### Author Topic: SORTED - Differential Amp - BJT working, JFET not working  (Read 1057 times)

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#### tron9000

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##### SORTED - Differential Amp - BJT working, JFET not working
« on: November 03, 2015, 08:50:01 pm »
So I have 2 Identical differential amp circuits:

R1 & R2 =10k, & R3 & R4 = 100k, giving a gain of 10. http://www.electronics-tutorials.ws/opamp/opamp_5.html

I have the inputs to the inputs to the amp across a 1R resistor with another 1R resistor in series with a variable power supply across both. adjust the voltage and the current increase in the circuit and the voltage out of the amplifier will be 10x the voltage across the resistor (V=IxR)

I'm running off a single rail, 12V supply for the diff amp cct.

If I use a BJT input op-amp: the LM324 - it works well; Vout = 10x (IxR)

If I use a JFET input op-amp: the TL074 - I get some weird results:
 Vin/V Vout/V Gain 0.1 11.327 107 0.2 11.328 56.64 0.3 11.253 37.51 0.4 4.755 11.887 0.5 4.005 8.01 0.6 3.885 6.475 0.7 3.659 5.227 0.8 4.150 5.187 0.9 6.160 6.844 1.0 8.168 8.168
where Vin is measure across the 1R resistor on input to amp, so the voltage goes from 0.2V to 2V, I've looked at the data sheets for both and the common mode ranges seem to confirm that they should work within that voltage.

Are JFET input op-amps just not designed for this roll? Or have I overlooked something?
« Last Edit: November 04, 2015, 12:22:07 am by tron9000 »
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#### Andy Watson

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##### Re: Differential Amp - BJT working, JFET not working
« Reply #1 on: November 03, 2015, 09:06:39 pm »
The common-mode voltage input range of the TL074 is not rail-to-rail. The datasheet suggests that it operates -12 to 15V with +/- 15V supplies, i.e. 3V above the most negative supply rail (0V in your case).

#### tron9000

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##### Re: Differential Amp - BJT working, JFET not working
« Reply #2 on: November 03, 2015, 09:15:13 pm »
So I've overlooked something - right I think I understand now where I've gone wrong: cos (as you stated) the common voltage range for a +/-15V goes -12v to +15v, and that cos I'm using a single supply rail that output is effectively 3 to 12V?
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#### tron9000

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##### Re: Differential Amp - BJT working, JFET not working
« Reply #3 on: November 04, 2015, 12:21:44 am »
yep, just stuck the TL074 on +/-12V supply rail and managed to get some sensible linear readings from 0 to 1A current. Fell into a trap!