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Old 23rd August 2012, 10:08 PM   #21
jcx is offline jcx  United States
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no one read the data sheet - the very 1st post basic premise is wrong

the 1794 has a -6.2 mA DC bias on both outputs - biasing the I/V op amp outputs into Class A
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Old 23rd August 2012, 10:28 PM   #22
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smms73, what about a current feedback Opamp like the LME49713 ? It is low noise, optimized for inverted ( shunt feedback ) and can throw around 100mA, not all in class a though but with a constant current source pulling at the output it can be biased into class a.
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Old 23rd August 2012, 10:42 PM   #23
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What has to be watched is that the feedback resistor has an effect on the frequency response.http://www.ti.com/lit/ds/symlink/lme49713.pdf
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Old 24th August 2012, 07:16 AM   #24
smms73 is offline smms73  Portugal
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Quote:
Originally Posted by jcx View Post
no one read the data sheet - the very 1st post basic premise is wrong

the 1794 has a -6.2 mA DC bias on both outputs - biasing the I/V op amp outputs into Class A
If the op amp U1 swings 21ma pp, how the -6.2ma are enough to keep it in class A?
Dont forget that U1 has to feed R4 (570ohm) plus the dac output current.

Think before you say someone is wrong.
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Old 24th August 2012, 07:22 AM   #25
smms73 is offline smms73  Portugal
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I will look to LME49713 Joachim, as your tips are always useful.
I have also to look for the fully differential op amps that texas have.
But not today (no time)
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Old 24th August 2012, 07:45 AM   #26
smms73 is offline smms73  Portugal
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Quote:
Originally Posted by georgehifi View Post
smms73, seeing your so handy at running sim's, could you do me a favour and run the sim on this, circuit.
Hi georgehifi,
I dont really have the time, but if you download the ltspice and create the circuit, then send it to me, i will run some simulations for you.
LTspice is a very powerfull tool and is very intuitive to create the circuit.
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Old 24th August 2012, 07:35 PM   #27
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Nice one, jcx.

Quote:
Originally Posted by smms73 View Post
If the op amp U1 swings 21ma pp, how the -6.2ma are enough to keep it in class A?
Dont forget that U1 has to feed R4 (570ohm) plus the dac output current
The DAC output swings 7.8 mA Pk-Pk, therefore the DAC output swings from -2.3mA to -10.1mA, i.e, the swing never crosses zero. The I/V opamp has its non-inverting input grounded. Since the current driving the inverting input never crosses zero, the opamp output never crosses zero. It never drives less than 2.3mA through Rf to maintain the voltage at the inverting input the same as the voltage at the grounded non-inverting input. Therefore it is always biassed into class A.

The fact that R4 on the output of U2 is grounded via R6 simply means that U2 is driven further into class A because it has to supply current through these 2 in addition to maintaining the net current at its inverting input at zero via its own Rf. A slightly more complicated situation pertains in regard to R3 and R5.

You guys must be reading a different datasheet, because U1 doesn't drive R4, and there's no 570R resistor.

The people at TI are no fools. Anyone who thinks they can be easily outdone might be a fool though.

I had a little think, someone is wrong.

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Old 24th August 2012, 08:40 PM   #28
smms73 is offline smms73  Portugal
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Quote:
Originally Posted by counter culture View Post
Nice one, jcx.



The DAC output swings 7.8 mA Pk-Pk, therefore the DAC output swings from -2.3mA to -10.1mA, i.e, the swing never crosses zero. The I/V opamp has its non-inverting input grounded. Since the current driving the inverting input never crosses zero, the opamp output never crosses zero. It never drives less than 2.3mA through Rf to maintain the voltage at the inverting input the same as the voltage at the grounded non-inverting input. Therefore it is always biassed into class A.

The fact that R4 on the output of U2 is grounded via R6 simply means that U2 is driven further into class A because it has to supply current through these 2 in addition to maintaining the net current at its inverting input at zero via its own Rf. A slightly more complicated situation pertains in regard to R3 and R5.

You guys must be reading a different datasheet, because U1 doesn't drive R4, and there's no 570R resistor.

The people at TI are no fools. Anyone who thinks they can be easily outdone might be a fool though.

I had a little think, someone is wrong.

Ok i admit, i´m only human... and i make mistakes, lots of them.

But i try to be polite.
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Old 24th August 2012, 09:37 PM   #29
smms73 is offline smms73  Portugal
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I have take a look to the simulation again,

U2 are working in class a for shure swings between 19ma and 4.33ma

U1 is swing between 22ma and 1ma

U3 swings to -12ma to +1ma and this make it work in class b not in class a.

At 5ma pk to pk all of them work in class A.

I will try to change some resistor values to see if i can make this better.

One way or another i still think buffers are better.

Maybe i´m really a fool try to make things better , nobody can for sure.
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Last edited by smms73; 24th August 2012 at 09:42 PM. Reason: missing
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Old 25th August 2012, 12:31 AM   #30
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Something is wrong with your sim if U1 swings to 1mA. This would mean U3 is sourcing 1.3mA of current thru R5, R3 and Rf of U1 to keep pin 2 of U1 at ground potential, and you said U3 swings -12mA to +1mA. Since you quote 4.33mA for U2, I presume +1mA is not an approximation.

You should be doing this kind of sanity checking yourself.
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