Bias currents for the LMH6703

Jose Delgado

Other Parts Discussed in Thread: LMH6703

Hello,

How were the LMH6703 Bias currents characterized? What was the circuit set up? When I tried measuring the Bias currents the part doesn't follow general op amp circuit set up.

Thanks for your response,

over 13 years ago

0 Loren Siebert 1 over 13 years ago

TI__Genius 13935 points

The LMH6703 is a current feedback amplifier. It has only one high impedance input (the non inverting input). The bias currents are measured on that input only.

Generally speaking, high bandwidth, current feedback amplifiers will exhibit fairly large bias currents compared to precision op amps or JFET or CMOS op amps.

To a first order approximation the non inverting input of the LMH6603 will look like the base of a fairly large NPN transistor. The inverting input will look like the emitter of an NPN transistor (low impedance).

0 Jose Delgado over 13 years ago in reply to Loren Siebert 1

Prodigy 225 points

Loren,

Why did National Semi characterize both inputs - Non-Inverting Typ = -7uA, Inverting Typ=-2uA?

Pleas see datasheet on page 3.

How was the inverting input characterized then?

Thanks for your reponse,

0 Loren Siebert 1 over 13 years ago in reply to Jose Delgado

TI__Genius 13935 points

I do not have access to the test schematics to give you an exact answer on how the bias currents are measured.

With a voltage feedback amplifier the inputs are symmetrical. With a current feedback amplifier the inputs are not symmetrical.

What is your application? Do you have specific bias current requirements?

0 Jose Delgado over 13 years ago in reply to Loren Siebert 1

Prodigy 225 points

Loren,

I am trying to characterize according to your datasheet because I need to test this part for environmental effects.

Who can I talk too in order for me to duplicate the datasheet's spec?

Thanks for your help,

0 Loren Siebert 1 over 13 years ago in reply to Jose Delgado

TI__Genius 13935 points

I am pretty sure our test equipment uses standard test fixtures like shown in this article: http://www.eetimes.com/design/automotive-design/4010017/SIGNAL-CHAIN-BASICS-Part-7--Op-Amp-Performance-Specifications-Bias-Current

What results are you getting? Could it be a sign error?

Again, the only real bias current in a current feedback amplifier is on the non inverting input. The current seen on the inverting input is not a bias current in the traditional sense of the word, though there will be some current on that pin.

http://www.ti.com/lit/an/snaa004/snaa004.pdf

http://www.ieee.li/pdf/viewgraphs/current_feedback_vs_voltage_feedback_amplifiers.pdf

See Figure 2. The + in pin can be connected to either a PNP or an NPN transistor.

0 Jose Delgado over 13 years ago in reply to Loren Siebert 1

Prodigy 225 points

Loren,

"What results are you getting?" The op amp is in an open loop. I connect one input to a current meter to measure the bias current, and the other input is set to ground. Then I reverse the connections to measure the other bias currents off the other pin. Output is open.

-In reading fails at about -64uA or about 50uA, but sometimes passes with a reading of 5uA.

+In reading passes at about -7.5uA

So I checked for oscillating output - scope connected at the output (passive probe Tektronix).

-In reading passes at about -2uA With the scope probe connected as a load the IC passes spec.

Is this test done with a load?

Thanks for your help,

0 Loren Siebert 1 over 13 years ago in reply to Jose Delgado

TI__Genius 13935 points

Bias currents should be measured closed loop.

A load is a good idea.

Passive scope probes are usually around 6pF and should not be used on high speed amplifiers.

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Bias currents for the LMH6703