Differential voltage measurement with op-amp

[Rithers]

Hello electro gurus!
I am trying to measure the difference in Vbe of 2 transistors, amplified through an op-amp to make it easier for my cheap-o multimeter to see.
I don't know how to correctly hook up the op amp for this,   here's what I have so far.
The readings I am getting seem like they're in the ballpark, 
but I am clearly in over my head too far to make an honest assessment.   
I dont mind if the measurement is inverted,    I am just going for 100x gain.    (for example to get a reading of -250.0mV rather than 0002.5mV)

any and all help appreciated!
/Etienne Stehelin
Los Angeles CA

the meat of this circuit is based on Ian Fritz's
http://www.dragonflyalley.com/synth/images/TransistorMatching/ianFritz-transmat0011_144.pdf

[suicidaleggroll]

This page goes through the setup to make an op-amp a difference amplifier.  I haven't studied your circuit enough to know whether or not the load this difference amp puts on the circuit would affect the measurements, you may want to look into an instrumentation amplifier to prevent that (also covered on this page):
http://www.electronics-tutorials.ws/opamp/opamp_5.html

[Rithers]

Thanks!    It looks like I was missing resistors between the non-inverting input, and ground.     If all the resistors surrounding the op-amp are equal, I have a unity gain amp; then if I change R7 to 100k  (as I have in the following drawing) should give me a gain of -100, correct
Can't wait to try this out when I get home. 

/Etienne

[suicidaleggroll]

Except now you're loading the emitter with 2k to ground.  Your 100k resistors in the circuit become pointless since they're in parallel with 2k, and the 2k and 3.9k on the collector side of the transistor becomes a voltage divider that drags the transistor down towards ground, affecting the bias.

You may need to add a pair of buffer amps to keep from loading down the circuit, turning the difference amp into an instrumentation amp.  With a gain of 100, you're going to have to watch out for op-amp offset voltage and drift as well.  The TL072 has a 3mV offset spec, multiply that by the gain and you have +/- 300mV of uncertainty in your result.

[Rithers]

"You may need to add a pair of buffer amps to keep from loading down the circuit, turning the difference amp into an instrumentation amp."

This is what I'll do.     
Thanks for the advice. 

/Etienne

EDIT -  after a couple hours of reading,  it looks like I might be better off spending $3 on an actual Instrumentation Amp IC than trying to piece this thing together with TL074s and resistors.   

[Rithers]

The TL072 has a 3mV offset spec, multiply that by the gain and you have +/- 300mV of uncertainty in your result."

Would it make sense for me to use a TL071,   and dial out any offset with an offset null trimmer?   Or will there always be uncertainty with a TL0xx at 100x gain?

Thanks
/etienne

[Zero999]

The TL072 has a 3mV offset spec, multiply that by the gain and you have +/- 300mV of uncertainty in your result."

Would it make sense for me to use a TL071,   and dial out any offset with an offset null trimmer?   Or will there always be uncertainty with a TL0xx at 100x gain?

Thanks
/etienne

Possible but lots of hassle.

Why not go for the OP07 which has a maximum offset of 150µV?
http://www.analog.com/media/en/technical-documentation/data-sheets/OP07.pdf

[Rithers]

I mainly have whats laying around...  but i might grab the OP7 in my next order o' parts.   
I was also considering a $3 instrumentation amp,  INA126:
http://tinyurl.com/ybozztgb

Although I've recently heard that transistors plucked from the same batch/tape reel have a close enough Vbe and maybe I'm spending too much energy on this.   

A fun learning experience nonetheless!

/rithers