished drawing.

 

[BradtheRad]

Q1 Q2 are arranged as a sziklai pair. It's made from NPN and PNP and operates similar to a Darlington pair. Both types have an advantage of high gain yet draw tiny bias current through those high-ohm resistors.

 

[D.A.(Tony)Stewart]

Q1 Q2 are arranged as a sziklai pair. It's made from NPN and PNP and operates similar to a Darlington pair. Both types have an advantage of high gain yet draw tiny bias current through those high-ohm resistors.

For this design to be accurate with unknown hFE it is not suitable. The bias is wrong and will not work with excess offset and gain introduced.