Help in understanding PROTEUS Simulation Result

Thread Starter

zeb59

Joined Oct 6, 2013
6
Attached is a circuit based on discreet BJTs in a differential amplifier configuration, with a current mirror load, simulated in PROTEUS.

I cannot understand the voltage level at Q2 Collector which is 53 volts while the VCC supply is 9 volts only. These voltages can be seen displayed on the DC voltmeters on the right side of the circuit.

Help much appreciated.

TIA

Zeb
 

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Thread Starter

zeb59

Joined Oct 6, 2013
6
Thanks for the answer.

Simulation file attached in zip format.

No I have not tried to change the BJT nor the 7809. Why should that make a difference?

Zeb
 

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Jony130

Joined Feb 17, 2009
5,313
I look at you circuit, and everything is OK with the simulation.
The voltage at Q1 and Q2 base is equal to 54V (R11(2)). And if so Q1 and Q2 are in saturation. And BC diode is forward biased and this is why Vc = Vb - Vbc.
So your differential amp doesn't work because both of a BJT are in saturation region.
 

Thread Starter

zeb59

Joined Oct 6, 2013
6
Jony130,

Thanks for your input.

I have modified the circuit by removing the voltage source. The purpose of the circuit is to measure the voltage drop across the 0.001 ohm resistor. The diff. amp must amplify the voltage so that it can be fed into a PIC microcontroller.

The revised circuit is attached. It still does not work.

Please help.

TIA.

Zeb
 

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Jony130

Joined Feb 17, 2009
5,313
You need to use a op amp, discrete differential amplifier has a pure CMRR and common mode input voltage.
Also in your diagram you short Q1 base to GND
Google low/high side current monitor op amp
 

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