BJT Differential Amplifier question?!

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hamsiii

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I have a homework states that we need to get a output of 100mV from 10mV input.
We need to use BJT differential amplifier with current mirror. The circuit will be single supplied with +12V only. I could not come up with an answer for the values as in spice simulation, i only amplified 1.2 times the input. Would you mind helping me to find the values of resistors?

https://i53.tinypic.com/4j2u14.png

R13 and C4 is HP filter for elimination of DC current.
Thank you all!
 
If you don't have access to negative voltage, then I suggest to invert the circuit if the circuit is working with higher input voltages. I assume the reference voltage is 0V so you need to amplify it by 10.

With inverting the circuit I mean to switch polarity on all diodes, caps and exchange NPN and PNP transistors. Then you should get out of the situation where Vbe limit the voltage range (useless for low voltages)

[edit]
Forget about previous statement, I see now that you're speaking AC voltage, not DC.
 
Last edited:

hi,
If you post your LTspice asc file, we can check what you are trying to do.
 
yes I need the AC voltage to be amplified. DC voltage is supplied only by +12 power supply. (If I have -12V below the circuit, everything will be great )
 
I'm using ORCAD Pspice 16.3 and it is on school's lab. I'm going to post the *.asc or the file for Orcad Pspice tomorrow. Thanks again for your concern.
 
I also wanted to say that when I do a DC analysis and AC analysis, IC values do not match the circuit.
Somewhere I found the formula for Gain = -Rc*gm/ (4*Vt). Also current mirror should be around 400mA from what the teachers say. (Q3,Q4, R5,R15 and R16 are for just current mirror purposes)
 


Hi there,

The main problem with your circuit is that Q2 is not biased properly. There is no voltage available for forward biasing of the base emitter junction.
To correct this, if you have Q1 biased with two 39k resistors (one to +Vcc and the other to ground as R13 and R14) then you can start by biasing Q2 that same way, also with two 39k resistors one to +Vcc and the other to ground. That will get you started.
 
Last edited:
oh thank you MrAl. I have biased Q1 and Q2 with two pair of 39k R's as you said and with RC = 22k, RE = 270 the circuit gives me 100mV at output! Thanks for the help!
 
hamsiii said:
oh thank you MrAl. I have biased Q1 and Q2 with two pair of 39k R's as you said and with RC = 22k, RE = 270 the circuit gives me 100mV at output! Thanks for the help!
Click to expand...

Oh you're welcome, and good luck with your project.

Take care for now,
Al
 
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